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Patients with critical illness often require mechanical ventilation. Mechanical ventilation can replace or supplement normal spontaneous ventilation. In some instances, the problem is primarily that of impaired CO2 elimination (ventilatory failure). In other instances, mechanical ventilation may be used as an adjunct (usually to positive-pressure therapy, as discussed next) in the treatment of hypoxemia. The decision to initiate mechanical ventilation is made on clinical grounds, but certain parameters have been suggested as guidelines (Table 58–4).
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Of the two available techniques, positive-pressure ventilation and negative-pressure ventilation (iron lung), the former has much wider applications and is almost universally used. Although negative-pressure ventilation does not require tracheal intubation, it cannot overcome substantial increases in airway resistance or decreases in pulmonary compliance, and it also limits access to the patient.
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During positive-pressure ventilation, lung inflation is achieved by periodically applying positive pressure to the upper airway through a tight-fitting mask (noninvasive mechanical ventilation) or through a tracheal or tracheostomy tube. Increased airway resistance and decreased lung compliance can be overcome by manipulating inspiratory gas flow and pressure. The major disadvantages of positive-pressure ventilation are altered ventilation-to-perfusion relationships, potentially adverse circulatory effects, and risk of pulmonary barotrauma and volutrauma. Positive-pressure ventilation increases physiological dead space because gas flow is preferentially directed to the more compliant, nondependent areas of the lungs, whereas blood flow (influenced by gravity) favors dependent areas. Reductions in cardiac output are primarily due to impaired venous return to the heart from increased intrathoracic pressure. Barotrauma is closely related to repetitive high peak inflation pressures and underlying lung disease, whereas volutrauma is related to the repetitive collapse and reexpansion of alveoli.
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1. Positive-Pressure Ventilators
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Positive-pressure ventilators periodically create a pressure gradient between the machine circuit and alveoli that results in inspiratory gas flow. Exhalation occurs passively. Ventilators and their control mechanisms can be powered pneumatically (by a pressurized gas source), electrically, or by both mechanisms. Gas flow is either derived directly from the pressurized gas source or produced by the action of a rotary or linear piston. This gas flow then either goes directly to the patient (single-circuit system) or, as commonly occurs with operating room ventilators, compresses a reservoir bag or bellows that is part of the patient circuit (double-circuit system).
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All ventilators have four phases: inspiration, the changeover from inspiration to expiration (cycling), expiration, and the changeover from expiration to inspiration (trigger) (see Chapter 4). These phases are defined by VT, ventilatory rate, inspiratory time, inspiratory gas flow, and expiratory time.
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Classification of Ventilators
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Modern ventilators are complicated and defy simple classification. Incorporation of microprocessors into ventilators has further complicated this task. Complexity of ventilator nomenclature combined with proprietary naming schema for similar ventilator functionality has led to calls for uniform taxonomy for ventilator modes. Chatburn provides one such nomenclature, and its salient points are used to frame the following discussion on ventilator modes.
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The breathing period can be divided into four phases: (1) change from expiration to inspiration (trigger), (2) inspiration (target), (3) change from inspiration to expiration (cycle), and (4) expiration. The trigger variable starts inspiration when it (pressure, volume, flow, or time) reaches a preset value. When time is the trigger, breaths are initiated on a defined frequency regardless of patient effort (see Figure 58–1). Alternatively, pressure, flow, or volume triggers initiate a breath when the ventilator detects a change in pressure, flow, or volume, respectively caused by patient effort.
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The target variable (pressure, volume, or flow) must reach a specified level before inspiration ends. This used to be called “limit” but nomenclatures have changed. The target variable does not define the end of inspiration, but only the upper boundary for each breath. When the cycle variable is reached, inspiration ends. Options include pressure, volume, flow, or time.
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Pressure-cycled ventilators cycle into the expiratory phase when airway pressure reaches a predetermined level. VT and inspiratory time vary, being related to airway resistance and pulmonary and circuit compliance. A significant leak in the patient circuit can prevent the necessary rise in circuit pressure and machine cycling. Conversely, an acute increase in airway resistance, or decrease in pulmonary compliance, or circuit compliance (eg, a kinked tube) causes premature cycling and decreases the delivered VT. Pressure-cycled ventilators have been most often used for short-term indications (eg, transport).
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Volume-cycled ventilators terminate inspiration when a preselected volume is delivered. Many adult ventilators are volume cycled but also have secondary limits on inspiratory pressure to guard against pulmonary barotrauma. If inspiratory pressure exceeds the pressure limit, the machine cycles into expiration even if the selected volume has not been delivered.
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Properly functioning volume-cycled ventilators do not deliver the set VT to the patient. A percentage of the set VT is always lost due to expansion of the breathing circuit during inspiration. Circuit compliance is usually about 3 to 5 mL/cm H2O; thus, if a pressure of 30 cm H2O is generated during inspiration, 90 to 150 mL of the set VT is lost to the circuit. Loss of VT from expansion of the breathing circuit is therefore inversely related to lung compliance. For accurate measurement of the exhaled VT, the spirometer must be placed at the tracheal tube rather than the exhalation valve of the ventilator.
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Flow-cycled ventilators have pressure and flow sensors that allow the ventilator to monitor inspiratory flow at a preselected fixed inspiratory pressure; when this flow reaches a predetermined level (usually 25% of the initial peak mechanical inspiratory flow rate), the ventilator cycles from inspiration into expiration (see the later sections on pressure support and pressure-controlled ventilation).
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Time-cycled ventilators cycle to the expiratory phase once a predetermined interval elapses from the start of inspiration. VT is the product of the set inspiratory time and inspiratory flow rate. Time-cycled ventilators are commonly used for neonates and in the operating room.
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A ventilator mode is a combination of control variable, breath sequence, and target scheme. Additionally, to understand ventilator mechanics one must consider the phase variables (introduced earlier).
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The control variable is the independent variable in the ventilator mode (see Figure 58–2). Choice are pressure, volume, and flow. In pressure-controlled ventilation (PCV), pressure is independent variable, and pressure waveform is specified (eg, rectangular waveform). In volume-controlled ventilation (VCV), similarly a volume waveform is defined. In common terminology, flow-controlled ventilation is not commonly used because flow is derivative of volume. When one directly controls volume, one indirectly controls flow.
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Targeting scheme is a feedback control design to deliver a specific pattern. A type of targeting scheme called set point targeting is the most basic. One sets a value, and ventilator seeks to deliver it. For VCV, set points would be VT and flow. For PCV, commonly it would be inspiratory pressure and inspiratory time.
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Breath sequence is the pattern of mandatory or spontaneous breaths in a ventilator mode, or both (see Figure 58–3). A spontaneous breath is defined as a breath in which a patient determines both timing and size of the breath. It is patient triggered and patient cycled. A mandatory breath is any breath that is not spontaneous. An assisted breath is a breath in which the ventilator does some of the work for a patient-initiated breath.
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There are three possible breath sequences. Continuous spontaneous ventilation (CSV) is a sequence in which all breaths are spontaneous. Intermittently mandatory ventilation (IMV) is a sequence in which spontaneous breaths are permitted in between mandatory breaths. If a mandatory breath is triggered by the patient, it a “synchronized” mandatory breath. In continuous mandatory ventilation (CMV), all breaths (including those by patient effort) are mandatory.
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Combining the two types of control variables (pressure control [PC] and volume control [VC]) with three breath sequences gives us five breathing patterns: VC-CMV, VC-IMV, PC-CMV, PC-IMV, and PC-CSV. The sixth, VC-CSV, would mean that patient would specify time and size of breath; however, in VC mode, a patient could not specify the size of breath.
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A. IMV Breath Sequences
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IMV permits spontaneous ventilation while the patient is on the ventilator. A selected number of mechanical breaths (with fixed VT) are given to supplement spontaneous breathing. At high mandatory rates (10–12 breaths/min), IMV essentially provides all of the patient’s ventilation; at low rates (1–2 breaths/min), it provides minimal mechanical ventilation and allows the patient to breathe relatively independently. The frequency and VT of spontaneous breaths are determined by the patient’s ventilatory drive and muscle strength. The IMV rate can be adjusted to maintain a desired minute ventilation. IMV has found greatest use as a weaning technique.
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Synchronized intermittent mandatory ventilation (SIMV) times the mechanical breath, whenever possible, to coincide with the beginning of a spontaneous effort. Proper synchronization prevents superimposing (stacking) a mechanical breath in the middle of a spontaneous breath, which might otherwise result in a very large VT. As with CMV and assist-control (AC) ventilation (discussed next), settings to limit inspiratory pressure guard against pulmonary barotrauma. The great advantage of SIMV over IMV is that it provides for increased patient comfort. When IMV or SIMV are used for weaning, the machine breaths provide a backup if the patient becomes fatigued. However, if the rate is set too low (4 breaths/min), the backup may be insufficient, particularly for weak patients who may not be able to overcome the added work of breathing during spontaneous breaths.
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IMV circuits provide a continuous flow of gas for spontaneous ventilation between mechanical breaths. Modern ventilators incorporate SIMV into their design, but older models required modification with a parallel circuit, a continuous flow system, or a demand flow valve. Regardless of the system, proper functioning of one-way valves and sufficient gas flow are necessary to prevent an increase in the patient’s work of breathing, particularly when PEEP is also used.
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Thus far the discussion of IMV has assumed this to be a volume-limited format; however, IMV can also be provided in pressure-limited format if desired (as described next).
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B. Pressure-Controlled Breath Sequences (PC-CMV, PC-IMV, and PC-CSV)
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PC breath sequences may be used in both the AC and IMV modes. In AC mode, all breaths (either machine initiated or patient initiated) are time cycled and pressure limited. In IMV, machine-initiated breaths are time cycled and pressure limited. The patient may breathe spontaneously between the set rate, and the VT of the spontaneous breaths is determined by the patient. The advantage of PCV is that by limiting inspiratory pressure, the risks of barotrauma and volutrauma may be decreased. Also, by extending inspiratory time, better mixing and recruitment of collapsed or flooded alveoli may be achieved, when used with adequate levels of PEEP levels.
The disadvantage of conventional PCV is that VT is not guaranteed (although there are modes in which the consistent delivered pressure of PCV can be combined with a predefined volume delivery). Changes in compliance or resistance will affect the delivered VT. This is a major issue in patients with acute lung injury because if the compliance decreases and the pressure limit is not increased, adequate VT may not be attained. PCV has been used for patients with acute lung injury or ARDS, often with a prolonged inspiratory time or inverse inspiratory/expiratory (I:E) ratio ventilation (IRV) (see later discussion) in an effort to recruit collapsed and flooded alveoli. The disadvantage of using IRV with PCV is that patients will require deep sedation, perhaps with paralysis to tolerate this ventilatory mode.
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With PCV, pressure and inspiratory time are preset, whereas flow and volume are variable and dependent on the patient’s resistance and compliance. With volume ventilation, on the other hand, inspiratory time is also preset but flow and VT are also preset, and with increased resistance or reduced compliance the inspiratory pressure can be greatly increased.
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PCV is similar to pressure support ventilation (PSV) in that peak airway pressure is controlled but is different in that a mandatory rate and inspiratory time are selected. As with pressure support, gas flow ceases when the pressure level is reached; however, the ventilator does not cycle to expiration until the preset inspiration time has elapsed.
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A. Continuous Mandatory Ventilation (Example of VC-CMV Breathing Pattern)
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In this mode, the ventilator cycles from expiration to inspiration after a fixed time interval. The interval determines the ventilatory rate. Typical settings on this mode provide a fixed VT and fixed rate (and, therefore, minute ventilation) regardless of patient effort, because the patient cannot breathe spontaneously. Settings to limit inspiratory pressure guard against pulmonary barotrauma, and indeed CMV can be provided in a pressure-limited (rather than volume-limited) way. Controlled ventilation is best reserved for patients capable of little or no ventilatory effort. Awake patients with active ventilatory effort require sedation, possibly with muscle paralysis, to safely receive CMV.
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B. Assist-Control Ventilation (Example of VC-CMV Breathing Pattern, or Can Be Set as PC-CMV)
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Incorporation of a pressure sensor in the breathing circuit of AC ventilators permits the patient’s inspiratory effort to be used to trigger inspiration. A sensitivity control allows selection of the inspiratory effort required. The ventilator can be set for a fixed ventilatory rate, but each patient effort of sufficient magnitude will trigger the set Vt. If spontaneous inspiratory efforts are not detected, the machine functions as if in the control mode. Most often, AC ventilation is used in a volume-limited format, but it can also be provided in a pressure-limited way (as discussed later).
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C. Pressure Support Ventilation (Example of PC-CSV Breathing Pattern)
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PSV was designed to augment the VT of spontaneously breathing patients and overcome any increased inspiratory resistance from the tracheal tube, breathing circuit (tubing, connectors, and humidifier), and ventilator (pneumatic circuitry and valves). Microprocessor-controlled machines have this mode, which delivers sufficient gas flow with every inspiratory effort to maintain a predetermined positive pressure throughout inspiration. When inspiratory flow decreases to a predetermined level, the ventilator’s feedback (servo) loop cycles the machine into the expiratory phase, and airway pressure returns to baseline (Figure 58–4). The only setting in this mode is inspiratory pressure. The patient determines the respiratory rate and VT varies according to inspiratory gas flow, lung mechanics, and the patient’s own inspiratory effort. Low levels of PSV (5–10 cm H2O) are usually sufficient to overcome any added resistance imposed by the breathing apparatus. Higher levels (10–40 cm H2O) can function as a standalone ventilatory mode if the patient has sufficient spontaneous ventilatory drive and stable lung mechanics. The principal advantages of PSV are its ability to augment spontaneous VT, decrease the work of breathing, and increase patient comfort. However, if the patient fatigues or lung mechanics change, VT may be inadequate, and there is no backup rate if the patient’s intrinsic respiratory rate decreases (eg, after opioid dosing). Note that one can add “pressure support” to IMV breathing breath sequences (Figure 58–5). The IMV machine mandatory breaths provide backup, and a low level of pressure support is used to offset the increased work of breathing resulting from the breathing circuit and machine during the spontaneous breaths.
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D. Inverse I:E Ratio Ventilation Modes, Including Airway Pressure Release Ventilation (Usually Examples of PC-IMV Breathing Pattern)
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IRV reverses the normal I:E time ratio of 1:3 or greater to a ratio of greater than 1:1 (eg, 1.5:1). This may be achieved by adding an end-inspiratory pause, by decreasing peak inspiratory flow during volume-controlled ventilation (CMV), or by setting an inspiratory time such that inspiration is longer than expiration during PCV (PC-IRV). Intrinsic PEEP may be produced during IRV and is caused by air trapping or incomplete emptying of the lung to the baseline pressure prior to the initiation of the next breath. This air trapping increases FRC until a new equilibrium is reached. This mode does not allow spontaneous breathing and requires heavy sedation or neuromuscular blockade. IRV with PEEP is effective for improving oxygenation in patients with decreased FRC.
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E. Airway Pressure Release Ventilation
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Airway pressure release ventilation (APRV), or bilevel ventilation, is a mode in which a relatively high PEEP is used, despite the patient being allowed to breathe spontaneously. Intermittently, the PEEP level decreases to help augment the elimination of CO2 (Figure 58–6). The inspiratory and expiratory times, high and low PEEP levels, and spontaneous respiratory activity determine minute ventilation. Initial settings include a minimum PEEP of 10 to 12 cm H2O and a release level of 5 to 10 cm H2O. Commonly, 10 to 12 releases are chosen as a starting point, along with a time at low PEEP to allow only 50% to 70% of expiratory flow (to provide “autoPEEP”). Advantages of APRV appear to be less circulatory depression and pulmonary barotrauma as well as less need for sedation. This technique appears to be an attractive alternative to PC-IRV for overcoming problems with high peak inspiratory pressures in patients with reduced lung compliance.
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Basic Mechanics of Ventilators
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Most modern ventilators behave like flow generators. Constant flow generators deliver a constant inspiratory gas flow regardless of airway circuit pressure. Constant flow is produced by the use of either a solenoid (on–off) valve with a high-pressure gas source (5–50 psi) or via a gas injector (Venturi) with a lower-pressure source. Machines with high-pressure gas sources allow inspiratory gas flow to remain constant despite large changes in airway resistance or pulmonary compliance. Ventilator performance varies with airway pressure for gas injectors. Nonconstant flow generators consistently vary inspiratory flow with each inspiratory cycle (such as by a rotary piston); a sine wave pattern of flow is typical.
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Constant-pressure generators maintain airway pressure constant throughout inspiration and irrespective of inspiratory gas flow. Gas flow ceases when airway pressure equals the set inspiratory pressure. Pressure generators typically operate at low gas pressures (just above peak inspiratory pressure).
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Cycling (Changeover from Inspiration to Expiration)
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Time-cycled ventilators cycle to the expiratory phase once a predetermined interval elapses from the start of inspiration. VT is the product of the set inspiratory time and inspiratory flow rate. Time-cycled ventilators are commonly used for neonates and in the operating room.
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Volume-cycled ventilators terminate inspiration when a preselected volume is delivered. Many adult ventilators are volume cycled but also have secondary limits on inspiratory pressure to guard against pulmonary barotrauma. If inspiratory pressure exceeds the pressure limit, the machine cycles into expiration even if the selected volume has not been delivered.
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Pressure-cycled ventilators cycle into the expiratory phase when airway pressure reaches a predetermined level. VT and inspiratory time vary, being related to airway resistance and pulmonary and circuit compliance. A significant leak in the patient circuit can prevent the necessary rise in circuit pressure and machine cycling. Conversely, an acute increase in airway resistance, or decrease in pulmonary compliance, or circuit compliance (kink) causes premature cycling and decreases the delivered VT. Pressure-cycled ventilators have been most often used for short-term indications (transport).
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Flow-cycled ventilators have pressure and flow sensors that allow the ventilator to monitor inspiratory flow at a preselected fixed inspiratory pressure; when this flow reaches a predetermined level (usually 25% of the initial peak mechanical inspiratory flow rate), the ventilator cycles from inspiration into expiration (see the earlier sections on pressure support and pressure-controlled ventilation).
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Microprocessor-Controlled Ventilators
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These versatile machines can be set to function in any one of a variety of inspiratory flow and cycling patterns. Microprocessor-controlled ventilators are the norm in modern critical care units and on newer anesthesia machines.
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High-Frequency Ventilation
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High-frequency ventilation is sufficiently different from conventional modes of mechanical ventilation that its mechanics require separate mention. Three forms of high-frequency ventilation (HFV) are available. High-frequency positive-pressure ventilation involves delivering a small “conventional” VT at a rate of 60 to 120 breaths/min. High-frequency jet ventilation (HFJV) utilizes a small cannula at or in the airway through which a pulsed jet of high-pressure gas is delivered at a set frequency of 120 to 600 times/min (2–10 Hz). The jet of gas may entrain air (Bernoulli effect), which may augment VT. High-frequency oscillation employs a driver (usually a piston) that creates to-and-fro gas movement in the airway at rates of 180 to 3000 times/min (3–50 Hz).
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These forms of ventilation all produce VT at or below anatomic dead space. The exact mechanism of gas exchange is unclear but is probably a combination of effects. Jet ventilation has found widest use in the operating room. It may be used for laryngeal, tracheal, and bronchial procedures and can be lifesaving in emergency airway management when tracheal intubation and conventional positive-pressure ventilation are unsuccessful (see Chapter 19). In the intensive care unit (ICU), HFJV may be useful in managing some patients with bronchopleural and tracheoesophageal fistulas when conventional ventilation has failed. Occasionally, HFJV or high-frequency oscillation is used in patients with ARDS to try to improve oxygenation. Inadequate heating and humidification of inspired gases during prolonged HFV, however, can be a problem. Initial settings for HFJV are typically a rate of 120 to 240 breaths/min, an inspiratory time of 33%, and a drive pressure of 15 to 30 psi. Mean airway pressure should be measured in the trachea at least 5 cm below the injector to avoid an artifactual error from gas entrainment. Carbon dioxide elimination is generally increased by increasing the drive pressure, whereas adequacy of oxygenation relates to the mean airway pressure. An intrinsic PEEP effect is seen during HFJV at high drive pressures and inspiratory times greater than 40%.
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Differential Lung Ventilation
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This technique, also referred to as independent lung ventilation, may be used in patients with severe unilateral lung disease or those with bronchopleural fistulae. Use of conventional positive-pressure ventilation and PEEP in such instances can aggravate ventilation/perfusion mismatching or, in patients with fistula, result in inadequate ventilation of the unaffected lung. In patients with restrictive disease of one lung, overdistention of the normal lung can lead to worsening hypoxemia or barotrauma. After separation of the lungs with a double-lumen tube, positive-pressure ventilation can be applied to each lung independently using two ventilators. When two ventilators are used, the timing of mechanical breaths is often synchronized, with one ventilator, the “master,” setting the rate for the “slave” ventilator.
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2. Care of Patients Requiring Mechanical Ventilation
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Tracheal intubation is most commonly undertaken in ICU patients to manage pulmonary failure with mechanical ventilation.
Both nasotracheal and orotracheal intubation appear to be relatively safe for at least 2 to 3 weeks. When compared with orotracheal intubation, nasotracheal intubation may be more comfortable for the patient and more secure (fewer instances of accidental extubation). Nasal intubation, however, has its own set of associated adverse events, including nasal bleeding, transient bacteremia, submucosal dissection of the nasopharynx or oropharynx, and sinusitis or otitis media (from obstruction of sinus outflow or of the auditory tubes). Nasal intubations will also generally incorporate a smaller diameter tube than orotracheal intubations, and this can make it more difficult to clear secretions and can force the clinician to use smaller fiberoptic bronchoscopes.
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Intubation often can be carried out without the use of sedation or muscle paralysis in agonal or unconscious patients. However, topical anesthesia of the airway and sedation are helpful in patients who still have active airway reflexes. More vigorous and uncooperative patients require varying degrees of sedation. Small doses of relatively short-acting agents are generally used; popular agents include midazolam, etomidate, dexmedetomidine, and propofol. If necessary, a neuromuscular blocker can be used for paralysis after a hypnotic is given.
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The time of tracheal intubation and initiation of mechanical ventilation can be a period of great hemodynamic instability. Hypertension or hypotension and bradycardia or tachycardia may be encountered. Responsible factors include activation of autonomic reflexes from stimulation of the airway, myocardial depression and vasodilation from sedative-hypnotic agents, straining by the patient, withdrawal of intense sympathetic activity, and reduced venous return due to positive pressure in the airways. Careful monitoring is required during and immediately following intubation of patients with critical illness.
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When left in place for more than 2 to 3 weeks, both orotracheal and nasotracheal tubes predispose patients to subglottic stenosis. If longer periods of mechanical ventilation are necessary, the tracheal tube should be replaced by a cuffed tracheostomy tube. If it is clear that a tracheal tube will be required for more than 2 weeks, a tracheostomy may be performed soon after intubation. While earlier tracheostomy does not reduce mortality, it may reduce the incidence of pneumonia, the duration of mechanical ventilation, and the length of stay.
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Initial Ventilator Settings
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Depending on the type of pulmonary failure, mechanical ventilation is used to provide either partial or full ventilatory support. For full ventilatory support, CMV, AC, or PCV is generally employed with a respiratory rate of 10 to 14 breaths/min, a VT of 6 mL/kg, and PEEP of 5 to 10 cm H2O. These settings reduce the likelihood of high peak inflation pressures (greater than 35 to 40 cm H2O), barotrauma, and volutrauma. High airway pressures that overdistend alveoli (transalveolar pressure >35 cm H2O) promote experimental lung injury.
Likewise, compared with a VT of 12 mL/kg, a VT of 6 mL/kg and plateau pressure (Pplt) less than 30 cm H2O have been associated with reduced mortality in patients with ARDS. Partial ventilatory support is usually provided by lower SIMV settings (<8 breaths/min), either with or without pressure support. Lower Pplt (<20–30 cm H2O) is recommended to better preserve cardiac output, and lessen adverse effects on ventilation/perfusion relationships.
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Patients breathing spontaneously on SIMV must overcome the additional resistances of the tracheal tube, demand valves, and breathing circuit of the ventilator. These imposed resistances increase the work of breathing. Smaller tubes (<7.0 mm internal diameter in adults) increase resistance and should be avoided if possible. The simultaneous use of pressure support of 5 to 15 cm H2O during SIMV can compensate for tube and circuit resistance.
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The addition of 5 to 8 cm H2O of PEEP during positive-pressure ventilation preserves FRC and gas exchange. This “physiological” PEEP is purported to compensate for the loss of a similar amount of intrinsic PEEP (and decrease in FRC) in patients following tracheal intubation. Periodic sigh breaths (large VT) are not necessary when a PEEP of 5 to 8 cm H2O accompanies VT of appropriate volumes.
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Sedation and paralysis may be necessary in patients who become agitated and “fight” the ventilator. Repetitive coughing (“bucking”) and straining can have adverse hemodynamic effects, can interfere with gas exchange, and may predispose to pulmonary barotrauma and self-inflicted injury. Sedation with or without paralysis may also be desirable when patients continue to be tachypneic despite high mechanical respiratory rates (>16–18 breaths/min).
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Commonly used sedatives include opioids (morphine or fentanyl), benzodiazepines (usually midazolam), propofol, and dexmedetomidine. These agents may be used alone or in combination and are often administered by continuous infusion. Propofol is uncommonly used for prolonged sedation due to concerns about propofol infusion syndrome (see Chapter 9). Nondepolarizing paralytic agents are used in combination with sedation when ventilation cannot otherwise be accomplished.
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Critically ill patients receiving mechanical ventilation should be cared for a head-up position to reduce the risk of ventilator-associated pneumonia. Such patients require continuous monitoring for adverse hemodynamic and pulmonary effects arising from positive pressure in the airways. Continuous electrocardiography, pulse oximetry, and capnometry are useful. Direct intraarterial pressure monitoring is often employed for sampling of arterial blood for respiratory gas analysis. Accurate recording of fluid intake and output is necessary to assess fluid balance. An indwelling urinary catheter will lead to an increased risk of urinary tract infections and should be avoided when possible, but it is helpful for monitoring urinary output. Chest radiographs are commonly obtained to confirm tracheal tube and central venous catheter positions, evaluate for evidence of pulmonary barotrauma or pulmonary disease, and determine whether there are signs of pulmonary edema.
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Airway pressures (baseline, peak, plateau, and mean), inhaled and exhaled VT (mechanical and spontaneous), and fractional concentration of oxygen should be closely monitored. Monitoring these parameters not only allows optimal adjustment of ventilator settings but helps detect problems with the tracheal tube, breathing circuit, and ventilator. For example, an increasing Pplt for a set VT can indicate worsening compliance. A declining blood pressure and increasing Pplt from dynamic hyperinflation (autoPEEP) can be quickly diagnosed (and treated) by disconnecting the patient from the ventilator. Inadequate suctioning of airway secretions and the presence of large mucus plugs are often manifested as increasing peak inflation pressures (a sign of increased resistance to gas flow) and decreasing exhaled VT. An abrupt increase in peak inflation pressure together with sudden hypotension strongly suggests a pneumothorax.
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3. Discontinuing Mechanical Ventilation
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There are two phases to discontinuing mechanical ventilation. In the first, “readiness testing,” so-called weaning parameters and other subjective and objective assessments are used to determine whether the patient can sustain progressive withdrawal of mechanical ventilator support. The second phase, “weaning” or “liberation,” describes the way in which mechanical support is removed.
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Readiness testing should include determining whether the process that necessitated mechanical ventilation has been reversed or controlled. Complicating factors such as bronchospasm, heart failure, infection, malnutrition, increased CO2 production due to increased carbohydrate loads, acid–base derangements, anemia, altered mental status, and sleep deprivation should be adequately treated. Chronic lung disease, respiratory muscle wasting from prolonged disuse, and heart failure may lead to complicated (or failed) weaning.
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Weaning from mechanical ventilation should be considered when patients no longer meet criteria for mechanical ventilation (see Table 58–4). In general, this occurs when patients have a pH greater than 7.25, show adequate arterial oxygen saturation while receiving FiO2 less than 0.5, are able to spontaneously breathe, are hemodynamically stable, and have no current signs of myocardial ischemia. Additional mechanical indices have also been suggested (Table 58–5). Useful weaning parameters include arterial blood gas tensions, respiratory rate, and rapid shallow breathing index (RSBI). Intact airway reflexes are mandatory and a cooperative patient is helpful for successful weaning and extubation, unless the patient will retain a cuffed tracheostomy tube. Similarly, adequate oxygenation (arterial oxygen saturation >90% on 40–50% O2 with <5 cm H2O of PEEP) is mandatory prior to extubation. When the patient is weaned from mechanical ventilation and extubation is planned, the RSBI is frequently used to help predict who can be successfully weaned from mechanical ventilation and extubated. With the patient breathing spontaneously on a Τ-piece, the VT (in liters) and respiratory rate (f) are measured:
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Most patients with an RSBI less than 105 can be successfully extubated. Those with an RSBI greater than 120 should retain some degree of mechanical ventilator support.
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The common techniques to wean a patient from the ventilator include SIMV, pressure support, or periods of spontaneous breathing alone on a Τ-piece or on low levels of CPAP. Many institutions use “automated tube compensation” to provide just enough pressure support to compensate for the resistance of breathing through an endotracheal tube. Newer mechanical ventilators have a setting that will automatically adjust gas flows to make this adjustment. In practice in adults breathing through conventionally sized tubes (7.5–8.5), the adjustment will typically amount to pressure support of 5 cm H2O and PEEP of 5 cm H2O.
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With SIMV the number of mechanical breaths is progressively decreased (by 1–2 breaths/min) as long as the arterial CO2 tension and respiratory rate remain acceptable (generally <45–50 mm Hg and <30 breaths/min, respectively). If pressure support is concomitantly used, it should generally be reduced to 5 to 8 cm H2O. In patients with acid–base disturbances or chronic CO2 retention, arterial blood pH (>7.35) is more useful than CO2 tension. Blood gas measurements can be checked after a minimum of 15 to 30 min at each setting. When an IMV of 2 to 4 breaths is reached, mechanical ventilation is discontinued if arterial oxygenation remains acceptable.
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Weaning with PSV alone is accomplished by gradually decreasing the pressure support level by 2 to 3 cm H2O while VT, arterial blood gas tensions, and respiratory rate are monitored (using the same criteria as for IMV). The goal is to try to ensure a VT of 4 to 6 mL/kg and an f of less than 30 with acceptable PaO2 and PaCO2. When a pressure support level of 5 to 8 cm H2O is reached, the patient is considered weaned.
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Weaning with a T-Piece or CPAP
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Τ-piece trials allow observation while the patient breathes spontaneously without any mechanical breaths. The Τ-piece attaches directly to the tracheal tube or tracheostomy tube and has corrugated tubing on the other two limbs. A humidified oxygen–air mixture flows into the proximal limb and exits from the distal limb. Sufficient gas flow must be given in the proximal limb to prevent the mist from being completely drawn back at the distal limb during inspiration; this ensures that the patient is receiving the desired oxygen concentration. The patient is observed closely during this period; obvious new signs of fatigue, chest retractions, tachypnea, tachycardia, arrhythmias, or hypertension or hypotension should terminate the trial. If the patient appears to tolerate the trial period and the RSBI is less than 100, mechanical ventilation can be discontinued permanently. If the patient can also protect and clear the airway, the tracheal tube can be removed.
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If the patient has been intubated for a prolonged period or has severe underlying lung disease, sequential Τ-piece trials may be necessary. Periodic trials of 10 to 30 min are initiated and progressively increased, typically by 5 to 10 min per trial as long as the patient appears comfortable, maintains acceptable arterial saturation, and does not become hypercarbic.
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Many patients develop progressive atelectasis during prolonged Τ-piece trials. This may reflect the absence of a normal “physiological” PEEP when the larynx is bypassed by a tracheal tube. If this is a concern, spontaneous breathing trials on low levels (5 cm H2O) of CPAP can be tried. The CPAP helps maintain FRC and prevent atelectasis.