Normal Preoperative Pulmonary Function
Surgical procedures that require general anesthesia and invade the thorax or abdominal cavity can result in impaired ventilatory function. As a result, well over 50% of cardiac and thoracic surgical patients show postoperative radiographic evidence of atelectasis.94 In patients with normal preoperative pulmonary function, this normally does not result in any compromise. But in patients with preexisting pulmonary disease, acute respiratory failure is more common.117
In patients without prior pulmonary disease, ventilatory management is normally uneventful. Pressure or volume assist/control is preferred with tidal volumes as large as the patient desires (6-10 mL/kg PBW) because lung function is normal and the period of total mechanical ventilation time is usually brief.37 Five cm H2O PEEP is applied to maintain the functional residual capacity, and the FIO2 and rate are adjusted to maintain adequate gas exchange.
Ventilatory management is short term in most patients. The FIO2 is titrated to maintain PaO2 above 70 mm Hg, and the rate or tidal volume is adjusted to maintain baseline PaCO2. Patients can frequently be rapidly transitioned to pressure support and extubated quickly.
Chronic Obstructive Pulmonary Disease
Postoperative chronic obstructive pulmonary disease (COPD) patients are commonly encountered in the ICU and are challenging to ventilate. Of primary concern is patient–ventilator synchrony.1 The single most significant factor affecting synchrony in these patients is air trapping and auto-PEEP.58,60 In COPD patients, auto-PEEP is caused by dynamic airway compression; that is, unstable airways dilate during positive-pressure inspiration but are compressed by the increased intrathoracic pressure during exhalation. Because auto-PEEP occurs distal to airways obstruction, the airway pressure proximal to the obstruction at end exhalation is normally equal to baseline ventilator circuit pressure. This means that during spontaneous inspiration the patient must first decompress the auto-PEEP level, and then trigger the ventilator. This results in a marked increase in patient effort to trigger the ventilator and dyssynchrony. Many of the patient's inspiratory efforts may not produce sufficient negative force to trigger the ventilator. The patient's actual respiratory rate is then greater than the ventilator's response rate. This increases ventilatory drive, patient respiratory rate, and the patient's overall ventilatory effort. The dyssynchrony associated with auto-PEEP can best be managed at the bedside by applying PEEP as discussed earlier (see Ventilation Settings and Patient–Ventilator Synchrony—Auto-PEEP section).45
A second major concern is ensuring that the ventilator's gas delivery rate meets the patient's inspiratory demand. COPD patients who are spontaneously triggering the ventilator have their ventilatory demands best supplied with pressure, as opposed to volume ventilation.47 However, in either case gas delivery (rise time during pressure ventilation, and peak flow and flow waveform during volume ventilation) should be set to match inspiratory demand. Inspiratory time should coincide with the patient's desired inspiratory time. In assist/control ventilation, the inspiratory time is generally set from 0.6 to 1.0 seconds, and with pressure-support ventilation, the inspiratory termination criteria should be adjusted to ensure a synchronous ending of inspiration.48
COPD patients require ventilatory support because of their inability to perform the work needed to maintain normal ("for them") gas exchange. As a result, the ventilator should ensure adequate rest but should always allow the patient to trigger the ventilator. In general, it is advisable not to use ventilatory modes that do not support every inspiratory effort, because they do not provide sufficient rest for the patient. Thus synchronized intermittent mandatory ventilation, airway pressure-release ventilation, and bilevel ventilation should not be used in COPD.
Initially, intubated, mechanically ventilated COPD patients should be managed with pressure support or assist/control (volume or pressure with pressure preferred). The delivered tidal volume should be 6 to 10 mL/kg PBW, dependent on plateau pressure and patient demand. Most COPD patients can be ventilated with a plateau pressure of 25 cm H2O or less. The respiratory rate is patient determined; however, in assist/control, a backup rate sufficient to maintain the appropriate PaCO2 should be set. In volume ventilation, the peak flow should be set at 80 L/min or higher with a decelerating waveform. With pressure ventilation, the rise time and inspiratory termination criteria (pressure support only) should be properly set. Inspiratory time is set equal to the patient's neuroinspiratory time (0.6-1.0 s) and the PEEP level adjusted to ensure that all of the patient's inspiratory efforts trigger the ventilator.48
In many COPD patients the key medical management issue is to provide rest for the patient. After 48 to 72 hours of ventilatory support, many patients are ready for ventilator discontinuation. In all cases, the focus should be on providing a level of ventilatory support consistent with minimal overall ventilatory effort to ensure recovery from ventilatory muscle dysfunction. As patients recover their PEEP level, pressure support or pressure assist/control level and FIO2 can be decreased, provided that ventilation targets can be met without markedly increasing the patient's work of breathing.
Patients requiring long-term ventilation should be considered for extensive rehabilitation, including generalized muscle retraining. Patients should be weaned with a spontaneous breathing trial, and most COPD patients can be extubated after a successful 30- to 120-minute trial. However, those requiring long-term ventilatory support (2-4 wk) should be given a tracheotomy and normally require successful 12- to 16-hour spontaneous breathing periods before being allowed to breathe without ventilatory support during the night.
For COPD patients who are ventilated for short periods, who meet all the criteria for weaning, and who clinically seem ready for ventilator discontinuation but continue to fail spontaneous breathing trials, carefully consider extubation to NPPV.118-120 Some COPD patients can be successfully transitioned to ventilator independence using NPPV. (See Weaning and Noninvasive Positive Pressure section.)
Acute Respiratory Distress Syndrome
ARDS and ALI are acute lung diseases characterized by atelectasis, edema, decreased compliance, and severe arterial hypoxemia. As a result, an important issue during mechanical ventilation is to avoid inducing greater lung injury either by end-inspiratory overdistension or by the repetitive opening and closing of unstable lung units. Other important issues are the recruitment of lung, the reversal of atelectasis, and the assurance of adequate systemic oxygenation and CO2 exchange while avoiding pulmonary oxygen toxicity.
The end-inspiratory plateau pressure should be less than 30 cm H2O to avoid lung injury unless chest wall compliance is decreased. This means that for most ARDS/ALI patients the tidal volume should be set to 4 to 8 mL/kg PBW.55,86,87 However, if the plateau pressure can be maintained below 25 cm H2O and the patient desires a tidal volume up to 10 mL/kg PBW, it may be more acceptable to meet the patient's ventilatory demands than to force the patient to receive a low tidal volume (which may require intravenous [IV] sedation to apnea).
Some of the atelectasis in ARDS/ALI patients is recruitable by the use of short-term application of a "high" airway pressure (a recruitment maneuver).97-99 These maneuvers work best on the initial day of identification of ARDS/ALI. The longer the patient has ARDS/ALI, the less likely that recruitment maneuvers will succeed and the greater the likelihood of hemodynamic compromise. Following a recruitment maneuver, sufficient levels of PEEP should be applied to maintain the benefits of lung recruitment.
Assist/control mode (Table 81-5), either pressure or volume, can be used with pressure targeting recommended if the patient is triggering each breath and has a variable ventilatory demand. A maximum plateau pressure of 30 cm H2O should be set at a tidal volume of 4 to 8 mL/kg PBW unless the plateau pressure is less than 25 cm H2O. The rate should be set to ensure adequate CO2 elimination. Respiratory rates can be set as high as 40 breaths/min. The rate-limiting factor is the development of auto-PEEP. In most patients, a normal PaCO2 of 35 to 50 mm Hg can be established by adjusting the rate. In some patients with a low compliance, permissive hypercapnia may be necessary. Tolerance of hypercapnia depends on the pH; it is better to allow the PaCO2 to gradually increase over a day or 2 to prevent the development of a marked respiratory acidosis.
Table 81-5 Initial Management of ARDS/ALI ||Download (.pdf)
Table 81-5 Initial Management of ARDS/ALI
Mode—assist/control (volume or pressure)
Plateau pressure—<30 cm H2O
Tidal volume—4-8 mL/kg ideal body weight
Inspiratory time—0.6-1.0 s, if triggering equal to neuroinspiratory time
Rate—≤40/min to manage PaCO2
PEEP—10-12 cm H2O
FIO2—1.0 once stabilized
Lung recruitment—pressure control peak pressure 40-50 cm H2O, PEEP 20-30 cm H2O for 1-3 min or 35-45 cm H2O CPAP for 30-40 s
FIO2—to maintain PaO2 >60 mm Hg
PEEP—set by decremental trial to minimal level maintaining benefits of lung recruitment
ARDS—usually 10-20 cm H2O
ALI—usually 8-15 cm H2O
FIO2 should initially be set at 1.0 and PEEP at 10 to 12 cm H2O until hemodynamic stability is achieved. Once the patient is hemodynamically stable, a lung recruitment maneuver should be considered, using either a CPAP or pressure-assist/control approach as discussed earlier (in the section Lung-Protective Ventilation). Before recruitment the patient should be stable hemodynamically, sedated to near apnea, and breathing 100% oxygen. Careful monitoring of the hemodynamic response and oxygenation level during the recruitment maneuver should be performed, and the maneuver should be stopped if hemodynamic compromise or desaturation is observed. Following the maneuver, the minimum PEEP level maintaining the oxygenation benefit of the recruitment maneuver should be applied. This is best determined by a decremental PEEP trial.96 First, the PEEP level is set at 20 cm H2O and the dynamic compliance determined. Then the PEEP level is decreased by 2 cm H2O and after the compliance has stabilized, the PEEP is decreased again by 2 cm H2O. In general it takes about 3 to 5 minutes for the compliance to stabilize. The decremental trail is stopped once the PEEP level associated with the best compliance can be identified. The PEEP is then set 2 cm H2O above this level because the best compliance PEEP underestimates the best oxygenation PEEP by about 2 cm H2O. At this point another recruitment maneuver is performed, after which PEEP is set at the identified level plus 2 cm H2O. If another method of setting PEEP is used and the oxygenation benefit of the recruitment maneuver is lost over a brief period, the PEEP level is set too low and should be increased. Many ARDS patients will require a PEEP level of 10 to 20 cm H2O and ALI patients often require a PEEP level of 8 to 15 cm H2O.
Regardless of the use of pressure or volume mode ventilation, the inspiratory time should be set at about 0.6 to 1.0 seconds, depending on the patient's ventilatory demand. In patients who are breathing spontaneously, the inspiratory time should be set equal to the patient's neuroinspiratory time. In volume ventilation, adequate peak flow should be provided to meet the patient's ventilatory demand (≥80 L/min) using a decelerating waveform.
A recently published report by Papazian et al121 demonstrated that paralysis as compared to standard care for the first 48 hours of ventilation in severe ARDS patients resulted in decreased mortality. It has been speculated that this was a result of decreased oxygen consumption and lack of fighting the ventilator that may have precipitated atelectasis and increased the likelihood of developing ventilator-associated pneumonia.122 In addition, paralysis was obtained with cis-atracurium, which has been shown to have anti-inflammatory properties.123 However, before this approach can be universally recommended additional studies are needed.
The FIO2 should be decreased before PEEP when oxygenation improves. PEEP generally should not be decreased until the FIO2 is less than 0.50. If the PaO2 decreases as PEEP is lowered, the prior PEEP level should be reestablished, and the FIO2 should not be increased. When this occurs, decreasing PEEP resulted in derecruitment of lung, indicating that the higher PEEP level is needed.
Once the patient's ventilatory drive has normalized and spontaneous triggering of the ventilator can be maintained, patients with ARDS/ALI can usually be maintained with pressure-support ventilation. As with assist/control, the peak pressure should be maintained as low as possible and always less than 30 cm H2O.
In patients with reduced chest wall compliances as a result of abdominal sepsis, ascites, obesity, thoracic deformity, or massive fluid resuscitation following trauma, a plateau pressure greater than 30 cm H2O may be necessary for adequate ventilation. This is acceptable, as the transpulmonary pressure is lower with the stiff chest wall preventing overdistension of the lung. However, when the plateau pressure is greater than 30 cm H2O, the tidal volume should be reduced to less than 6 mL/kg PBW.
In ARDS/ALI, care should be exercised to never disconnect the ventilator circuit. Whenever the circuit is disconnected from the airway, lung derecruitment occurs within seconds. Inline suction catheters should always be used, and the use of manual resuscitators (Ambu bags) should be avoided. Suctioning should only occur when the presence of secretions is observed, and when performed, the suction pressure should be regulated to 120 mm Hg and performed gently to avoid lung derecruitment.
If lung recruitment and an appropriate PEEP setting are achieved but the FIO2 remains greater than 0.60 to achieve adequate oxygenation, then consider prone positioning (if the PaO2/FIO2 is <100 mm Hg) of the patient. Although neither prone positioning nor lung recruitment have been shown to improve survival rates, both can recruit lung and may improve oxygenation. The recruitment of regions of atelectasis reduces the plateau pressure and FIO2, improves surfactant function, and decreases the risk of pneumonia. At this time, the use of high-frequency oscillation or airway pressure-release ventilation does not appear to improve outcome in ARDS, but both modes can provide adequate management of ARDS/ALI patients. Neither mode has been shown to be superior to conventional assist/control ventilation.
Patients with unilateral lung disease present a particularly difficult challenge for mechanical ventilation. The postoperative patient after single-lung transplantation illustrates these problems. One lung has relatively normal pulmonary mechanics (the transplanted lung), whereas the native lung has mechanics reflecting either obstructive or restrictive disease. In these patients, the ventilator should be set to ensure maximum function of the native lung, as this lung will present the greatest ventilatory challenge. If the native lung has chronic obstruction, ventilate with moderate volume and slow rates. With pulmonary fibrosis of the native lung, a smaller VT and more rapid rate are indicated. In the case of pulmonary fibrosis, there is less concern about air trapping. However, peak alveolar pressure may be high because of the reduced lung compliance.
The greatest ventilatory challenge is the patient with a single-lung transplant where the native lung is obstructed and the transplanted lung has become stiff (low compliance) as a consequence of infection, rejection, or acute lung injury. In this setting, it is difficult to determine the ideal ventilator settings because of the differing pathology of each lung. Careful attention needs to be paid to 2 variables as adjustments are made: first, concern about peak alveolar pressure because of ventilator-imposed lung injury, damaging the transplanted lung and its bronchial anastomosis, and second, air trapping in the obstructed lung resulting in grossly compromised ventilation/perfusion ratios, overdistension, and the like. In this setting, permissive hypercapnia is often necessary with the final ventilator settings being a compromise between the conflicting needs of each lung.