Chapter 6: Traditional Modes of Mechanical Ventilation

The relationship between breath types and phase variables is referred to as a mode of ventilation. During mechanical ventilation, the mode is one of the principal ventilator settings. Although many modes are available, the choice of mode is usually based on clinician's preference or institutional bias, since evidence is lacking that one mode is clearly superior to others. This chapter describes traditional ventilator modes (Table 6-1), which include continuous mandatory ventilation (CMV), continuous spontaneous ventilation (CSV), and synchronized intermittent mandatory ventilation (SMV).

The two general approaches to ventilatory support are volume-controlled ventilation and pressure-controlled ventilation. At any time, the ventilator controls either volume (flow) or pressure applied to the airway. Some volume-targeted modes such as pressure-regulated volume control and adaptive support ventilation actually adjust the level of pressure control to achieve the set tidal volume. Although the term volume control is usually used, in reality the ventilator controls the inspiratory flow. The important variables for volume-controlled ventilation are shown in Figure 6-1. During pressure-controlled ventilation, the inspiratory flow decreases as the alveolar pressure approaches the pressure applied to the airway. The important variables affecting pressure-controlled ventilation are illustrated in Figure 6-2.

A minimal rate is set with CMV (Figure 6-3). The patient can trigger the ventilator at a more rapid rate, but every breath delivered is a mandatory breath type. Note that the mandatory breaths can be either volume-controlled or pressure-controlled. CMV is commonly called assist/control (A/C) ventilation—the terms CMV and A/C are used interchangeably. Note that, from the perspective of the ventilator, there is no controlled-mechanical ventilation mode. If the patient's breathing is completely controlled by the ventilator, this is the result of pharmacologic support or pathophysiology, not the ventilator mode. Also note that the ventilator always assists the patient's breathing regardless of the ventilator mode.

With CSV, every breath is a spontaneous type; ie, every breath is triggered and cycled by the patient. The two most common forms of CSV are continuous positive airway pressure (CPAP) and pressure support ventilation (PSV).

Continuous Positive Airway Pressure

This is a spontaneous breathing mode; no mandatory breaths are delivered (Figure 6-4). A clinician-determined level of positive pressure is maintained throughout the ventilatory cycle. However, it is possible to set CPAP to 0, where the pressure applied to the airway is ambient. The CPAP mode is most commonly used to evaluate whether the patient can be liberated from the ventilator. It is interesting to note that the performance of many current generation ventilators is such that a small level of PSV (1-2 cm H₂O) is applied during CPAP. Ventilator performance during CPAP is better with flow-triggering than with pressure-triggering. For that reason, flow-triggering is recommended when CPAP is used.

Pressure Support Ventilation

The patient's inspiratory effort is assisted by the ventilator at a preset level of inspiratory pressure with PSV. Inspiration is triggered and cycled by patient's effort. During PSV, the patient determines the respiratory rate, inspiratory time, and tidal volume (Figure 6-5). Current generation ventilators provide backup ventilation (volume-controlled or pressure-controlled CMV) should apnea occur during PSV, but this is an alarm condition. PSV is normally flow-cycled. Secondary cycling mechanisms with PSV are pressure and time. In other words, PSV will cycle to the expiratory phase when the flow decreases to a ventilator-determined level, when the pressure rises to a ventilator-determined level, or when the inspiratory time reaches a ventilator-determined limit. The flow at which the ventilator cycles to the expiratory phase can be either a fixed flow, a flow based on the peak inspiratory flow, or a flow based on peak inspiratory flow and elapsed inspiratory time. Newer generation ventilators allow the clinician to adjust the termination flow at which the ventilator cycles to a level appropriate for the patient. Newer generation ventilators also allow adjustment of rise time at the beginning of the pressure support breath and the maximum time of the inspiratory phase. Rise time is the amount of time required to reach the pressure support level at the beginning of inspiration.

Synchronized intermittent mandatory ventilation (SIMV) is a ventilator mode where mandatory breaths are delivered intermittently with volume-controlled or pressure-controlled ventilation. Between the mandatory breaths, the patient is allowed to breathe spontaneously. The ventilator delivers the mandatory breaths in synchrony with the patient's inspiratory effort (Figure 6-6). If no inspiratory effort is detected, the ventilator delivers a mandatory breath at the scheduled time. This is usually achieved by use of an assist window (Figure 6-7). This window opens at intervals determined by the SIMV rate, and remains open for a manufacturer-specific period of time. If a patient-generated effort is detected while this window is open, a mandatory breath is delivered. If no patient effort is detected in the time that the window is open, the ventilator delivers a mandatory breath. With SIMV, the spontaneous breaths are usually pressure-supported (Figure 6-8). SIMV has been suggested to partition the work-of-breathing between the ventilator and the patient; in other words, for the ventilator to do the work during mandatory breaths with the patient doing the work during spontaneous breaths. But this often does not occur. Particularly at a low mandatory rate, SIMV requires the patient to exert as much effort during the mandatory breath as during the spontaneous breaths.

Mechanical ventilation can be referred to as full or partial ventilatory support. This can be described by the equation of motion:

With full support, there is no respiratory muscle activity and thus Pmus is 0. With partial support, there is a contribution by the respiratory muscles, and thus, Pmus makes a contribution to the equation of motion.

With full support, the ventilator does all of the work needed for ventilation of the patient; the patient does not trigger the ventilator or breathe spontaneously. This can be achieved as the result of the patient's primary disease process (eg, neuromuscular disease), pharmacologic therapy (eg, paralysis), or a minute ventilation high enough to suppress the patient's respiratory drive (eg, hyperventilation). Full support is often preferred for patients who are severely ill to decrease the oxygen cost of breathing and achieve control of the patient's ventilatory pattern. Full support is provided by CMV.

With partial support, some of the work-of-breathing is provided by the ventilator and the remainder is provided by the patient. Partial ventilatory support is commonly used during weaning from mechanical ventilation. Partial support is also preferred by clinicians who believe that this maintains respiratory muscle tone, allows the patient to maintain some control of the ventilatory pattern, and improves patient comfort and synchrony. Total respiratory muscle suppression can quickly lead to atrophy and weakness. Partial ventilatory support can be achieved with CMV, SIMV, or PSV.