Static Pressure-Volume Curve

As noted earlier, changes in lung volume above FRC during inspiration relate to the recoil pressure of the respiratory system (Figure 2.3A). The compliance of the respiratory system incorporates both lung compliance and chest wall compliance and is expressed as follows:

(Equation 2)

In general, P equals the plateau pressure (Pplat) minus the end-expiratory pressure (which equals zero unless positive end-expiratory pressure [PEEP] is present). One measures Pplat as the airway pressure after a certain tidal volume has been delivered by use of an inspiratory pause (0.5-second pause) at end inspiration. While monitoring Cstat over time, one should use the same tidal volume for each Pplat measurement. Normal respiratory system compliance is 50 to 100 mL/cm H₂O—that is, the lungs and chest wall expand by 50 to 100 mL for every 1 cm H₂O of static distending pressure. Decreased respiratory system compliance is common in ICU patients and, if present, needs special consideration during ventilator management. Such a Cstat indicates that the respiratory system is “stiffer” than normal. This may be due to stiff lungs, such as those caused by pulmonary edema, or a stiff chest wall, resulting from edema of the chest wall or taut, dilated loops of bowel encroaching into the chest.

A high Cstat usually does not affect ventilator care as much as a low Cstat. A high Cstat may reflect abnormal lungs that have lost their elastic recoil because of emphysema or an abnormal chest wall that has low compliance, such as that caused by neuromuscular blocker–induced paralysis or multiple rib fractures and a “flail chest.”

Dynamic Pressure-Volume Curve

The static pressure-volume (P-V) curve of the respiratory system, discussed earlier, applies only when there is no airflow—that is, at end inspiration or end expiration. When air is flowing into or out of the lung, resistance to airflow comes into play as can be seen by the patient’s dynamic P-V curves (see Equation 1 and Figure 2.3B).

Settings

The control mode is straightforward because the operator sets only two primary parameters: respiratory rate and tidal volume (Figure 2.4A). Most ventilators function as volume-cycled ventilators, in which inspiration ceases after the preset tidal volume is delivered to the patient. Unless there is a leak in the system, such as around the cuff of a tracheal tube or through a chest tube, or unless peak inspiratory pressure exceeds the “pop-off” pressure (the threshold set for the peak pressure alarm [see Chapter 47]), the patient should receive all of the preset tidal volume. The combination of rate and tidal volume, therefore, defines the minute ventilation that the patient receives (Table 1.1).

The assist mode adds two features of operation to the control mode (Figure 2.4B). First, the ventilator can detect a patient’s inspiratory effort (by detecting the negative pressure deflection from baseline in the inspiratory circuit or by detecting the beginning of patient-initiated airflow through the ventilator’s demand valve). When that inspiratory effort exceeds a certain threshold, the patient “triggers” the ventilator to begin delivery of the preset inspiratory tidal volume. The timing of the start of the next inspiration is defined by the set respiratory rate or by the patient’s spontaneous rate, whichever is higher. For example, if the preset rate is 10 breaths/min, the ventilator provides a tidal volume every 6 seconds unless it detects the patient’s spontaneous inspiratory effort earlier. If it does, it resets the start of the next ventilator-initiated breath to be 6 seconds from the start of the patient-initiated breath.

The second additional feature of the assist mode allows the patient to inspire from the ventilator’s demand valve (which is similar to the demand valve of SCUBA [self-contained underwater breathing apparatus] gear). This occurs during inspiration if the patient’s inspiratory flows exceed those generated by the ventilator or if the patient continues to inspire after the preset tidal volume has been delivered. In this manner, the patient’s actual tidal volumes can be consistently larger than the ventilator’s set tidal volume (see Figure 2.4B). On occasion, patients breathing vigorously can also set off the ventilator’s low inspiratory pressure alarm by continually inspiring during machine inspiration (see Chapter 47 for more information about alarms).

Mechanical Considerations

In volume-cycled ventilation, such as in A/C mode, the delivered tidal volume is less than the preset tidal volume if the peak inspiratory pressure (PIP) exceeds the threshold for the peak pressure alarm. For any given tidal volume and inspiratory flow rate, PIP is determined by two factors relating to the patient’s mechanics (as expressed in Equations 1 and 2): airway resistance and static compliance of the respiratory system. If the PIP exceeds the threshold for the ventilator’s peak pressure alarm, the remaining tidal volume is “dumped” (allowed to escape from the system) and is not delivered to the patient. This may occur if the patient actively resists the machine-delivered inspiration (termed bucking or fighting the ventilator) or coughs (both of which decrease compliance of the respiratory system). Another common cause of high PIPs is if the patient’s airway resistance increases, such as in an airway that is partially blocked by respiratory secretions or by the patient biting the endotracheal tube.

Clinical Considerations

The A/C mode of ventilation is indicated for patients without spontaneous respiratory efforts, such as in paralyzed and apneic patients, or for patients with potential loss of their breathing efforts. The latter include patients with drug overdoses that wax and wane in their depressive effects on central nervous system respiratory drive. It is also the mode of choice to provide therapeutic hyperventilation.

Numerous studies have indicated that spontaneously breathing patients with a high ventilatory drive often continue to have a high work of breathing while on A/C mode. In these conditions, they may make strenuous efforts to breathe during machine-delivered inspirations, as reflected by negative esophageal pressure swings during machine inspiration. As a consequence, if such patients were put on A/C mode because of respiratory muscle fatigue, this mode may not effectively “unload” their respiratory muscles—that is, put their respiratory muscles at rest to allow for full recovery from skeletal muscle fatigue.

In the case of patients who have high respiratory rates while on A/C mode, one must first check that the ventilator’s triggering sensitivity is not excessive (which could lead to ventilator self-cycling and high respiratory rates). One can address the problem of ventilated patients who continue to make strong inspiratory efforts on A/C mode by increasing inspiratory flow rates and checking for auto-PEEP (auto–positive end-expiratory pressure, described later). If auto-PEEP is present, it should be treated as discussed later. If the tachypnea persists, one may need to suppress the patient’s high respiratory drive pharmacologically, such as with more sedation (see Chapter 5), in order to provide a respite for the patient’s respiratory muscles. Changing to the PS mode of ventilation is another alternative.