一個呼吸的距離

(from uptodate-modes of mechanical ventilator)

PRESSURE-LIMITED VENTILATION — Pressure-limited ventilation (also called pressure-cycled ventilation)需要設定inspiratory pressure level, I:E ratio, respiratory rate, applied PEEP, and FiO₂ 。吸氣停止於delivery of the set inspiratory pressure後。

潮氣容積(tidal volume)在pressure-limited ventilation是變動的。它與inspiratory pressure level, compliance, airway resistance, and tubing resistance有關。Specifically, tidal volumes會變大：當the set inspiratory pressure level is high 或 there is good compliance, little airway resistance, or little resistance from the ventilator tubing.

相對地，the peak airway pressure是穩定的during pressure-limited ventilation. 它的壓力值相當於the set inspiratory pressure level 與 the applied PEEP的總合. As an example, a patient with a set inspiratory pressure level of 20 cm H₂O and an applied PEEP of 10 cm H₂O will have a peak airway pressure of 30 cm H₂O.

Pressure-limited ventilation can be delivered using the same modes of ventilation that deliver volume-limited ventilation (table 1):

• During pressure-limited CMV (also called pressure control ventilation), the minute ventilation is determined entirely by the set respiratory rate and inspiratory pressure level. The patient does not initiate additional minute ventilation above that set on the ventilator.
• During pressure-limited AC, the set respiratory rate and inspiratory pressure level determine the minimum minute ventilation. The patient is able to increase the minute ventilation by triggering additional ventilator-assisted, pressure-limited breaths.
• During pressure-limited IMV or SIMV, the set respiratory rate and inspiratory pressure level determine the minimum minute ventilation. The patient is able to increase the minute ventilation by initiating spontaneous breaths.

VOLUME-LIMITED V.S. PRESSURE-LIMITED — Pressure-limited ventilation was compared to volume-limited ventilation in a randomized trial and several observational studies: 

1. 沒有統計學上的差異 in mortality, oxygenation, or work of breathing

2. 以下狀況使用pressure-limited ventilation較好: it was associated with lower peak airway pressures, a more homogeneous gas distribution (less regional alveolar overdistension), improved patient-ventilator synchrony, and earlier liberation from mechanical ventilation than volume-limited ventilation

• 3. 以下狀況使用volume-limited ventilation較好: guarantee a constant tidal volume & ensuring a minimum minute ventilation

Most studies comparing pressure-limited and volume-limited ventilation used a square wave (constant flow) pattern for both modes. When volume-limited mechanical ventilation with a ramp wave (decelerating flow) pattern was compared to pressure-limited ventilation, lower peak airway pressures 不再是 pressure-limited ventilation的優點。

PRESSURE SUPPORT — Pressure support ventilation (PSV) is a flow-limited mode of ventilation that delivers inspiratory pressure until the inspiratory flow decreases to a predetermined percentage of its peak value. This is usually 25 percent (figure 3) [13].

For PSV, the clinician sets the pressure support level (inspiratory pressure level), applied PEEP, and FiO₂. The patient must trigger each breath because there is no set respiratory rate. The tidal volume, respiratory rate, and minute ventilation are dependent on multiple factors, including the ventilator settings and patient-related variables (eg, compliance, sedation). In general, a high pressure support level results in large tidal volumes and a low respiratory rate.

The work of breathing is inversely proportional to the pressure support level, provided that inspiratory flow is sufficient to meet patient demand [13,14]. In other words, increasing the level of pressure support decreases the work of breathing. The work of breathing is also inversely proportional to the inspiratory flow rate. Increasing the inspiratory flow rate shortens the time until the maximal airway pressures are achieved, which decreases the work of breathing [15].

Potential uses — PSV seems particularly well suited for weaning from mechanical ventilation because it tends to be a comfortable mode, giving the patient greater control over the inspiratory flow rate and respiratory rate. However, clinical studies have failed to show that PSV improves weaning. (See "Methods of weaning from mechanical ventilation", section on 'Choosing a weaning method'.)

PSV is frequently combined with SIMV. The ventilator delivers the set respiratory rate using SIMV, but patient-initiated breaths beyond the set respiratory rate are delivered using PSV. The purpose of adding PSV for patient-initiated breaths is to overcome the resistance of the endotracheal tube and ventilator circuit. The necessary level of pressure support is unknown and generally estimated. Resistance of the endotracheal tube is related to the tube diameter and inspiratory flow rate [16]. With small endotracheal tubes (eg, <7 mm), a pressure support level ≥10 cm H₂O may be needed to overcome the resistance [17,18]. Levels of pressure support higher than that required to overcome resistance will augment tidal volume.

Disadvantages — PSV is poorly suited to provide full or near-full ventilatory support. The following characteristics of PSV are disadvantages in that setting:

• Each breath must be initiated by the patient. Central apnea may occur if the respiratory drive is depressed due to sedatives, critical illness, or hypocapnia due to excessive ventilation [19].
• An adequate minute ventilation cannot be guaranteed because tidal volume and respiratory rate are variable.
• Ventilator asynchrony can occur when PSV is employed for full ventilatory support, potentially prolonging the duration of mechanical ventilation [20,21].
• PSV is associated with poorer sleep than AC. Specifically, there is greater sleep fragmentation, less stage 1 and 2 non-rapid eye movement (NREM) sleep, more wakefulness during the first part of the night, and less stage 3 and 4 NREM sleep during the second part of the night [22].
• Relatively high levels of pressure support (eg, >20 cm H₂O) are required during full ventilatory support to prevent alveolar collapse (which can lead to cyclic atelectasis and ventilator-associated lung injury) and to attain a stable breathing pattern [23,24]. Such high levels of pressure support are not as comfortable as moderate levels (eg, 10 to 15 cm H₂O) [25]. (See "Ventilator-associated lung injury".)

While PSV is poorly suited to provide full or nearly full ventilatory support in general, it is a particularly poor choice for patients who also have increased airway resistance (eg, COPD or asthma exacerbation). Minute ventilation is more likely to be insufficient when airway resistance is high, which may be related to decreased airflow causing inspiration to be terminated after a smaller than optimal tidal volume has been delivered [26,27]. In addition, PSV does little to decrease auto-PEEP, which can increase patient work and worsen respiratory muscle fatigue [28]. Choosing a higher percentage of the peak inspiratory flow as the trigger to end inspiration may improve auto-PEEP slightly [29]. (See "Mechanical ventilation in acute respiratory failure complicating COPD".)

Tube compensation — Many ventilators can be set to a mode called automatic tube compensation. This mode is a type of PSV that applies a sufficient level of positive pressure to overcome the work of breathing imposed by the endotracheal tube, which can vary from breath to breath. Automatic tube compensation is often used for a spontaneous breathing trial. Patients who undergo a spontaneous breathing trial with automatic tube compensation are more likely to successfully tolerate their trial than those who receive continuous positive airway pressure alone [30]. In addition, many ventilators have the option of combining automatic tube compensation with other modes, so that resistance of the endotracheal tube has no impact on ventilation.