一個呼吸的距離

(轉錄自uptodate; FROM UPTODATE-MODES OF MECHANICAL VENTILATOR)

CONTINUOUS POSITIVE AIRWAY PRESSURE — Continuous positive airway pressure (CPAP) refers to the delivery of a continuous level of positive airway pressure (持續給予呼吸道正壓). It is functionally similar to PEEP (基本上有點像PEEP). The ventilator does not cycle during CPAP, no additional pressure above the level of CPAP is provided, and patients must initiate all breaths (病人需啟始所有呼吸).

CPAP is most commonly used (多用於) in the management of sleep related breathing disorders (睡眠相關疾病), cardiogenic pulmonary edema (心因性肺永腫), and obesity hypoventilation syndrome (肥胖致呼吸低下症候群). (See "Initiation of positive airway pressure therapy for obstructive sleep apnea in adults" and "Noninvasive positive pressure ventilation in acute respiratory failure in adults", section on 'Cardiogenic pulmonary edema' and "Noninvasive positive pressure therapy of the obesity hypoventilation syndrome".)

BILEVEL POSITIVE AIRWAY PRESSURE — Bilevel positive airway pressure (BPAP，又名BiPAP) is a mode used during noninvasive positive pressure ventilation (NPPV, 為一種非侵入性正壓呼吸器). It delivers a preset inspiratory positive airway pressure (IPAP) and expiratory positive airway pressure (EPAP) (提供IPAP與EPAP, 即吸氣與吐氣壓力兩種壓力交替). The tidal volume correlates with the difference between the IPAP and the EPAP (tidal volume, 潮氣容積與I/EPAP兩種壓力差有關). As an example, the tidal volume is greater using an IPAP of 15 cm H₂O and an EPAP of 5 cm H₂O (difference of 10 cm H₂O), than an IPAP of 10 cm H₂O and an EPAP of 5 cm H₂O (difference of 5 cm H₂O). Most BPAP devices also permit a backup respiratory rate to be set (機器通常設定有back-up rate，避免病人不呼吸).

The term "BiPAP" is often used incorrectly to refer to the BPAP mode. BiPAP is the name of a portable ventilator manufactured by Respironics Corporation; it is just one of many ventilators that can deliver BPAP. BPAP is discussed in more detail separately. (See "Noninvasive positive pressure ventilation in acute respiratory failure in adults", section on 'Mode' and "Noninvasive positive pressure therapy of the obesity hypoventilation syndrome", section on 'Bilevel positive airway pressure'.)

(上圖為Biphasic intermittent positive airway pressure ventilation); Data from: Seymour, CW, Frazer M, Reilly, PM, Fuchs, BD. Airway pressure release and biphasic intermittent positive airway pressure ventilation: are they ready for prime time? J Trauma 2007; 62:1298.

AIRWAY PRESSURE RELEASE VENTILATION (APRV) — a high continuous positive airway pressure (P high) is delivered for a long duration (T high) and then falls to a lower pressure (P low) for a shorter duration (T low) (下圖)

The transition from P high to P low 使肺部洩氣，然後排出CO2。相反地，從P low to P high的轉換使肺部充氣。Alveolar recruitment可以達到最大，透過the high continuous positive airway pressure。

The difference between P high and P low is the driving pressure. Larger differences are associated with greater inflation and deflation, while smaller differences are associated with smaller inflation and deflation. The exact size of the tidal volume is related to both the driving pressure and the compliance.

T high and T low determine the frequency of inflations and deflations. As an example, a patient whose T high is set to 12 seconds and whose T low is set to 3 seconds has an inflation-deflation cycle lasting 15 seconds. This allows four inflations and deflations to be completed each minute.

Spontaneous breathing is possible at both P high and P low, although most spontaneous breathing occurs at P high because the time spent at P low is brief. (可以在P high時自主呼吸成了APRV的特色。) This is a novel feature that distinguishes APRV from other types of inverse ratio ventilation (IRV). In a patient who is not spontaneously breathing, APRV is identical to PC-IRV.

Efficacy — APRV has not been shown to improve mortality(對於mortality沒有助益). However, it may improve alternative important clinical outcomes compared to other modes of ventilation. In one trial, 30 patients being mechanically ventilated because of trauma(創傷) were randomly assigned to receive APRV alone or pressure-limited ventilation for 72 hours followed by APRV [33]. The APRV alone group had a shorter duration of mechanical ventilation, a shorter ICU stay, and required less sedation and pharmacologic paralysis. Mortality did not differ between groups.

Numerous observational studies suggest that APRV may decrease the peak airway pressure, improve alveolar recruitment, increase ventilation of the dependent lung zones and improve oxygenation [34-39]. However, such findings have not been universal. In one clinical trial, 58 patients being mechanically ventilated for acute lung injury were randomly assigned to receive either APRV or SIMV plus PSV: 沒有差異 in physiologic or clinical outcomes.

APRV is well tolerated hemodynamically. In a trial of 12 patients who were being mechanically ventilated for ARDS, the patients were switched from PC-IRV to APRV [41]. The initial P high was 75 percent of the peak airway pressure during PC-IRV. Following the change, there was significant improvement in the cardiac index and oxygen delivery, as well as a diminished need for vasopressors. Although much of the hemodynamic improvement in this trial was probably related to lower airway pressures during APRV compared to PC-IRV, spontaneous breathing also confers a hemodynamic advantage [42].

Indications — There are no universally accepted indications. Use of APRV and its related modes – intermittent mandatory airway pressure release ventilation and biphasic intermittent positive airway pressure (described below) – has been best described in patients who have ARDS. In theory, APRV may recruit alveoli and improve oxygenation.

Contraindications — APRV and its related modes are infrequently used in patients with severe obstructive airways disease or a high ventilatory requirement because hyperinflation, high alveolar pressure, and pulmonary barotrauma may result. (See "Pulmonary barotrauma during mechanical ventilation".)

Related modes — Intermittent mandatory airway pressure release ventilation (IMPRV) and biphasic intermittent positive airway pressure (herein called biphasic ventilation) are similar to APRV. Specifically, they allow spontaneous breathing and have cyclic inflations and deflations due to transitions between P high and P low.

• During IMPRV, the cyclic inflations and deflations are synchronized to occur after every few spontaneous breaths [43].
• The principal difference between biphasic ventilation and APRV is that T low is longer during biphasic ventilation, allowing more spontaneous breaths to occur at P low (figure 6) [44,45]. Another difference is that inverse ratio ventilation is more often performed using APRV than biphasic ventilation. Biphasic ventilation is also referred to as Bi-Vent, BiLevel, BiPhasic, and DuoPAP ventilation. It is different than bilevel positive airway pressure, a common type of noninvasive positive pressure ventilation. (See "Noninvasive positive pressure ventilation in acute respiratory failure in adults".)

HIGH FREQUENCY VENTILATION — High frequency mechanical ventilation employs very high respiratory rates and small tidal volumes. It is described in detail separately. (See "High-frequency ventilation in adults".)

ADAPTIVE SUPPORT VENTILATION — Adaptive support ventilation (ASV) is a ventilatory mode in which respiratory mechanics dictate adjustments to the respiratory rate and inspiratory pressure that are necessary to achieve a desired minute ventilation:

• Patients who are unable to trigger the ventilator are given pressure-control breaths. (See 'Pressure-limited ventilation' above.)
• Patients who are able to trigger the ventilator are given pressure support for the triggered breaths, supplemented with pressure-control breaths as needed to achieve the desired respiratory rate. (See 'Pressure support' above and 'Pressure-limited ventilation' above.)

The basis for the adjustments is an equation that determines the respiratory rate that minimizes the work of inspiration at a given minute ventilation. This equation relies on an expiratory time constant, which can be obtained from the expiratory limb of the flow volume loop on a breath by breath basis [46,47]. Patients who have a long expiratory time constant (eg, COPD) receive a higher tidal volume and a lower respiratory rate when ventilated by ASV than patients with stiff lungs (eg, ARDS) or chest wall stiffness (eg, kyphoscoliosis, morbid obesity, neuromuscular disorder) who expire quickly [48,49]. The effect of ASV has not been directly compared to the effect of other modes of mechanical ventilation on important clinical outcomes.

INVERSE RATIO VENTILATION — Inverse ratio ventilation (IRV) is not a mode of mechanical ventilation, but rather a strategy employed during volume-limited or pressure-limited mechanical ventilation. The inspiratory time exceeds the expiratory time during IRV (the I:E ratio is inversed), increasing the mean airway pressure and potentially improving oxygenation. A trial of IRV may be warranted when a patient is severely hypoxemic despite optimal PEEP and FiO₂.

IRV has never been shown to improve important clinical outcomes, such as mortality, duration of mechanical ventilation, or duration of ICU stay. The preponderance of evidence suggests that IRV improves oxygenation, although the evidence is weak and characterized by low quality, conflicting studies [50-61]. The following studies are illustrative of the data that exist:

• In an observational study of 31 patients undergoing pressure control ventilation, initiation of IRV was followed by a significant increase in the mean airway pressure and the PaO₂ (from 69 to 80 mmHg), despite reduction of the PEEP [59].
• A crossover trial randomly assigned 16 patients with ARDS to received IRV or no IRV [51]. IRV increased the mean airway pressure, but the improvement in the PaO₂ did not reach statistical significance (93 versus 86 mmHg).

IRV generally requires heavy sedation or neuromuscular paralysis because the inverse I:E ratio is unnatural and uncomfortable. It is usually well tolerated hemodynamically [56,62].

Types — IRV can be performed during pressure-limited ventilation (PL-IRV) or volume-limited ventilation (VL-IRV). Neither is clearly superior to the other. In a multicenter, randomized trial that compared PC-IRV to VC-IRV in patients with ARDS, the type of IRV did not affect mortality [63].

Pressure-limited — During pressure-limited ventilation, IRV is initiated by increasing the I:E ratio until the inspiratory time exceeds the expiratory time. The primary advantage of PC-IRV is the ability to guarantee that a maximal plateau airway pressure will not be exceeded. This may limit the risk of pulmonary barotrauma or ventilator-associated lung injury. In addition, many clinicians believe that clinically significant auto-PEEP is less likely with PC-IRV than VC-IRV, although this is unproven [64]. (See "Ventilator-associated lung injury" and "Pulmonary barotrauma during mechanical ventilation".)

Volume-limited — During volume-limited ventilation, IRV can be initiated using a ramp wave (decelerating flow) or a square wave (constant flow) flow pattern:

• With the ramp wave, the peak inspiratory flow rate is initially set at least four times higher than the minute ventilation and then slowly decreased until the inspiratory time exceeds the expiratory time
• With the square wave, an end-inspiratory pause is added (0.2 sec works well) and then slowly lengthened until the inspiratory time exceeds the expiratory time

Risks — The shorter expiratory time during IRV increases the risk of auto-PEEP and its adverse sequelae (eg, pulmonary barotrauma, hypotension) (figure 7) [59]. IRV also appears to increase the risk of pulmonary barotrauma independent of auto-PEEP. In a study of 14 patients undergoing mechanical ventilation with PC-IRV, the incidence of pneumothorax was 29 percent despite the lack of measurable auto-PEEP [65].

SUMMARY AND RECOMMENDATIONS

• The mode of mechanical ventilation refers to the method of inspiratory support. (See 'Introduction' above.)
• During volume-limited ventilation, inspiration ends after delivery of the set tidal volume. During pressure-limited ventilation, inspiration ends after delivery of the set inspiratory pressure. Each has unique advantages and disadvantages. (See 'Volume-limited ventilation' above and 'Pressure-limited ventilation' above and 'Volume-limited versus pressure-limited' above.)
• Pressure support ventilation (PSV) is neither volume-limited nor pressure-limited. Once a breath is triggered by the patient, inspiratory pressure is delivered until the inspiratory flow decreases to a predetermined percentage of its peak value. (See 'Pressure support' above.)
• Continuous positive airway pressure (CPAP) refers to the delivery of a continuous level of positive airway pressure. (See 'Continuous positive airway pressure' above.)
• Bilevel positive airway pressure (BPAP) is a mode used during noninvasive positive pressure ventilation. It delivers a preset inspiratory positive airway pressure (IPAP) and an expiratory positive airway pressure (EPAP). (See 'Bilevel positive airway pressure' above.)
• Airway pressure release ventilation (APRV) cycles between a high continuous positive airway pressure (P high) and a low continuous positive airway pressure (P low). Spontaneous breathing can occur during APRV. Variants include intermittent mandatory airway pressure release ventilation and biphasic intermittent positive airway pressure. (See 'Airway pressure release ventilation' above.)
• Inverse ratio ventilation (IRV) is not a mode of mechanical ventilation, but rather a strategy employed during volume-limited or pressure-limited mechanical ventilation. The inspiratory time exceeds the expiratory time during IRV (the I:E ratio is inversed), increasing the mean airway pressure and potentially improving oxygenation. (See 'Inverse ratio ventilation' above.)