一個呼吸的距離

(from uptodate-Modes of mechanical ventilation)

Volume-limited ventilation (also called volume-controlled or volume-cycled ventilation)

1. 需要設 peak flow rate, flow pattern, tidal volume, respiratory rate, positive end-expiratory pressure (applied PEEP), & FiO₂

2. Inspiration ends after delivery of the set tidal volume.

3. (I:E) ratio 由the peak inspiratory flow rate決定：增加的peak inspiratory flow rate會減少inspiratory time而增加expiratory time

4. 氣道壓力(peak, plateau, and mean) 決定於both the ventilator settings and patient-related variables (eg, compliance, airway resistance).

5. 高氣道壓力的原因有許多：large tidal volumes, a high peak flow, poor compliance (eg, acute respiratory distress syndrome, minimal sedation), or increased airway resistance

模式: controlled mechanical ventilation (CMV), assist control (AC), intermittent mandatory ventilation (IMV), and synchronized intermittent mandatory ventilation (SIMV)

CMV — During CMV, the minute ventilation is determined entirely by the set respiratory rate and tidal volume. The patient does not initiate additional minute ventilation above that set on the ventilator. This may be due to pharmacologic paralysis, heavy sedation, coma, or lack of incentive to increase the minute ventilation because the set minute ventilation meets or exceeds physiologic need. CMV does not require any patient work. (病人不用費力，全靠機器)

AC — 醫師決定minimal minute ventilation，透過respiratory rate and tidal volume. The patient can increase the minute ventilation by triggering additional breaths. Each patient-initiated breath receives the set tidal volume from the ventilator.

Consider the following example. If the clinician sets the respiratory rate to 20 breaths per minute and the tidal volume to 500 mL, the lowest possible minute ventilation is 10 L per minute (20 breaths per minute times 500 mL per breath). If the patient triggers an additional 5 breaths beyond the preset 20 breaths, the ventilator will deliver 500 mL for each additional breath and the minute ventilation will be 12.5 L per minute (25 breaths per minute times 500 mL per breath).

Pressure regulated volume control (PRVC) is similar to AC. 與AC最大不同處在於呼吸器可以自動調節 the inspiratory time and flow，如此一來tidal volume 只會產生一些些smaller rise in the plateau airway pressure.

IMV — 與AC相同之處有兩點: (1)the clinician determines the minimal minute ventilation (by setting the respiratory rate and tidal volume) and (2) the patient is able to increase the minute ventilation. 然而不同處在於the minute ventilation是增加的。Specifically, patients increase the minute ventilation by spontaneous breathing, rather than patient-initiated ventilator breaths. ??

(以下舉一個例子)Consider the following example. If the clinician sets the respiratory rate to 10 breaths per minute and the tidal volume to 500 mL per breath, the lowest possible minute ventilation is 5 L per minute (10 breaths per minute times 500 mL per breath). If the patient initiates an additional 5 breaths beyond the preset 10 breaths, the tidal volume for each additional breath will be whatever size the patient is able to generate and the minute ventilation will be some amount greater than 5 L per minute. The precise minute ventilation depends on the size of the tidal volume for each spontaneous breath.

SIMV — SIMV is a variation of IMV, in which the ventilator breaths are synchronized with patient inspiratory effort [1,2]. SIMV (or IMV) can be used to titrate the level of ventilatory support over a wide range (figure 2) [3]. This is an advantage unique to these modes. Ventilatory support can range from full support (set respiratory rate is high enough that the patient does not overbreathe) to no ventilatory support (set respiratory rate is zero).

The level of support may need to be modified if hemodynamic consequences of positive pressure ventilation develop. In one study, cardiac output, mean blood pressure, pulmonary capillary wedge pressure, and oxygen consumption were all better when the level of support provided by SIMV was less than 50 percent [4]. (See "Physiologic and pathophysiologic consequences of mechanical ventilation", section on 'Hemodynamics'.)

比較 — SIMV and AC 是最常用的volume-limited mechanical ventilation. SIMV較AC好的優點包括better patient-ventilator synchrony, better preservation of respiratory muscle function, lower mean airway pressures, and greater control over the level of support. 此外auto-PEEP於SIMV較少出現。但AC較適合critically ill patients who require a constant tidal volume or full or near-maximal ventilatory support.