Alternative modes of mechanical ventilation: A review for the hospitalist

ADAPTIVE SUPPORT VENTILATION

Adaptive support ventilation (ASV) evolved as a form of mandatory minute ventilation implemented with adaptive pressure control. Mandatory minute ventilation is a mode that allows the operator to preset a target minute ventilation, and the ventilator then supplies mandatory breaths, either volume- or pressure-controlled, if the patient’s spontaneous breaths generate a lower minute ventilation.

ASV automatically selects the appropriate tidal volume and frequency for mandatory breaths and the appropriate tidal volume for spontaneous breaths on the basis of the respiratory system mechanics and target minute alveolar ventilation.

Described in 1994 by Laubscher et al,^8,9 ASV became commercially available in 1998 in Europe and in 2007 in the United States (Hamilton Galileo ventilator, Hamilton Medical AG). This is the first commercially available ventilator that uses an “optimal” targeting scheme (see below).

What does adaptive support ventilation do?

ASV delivers pressure-controlled breaths using an adaptive (optimal) scheme (Table 2). “Optimal,” in this context, means minimizing the mechanical work of breathing: the machine selects a tidal volume and frequency that the patient’s brain would presumably select if the patient were not connected to a ventilator. This pattern is assumed to encourage the patient to generate spontaneous breaths.

The ventilator calculates the normal required minute ventilation based on the patient’s ideal weight and estimated dead space volume (ie, 2.2 mL/kg). This calculation represents 100% of minute ventilation. The clinician at the bedside sets a target percent of minute ventilation that the ventilator will support—higher than 100% if the patient has increased requirements due, eg, to sepsis or increased dead space, or less than 100% during weaning.

The ventilator initially delivers test breaths, in which it measures the expiratory time constant for the respiratory system and then uses this along with the estimated dead space and normal minute ventilation to calculate an optimal breathing frequency in terms of mechanical work.

The optimal or target tidal volume is calculated as the normal minute ventilation divided by the optimal frequency. The target tidal volume is achieved by the use of APC (see above) (Figure 2). This means that the pressure limit is automatically adjusted to achieve an average delivered tidal volume equal to the target. The ventilator continuously monitors the respiratory system mechanics and adjusts its settings accordingly.

The ventilator adjusts its breaths to avoid air trapping by allowing enough time to exhale, to avoid hypoventilation by delivering tidal volume greater than the dead space, and to avoid volutrauma by avoiding large tidal volumes.

Ventilator settings in adaptive support ventilation

Ventilator settings in ASV are:

• Patient height (to calculate the ideal body weight)
• Sex
• Percent of normal predicted minute ventilation goal
• Fio₂
• PEEP.

Clinical applications of adaptive support ventilation

ASV is intended as a sole mode of ventilation, from initial support to weaning.

Theoretical benefits of adaptive support ventilation

In theory, ASV offers automatic selection of ventilator settings, automatic adaptation to changing patient lung mechanics, less need for human manipulation of the machine, improved synchrony, and automatic weaning.

Evidence of benefit of adaptive support ventilation

Physiologic benefits. Ventilator settings are adjusted automatically. ASV selects different tidal volume-respiratory rate combinations based on respiratory mechanics in passive and paralyzed patients.^10–12 In actively breathing patients, there was no difference in the ventilator settings chosen by ASV for different clinical scenarios (and lung physiology).¹⁰ Compared with pressure-controlled intermittent mandatory ventilation, with ASV, the inspiratory load is less and patient-ventilator interaction is better.¹³

Patient-ventilator synchrony and comfort have not been studied.

Outcomes. Two trials suggest that ASV may decrease time on mechanical ventilation.^14,15 However, in another trial,¹⁶ compared with a standard protocol, ASV led to fewer ventilator adjustments but achieved similar postsurgical weaning outcomes. The effect of this mode on the death rate has not been examined.^17,18

Adaptive support ventilation: Bottom line

ASV is the first commercially available mode that automatically selects all the ventilator settings except PEEP and Fio₂. These seem appropriate for different clinical scenarios in patients with poor respiratory effort or in paralyzed patients. Evidence of the effect in actively breathing patients and on outcomes such as length of stay or death is still lacking.

PROPORTIONAL ASSIST VENTILATION

Patients who have normal respiratory drive but who have difficulty sustaining adequate spontaneous ventilation are often subjected to pressure support ventilation (PSV), in which the ventilator generates a constant pressure throughout inspiration regardless of the intensity of the patient’s effort.

In 1992, Younes and colleagues^19,20 developed proportional assist ventilation (PAV) as an alternative in which the ventilator generates pressure in proportion to the patient’s effort. PAV became commercially available in Europe in 1999 and was approved in the United States in 2006, available on the Puritan Bennett 840 ventilator (Puritan Bennett Co, Boulder, CO). PAV has also been used for noninvasive ventilation, but this is not available in the United States.

Other names for proportional assist ventilation

Proportional Pressure Support (Dräger Medical; not yet available in the United States).