Alternative modes of mechanical ventilation: A review for the hospitalist
ABSTRACTNewer ventilators can be set to modes other than the pressure-control and volume-control modes of older machines. In this paper, the authors review several of these alternative modes (adaptive pressure control, adaptive support ventilation, proportional assist ventilation, airway pressure-release ventilation, biphasic positive airway pressure, and high-frequency oscillatory ventilation), explaining how they work and contrasting their theoretical benefits and the actual evidence of benefit.
KEY POINTS
- The alternative modes of ventilation were developed to prevent lung injury and asynchrony, promote better oxygenation and faster weaning, and be easier to use. However, evidence of their benefit is scant.
- Until now, we have lacked a standard nomenclature for mechanical ventilation, leading to confusion.
- Regardless of the mode used, the goals are to avoid lung injury, keep the patient comfortable, and wean the patient from mechanical ventilation as soon as possible.
HIGH-FREQUENCY OSCILLATORY VENTILATION
High-frequency oscillatory ventilation (HFOV) was first described and patented in 1952 by Emerson and was clinically developed in the early 1970s by Lunkenheimer.45
The goal of HFOV is to minimize lung injury; its characteristics (discussed below) make it useful in patients with severe ARDS. The US Food and Drug Administration approved it for infants in 1991 and for children in 1995. The adult model has been available since 1993, but it was not approved until 2001 (SensorMedics 3100B, Cardinal Health, Inc).
Other names for high-frequency oscillatory ventilation
While HFOV has no alternative names, the following acronyms describe similar modes:
- HFPPV (high-frequency positive pressure ventilation)
- HFJV (high-frequency jet ventilation)
- HFFI (high-frequency flow interruption)
- HFPV (high-frequency percussive ventilation)
- HFCWO (high-frequency chest wall oscillation).
All of these modes require different specialized ventilators.
What does high-frequency oscillatory ventilation do?
Conceptually, HFOV is a form of pressure-controlled intermittent mandatory ventilation with a set-point control scheme. In contrast to conventional pressure-controlled intermittent mandatory ventilation, in which relatively small spontaneous breaths may be superimposed on relatively large mandatory breaths, HFOV superimposes very small mandatory breaths (oscillations) on top of spontaneous breaths.
Adult patients are usually paralyzed or deeply sedated, since deep spontaneous breathing will trigger alarms and affect ventilator performance.
To manage ventilation (CO2 clearance), one or several of the following maneuvers can be done: decrease the oscillation frequency, increase the amplitude of the oscillations, increase the inspiratory time, or increase bias flow (while allowing an endotracheal tube cuff leak). Oxygenation adjustments are controlled by manipulating the mean airway pressure and the Fio2.
Ventilator settings in high-frequency oscillatory ventilation
Ventilator settings in HFOV are46:
- Airway pressure amplitude (delta P or power)
- Mean airway pressure
- Percent inspiration
- Inspiratory bias flow
- Fio2.
Clinical applications of high-frequency oscillatory ventilation
This mode is usually reserved for ARDS patients for whom conventional ventilation is failing. A recently published protocol46 suggests considering HFOV when there is oxygenation failure (Fio2 ≥ 0.7 and PEEP ≥ 14 cm H2O) or ventilation failure (pH < 7.25 with tidal volume ≥ 6 mL/kg predicted body weight and plateau airway pressure ≥ 30 cm H2O).
This mode is contraindicated when there is known severe airflow obstruction or intracranial hypertension.
Theoretical benefits of high-frequency oscillatory ventilation
Conceptually, HFOV can provide the highest mean airway pressure paired with the lowest tidal volume of any mode. These benefits might make HFOV the ideal lung-protective ventilation strategy.
Evidence of benefit of high-frequency oscillatory ventilation
Physiologic benefits. Animal models have shown less histologic damage and lung inflammation with HFOV than with high-tidal-volume conventional ventilation47,48 and low-tidal-volume conventional ventilation.49
Patient comfort has not been studied. However, current technology does impose undue work of breathing in spontaneously breathing patients.50
Outcomes. Several retrospective case series have described better oxygenation with HFOV as rescue therapy for severe ARDS than with conventional mechanical ventilation. Two randomized controlled trials have studied HFOV vs high-tidal-volume conventional mechanical ventilation for early severe ARDS; HFOV was safe but made no difference in terms of deaths.42,51–54
High-frequency oscillatory ventilation: Bottom line
In theory, HFOV provides all the benefits of an ideal lung-protective strategy, at least for paralyzed or deeply sedated patients. Animal studies support these concepts. In human adults, HFOV has been shown to be safe and to provide better oxygenation but no improvement in death rates compared with conventional mechanical ventilation. Currently, HFOV is better reserved for patients with severe ARDS for whom conventional mechanical ventilation is failing.
