Lung protective strategies, patient ventilator synchrony, and weaning may depend on ventilator mode selection. Mechanical ventilation is routinely applied to patients admitted in critical care units. Primary goal of ventilation is to reduce work of breathing, improve oxygenation, or correct carbon dioxide retention.

Conventional modes of mechanical ventilation provide adequate control of ventilation in most cases. However, clinician may encounter problems in some patients. As the numbers of chronically ill and elderly patients who are mechanically ventilated has increased, the average length of time that patients stay ventilator-dependent has also increased.

The key development since invasive positive pressure ventilation replaced the iron lung has been the development of the servo-controlled valve, or electronic ventilator. This has made ventilators that respond rapidly to patient effort and adapt to changes in patient physiology without always needing human intervention. In addition, attention has been on improving ventilator-patient interaction and reducing potential harmful effects of positive pressure ventilation. Newer modes of ventilation were developed to prevent lung injury and asynchrony, promote better oxygenation, faster weaning, and ease of use.

APRV

Airway pressure release ventilation (APRV) is basically a set level of CPAP that intermittently releases to a lower level using a time-controlled release valve. The clinical benefits of APRV include the preservation of spontaneous breathing and comfort with most spontaneous breathing occurring at high CPAP but it can occur throughout the ventilator cycle with little work of breathing being imposed by the ventilator. This results in less barotraumas, reduction in circulatory compromise, and a better matching of pulmonary ventilation and perfusion.

Certain patients are poor candidates for APRV due to their underlying pulmonary state like patients with increased airway resistance or those unable to empty their lungs in 2 seconds (asthma or severe COPD).

BIPAP Bi-Level

Biphasic positive airway pressure (BIPAP) is pressure-controlled ventilation, which allows unrestricted spontaneous breathing throughout the respiratory cycle. In BiPAP the circuit switches between a high and low airway pressure in an adjustable time sequence.

In BiLevel the ventilator cycling between the two pressure levels can be synchronized with the patient to prevent a cycle to low pressure just as a patient takes a breath. Spontaneous breaths can be pressure supported at the high and low pressure levels. The advantages of this method of ventilation are the decreased level of sedation required to facilitate patient acceptance of the ventilator and the concept of a single modality to ventilate the patient.

Proportional assist ventilation

Proportional assist ventilation (PAV) is conceptually different from all available modes of mechanical ventilation. It is a patient-triggered and pressure-controlled mode which adapts assistance according to patient's effort and mechanics properties within the inspiratory cycle. In contrast with previously cited modes of ventilation, PAV controlled variable (pressure) is changed all around the inspiratory phase following inspiratory flow and volume instantaneous values. PAV is based on the equation of motion of the respiratory system. Several physiological studies showed that PAV efficiently decreases respiratory effort. It may also preserve physiological breathing pattern better than PSV, allowing a greater variability of tidal volume with different degrees of assistance.

Several drawbacks have been signaled for PAV. As it was previously described, elastance and resistance must be known by the ventilator to work correctly. This may be difficult during assisted ventilation. If incorrect high values are introduced and high percentage of assistance is used, the ventilator can over assist the patient and fail to recognize the end of patient breath. This situation is known as runaway. PAV is also affected by intrinsic PEEP; even if inspiratory trigger is set at the minimum value. The latter may induce ineffective.

High frequency ventilators

High frequency ventilation can be either jet ventilation or oscillation. In jet ventilation high-pressure gas (30-300 kPa) is delivered into the airway via a small-bore catheter at high frequencies (60-300/min). Air or supplementary O2 is entrained with the jetted ventilatory breath and expiration is purely passive. This form of ventilation is mostly used in the operating room where a surgeon is working in the airway (laser Rx of papilloma on vocal cords). Another situation where it may come in handy is when you are treating a disrupted airway or massive bronchopleural fistula as the non bulk flow of gas decreases the amount of gas escaping out of the fistula. Always ensure that you monitor expiration as it is passive and if you do not allow enough time for full exhalation you may cause iatrogenic barotrauma and pneumothoracies.

High frequency oscillation is a difficult concept to explain. Oscillators operate with a to-and-fro application of pressure on the airway opening at high frequency (100-1000/min). The goal of HFOV is to minimize lung injury. 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 has been observed. Currently, HFOV is better reserved for patients with severe ARDS for whom conventional mechanical ventilation is failing.

NAVA

Neurally adjusted ventilator assist (NAVA) is a new mode of mechanical ventilation that uses the electrical activity of the diaphragm (EAdi) to control the ventilator. First experimental studies showed an improved patient-ventilator synchrony and an efficient unloading of the respiratory muscles. Future clinical studies will have to show that NAVA is of clinical advantage when compared to conventional modes of assisted mechanical ventilation.