Mechanical Ventilation Medley of Modes

Why are there so many different ways to ventilate a patient?

There really is only two ways to ventilate a patient, using (conventional) positive pressure or negative pressure. Some of the earliest ventilators were negative pressure chambers (iron lungs).

A severe poliomyelitis epidemic broke out in Northern Europe in the mid 1950s. Patients suffering with this virus die from asphyxia - respiratory muscle paralysis and failure to ventilate. Medical students were assigned to manually ventilate paralysis victims until restoration of neuromuscular activity occurred. Iron lungs mimicked the chest cage’s activity in generating minute ventilation, but were of little value in diseases characterized by failure to oxygenate. The machines were bulky, expensive and somewhat unhygienic.

The first positive pressure ventilators were pressure controlled. This made sense as the chest is a negative pressure ventilator. Volume controlled ventilators became ubiquitous in the 1960s as this mechanism was perceived to be more reliable at delivering minute ventilation, and thus normalizing blood gases.

During the 1970s and 1980s ventilators were developed which allowed patients breathe spontaneously, initially with assisted breaths (assist control ventilation) and subsequently with spontaneous breathing limbs – (synchronized) intermittent mandatory ventilation (SIMV). The latter was the first mode to allow partial ventilatory support and thus gradual liberation from the ventilator. Pressure support was initially developed as a method of lending partial support to the patient’s spontaneous breaths, and interactivity became a function of microprocessor driven  ventilators. Physicians rapidly discovered that this could be used as a primary ventilation mode, with full patient interaction. Using the ventilator as an interactive weaning device emerged at this time.

During the 1990s widespread concern developed about ventilator induced lung injury. Accumulating evidence revealed that larger tidal volume, low PEEP, ventilation strategies were damaging the lungs. This has led to the development of lung protective ventilator strategies, using PEEP to maintain alveolar recruitment (the “open lung” approach), and lower tidal volumes, leading to reduced end inspiratory volumes, to prevent stretch injury. There was renewed interest in plateau pressure limitation and increasing mean airway pressures. Various strategies have been developed to achieve this goal. Pressure controlled ventilation has emerged as a viable alternative, although all strategies involve tidal volume targeting.

Technology has played a large part in the development of modern ventilators. However, the introduction of a multitude of new modes has not been accompanied by good quality outcomes research. Dual modes, combining pressure limitation with guaranteed tidal volume, have been developed. Physicians are now demanding more control over gas flow than before - hence the development of active exhalation valves, dynamic inspiration valves, rise time control, automatic tube compensation and, of course, waveform analysis. Modern ventilators deliver enhanced patient interactivity using better triggering sensors, and more comfortable spontaneous breathing - even in inverse ratio ventilation. AN exciting prospect is the gradual arrival of high frequency oscillation into adult critical care units. Using this technique, the physician sets the mean airway pressure, and there is minimal tidal gas movement.

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