This is the second tutorial on Pressure Assist Control (PAC).

The tutorial focuses on the use of PAC in acute hypoxic respiratory failure (AHRF or ARDS). In this tutorial I cover the use of Pressure Assist Control in Acute Hypoxic Respiratory Failure (AHRF or ARDS). My major objective is to ensure that you understand that cookbook approaches to ARDS using volume control cannot be ported over to pressure control – in particular the ever increasing use of PEEP.

During previous tutorials I explained that increasing mean airway pressure (Pmean) can be achieved more effectively with increased inspiratory time (Ti) than by increasing PEEP. Pmean can also be increased by increasing respiratory rate but you must be really careful with Auto-PEEP as that reduces tidal ventilation, Pmean and increases dead space. This tutorial starts with an explanation of the Pendulluft effect in hypoxemia and with increased airway resistance – basically prolonging inspiration results in better overall gas distribution. How one manages worsening hypoxemia and lung compliance is key to your skill as an operator of a mechanical ventilator.

Early in the course one tends to maintain driving pressure and inspiratory time in PAC while increasing PEEP. However, ultimately one runs into the 30cmH2O barrier. At that point one must adjust. An important adjustment is to stop the patient from breathing – or more likely – gasping. “Gasping” is a term that I will use in these tutorials to describe the patient generating massive transpulmonary pressures (and likely lung stretch) with minimal impact on ventilation. In fact, the increased work of breathing causes a deterioration in oxygenation due to lower mixed venous oxygen tensions consequent of increased oxygen consumption (not covered in this tutorial).

The second adjustment one must make is to increase the inspiratory time and reduce the PEEP – keeping in mind your tidal volume target. At this point respiratory rate must fall and Auto-PEEP controlled. The final part of the tutorial covers the major drawback of PAC ventilation – expiratory dys-synchrony: what happens when the patient wants to exhale during inspiration.

I guarantee you’ll learn something. @ccmturorials http://www.ccmtutorials.org