Most bedside practitioners pay little attention to ventilator waveforms – usually just the tidal volume and, occasionally, the pressure waveform. However, mechanical ventilation is all about flow – if there is no flow there is no breath. In this tutorial I will look at flow patterns in patients attached to a ventilator. Patients who breathe spontaneously, without assistance, draw flow from the ventilator, the positive flow in inspiration is hemispheric in appearance, exhalation is a v shape – reflecting elastic recoil. Volume controlled ventilation may be delivered by either constant or decelerating flow, with or without an inspiratory hold (also known as a pause). The flow pattern in pressure control is always decelerating – as airway pressure rises, flow falls. Tidal volumes are variable in pressure control, as the negative pressure deflection during inspiration increases the inspiratory ramp and and hence the tidal volume.

I guarantee you will learn something from this tutorial and will never look at a ventilator the same way again.