Interpreting the shape of the ventilator flow waveform

The flow waveform is the most interesting waveform. Much information can be derived from its shape. When flow is being used to generate a controlled level of pressure, the shape of the inspiratory flow waveform is informative regarding the necessary inspiratory time (if flow reaches zero, then the inspiratory time could be shorter without compromising volume). The expiratory flow pattern is also informative, as a slow return to baseline is an indication of the resistance to airflow.

Though there have never been any CICM Part I questions about flow-time curves, the college WCA document ("Ventilation") clearly states that a mid-program trainee needs to be somebody who "outlines information available from plots of ...Flow vs time". This chapter is supposed to address this criterion.

The following diagram represents a constant flow and a decelerating flow waveform, and the various features of the curve.

Maximum flow

This is the flow generated during the inspiratory phase. Much of the time it is irrelevant from the diagnostic point of view, as it is something wholly machine-related. The ventilator generates this flow. In a volume-controlled mode of ventilation, the inspiratory flow waveform is rectangular (as the machine delivers a steady flow of gas, waiting for the target volume to be achieved).

In a pressure-controlled mode, the inspiratory flow waveform is decelerating, as the machine delivers just enough flow to produce the desired pressure. A rapid deceleration of this flow indicates either that the lung compliance is poor (i.e. the desired inspiratory pressure is achieved with minimal flow) or that the airway resistance is high (same reason).

In the graphic displayed here, the flow rate reaches zero very early in the (extremely prolonged) inspiratory phase. From this, one might make the inference that if one were to shorten the inspiratory phase somewhat, the inspiratory flow would still reach zero long before it is finished, and one might still have the same tidal volume as the result. However, the expiratory clearance of CO₂ might improve as the result of making the expiratory phase longer. This is an important method of assessing the adequacy of one's I:E ratio strategy.

End-inspiratory flow

The last point can be reversed: if the inspiratory flow is still above zero before the end of inspiration, then increasing the inspiratory time will produce larger tidal volumes. 

In pressure-controlled ventilation which is time cycled, one can see the flow curve return to zero during inspiration.  This means that the target pressure has been achieved and that there is no flow (a tidal volume is breath-held) for some period of time. If the flow curve does not return to zero before expiration, there is still some room to go with the inspiratory phase:  a larger tidal volume can still be achieved if flow is allowed to continue.

Peak expiratory flow

The rate of expiratory flow is determined by the resistance of the airways and the elastic recoil of the lungs and chest wall. The height of this waveform reflects these features; a low expiratory peak flow suggests there may be some bronchospasm.

In the same way, as peak flow measurements can be used to assess an asthmatic, so can the expiratory peak flow of a ventilated patient inform you about the airway resistance.  A low expiratory flow obviously suggests you have an airway obstruction (or, more freakishly, an abnormally over-compliant chest wall).  Similarly, if your expiratory flow is prolonged, your airway resistance must be increased. Furthermore, if the flow does not reach zero by the beginning of the next breath, there must be gas trapping (i.e. the patient has not finished exhaling fully before they are forced to take another breath).