The results are in, and the correct answer was Flow Starvation
Flow starvation is a form of patient-ventilator dyssynchrony that occurs when a patient is demanding more flow than the ventilator provides.
Visual inspection of the inspiratory waveforms is often sufficient to detect flow starvation. This form of asynchrony is most often seen in volume assist controlled ventilation due to the fact that flow is controlled. The patient pulls flow from the circuit which drops the pressure in the system. Some ventilators may allow additional flow when the pressure drop in the circuit exceeds a predetermined amount, but the same scooping of the pressure-time curve is often seen.
Flow starvation does not necessarily imply that the patient has excessive effort. The patient’s effort and drive can often be changed by increasing flow or tidal volume (within safe limits).
Figure1 Orange arrows show a significant drop in pressure during inspiration due to the patient’s effort that demands more flow than the ventilator is set to deliver Red arrow represents the swing in esophageal pressure for the effort that is approx. -15 cm H2O.
It is important to set the inspiratory flow appropriately when using volume assist control. Most adult patients will be comfortable with a flow rate between 50-60 L/min. The use of lower flow rates reduce peak inspiratory pressure (which is due to the resistive pressure), but may not meet the inspiratory demand of a patient when they begin to interact with the ventilator. Ultimately, when a patient begins to interact with the ventilator they should be tested for the ability of liberation from the ventilator (spontaneous breathing trial), or attempt transition to a spontaneous mode of ventilation. If pressure assist control is used instead, clinicians will need to more closely monitor the appropriateness of the delivered tidal volume to ensure tidal volume is still lung protective.
If the patient has an excessive drive, there may be an increased risk of diaphragm and lung injury. Consider monitoring values such as the occlusion pressure (P0.1; available for all modern ICU ventilators), esophageal pressure swings, electrical activity of the diaphragm (EDi), or Pmus if you have these monitoring capabilities.
Pham T, Telias I, Piraino T, Yoshida T, Brochard LJ. Asynchrony Consequences and Management. Crit Care Clin 2018; 34: 325–41.
3 thoughts on “Flow Starvation”
Really useful. Please keep these posts coming.
Notice that this if volume control with dual targeting (eg, Servo-I or Servo-U vents)
That means the flow starts out constant (VC) but the drop in airway pressure signals the vent to switch to PC with increased flow
What is not shown is that the tidal volume has increased above the set value
See Respir Care 2012;57(8):1297–1304
Although the manual does not state this clearly in our lab we can only see this “dual-targeting” when flow adapatation is turned on (the rainbow waveform seleceted). If just the square waveform is selected you cannot access extra flow even if you drop the pressure by large amounts.