The results are in, and the correct answer was Premature/Short Cycling
Short cycling is a common form of patient-ventilator dyssynchrony. It occurs when a patient-triggered breath cycles off prior to the patient effort being completed.
Visual inspection of the inspiratory and expiratory waveforms is often sufficient to detect short cycling. First, look at the inspiratory pressure waveform to determine that the breath was indeed triggered by the patient. This is evident by an abrupt (albeit usually small) drop in airway pressure prior the breath delivery, which is the patient effort to trigger the ventilator. Most of the modern ventilators also display on their screen a specific sign indicating that the breath was triggered by the patient
Next, assess the expiratory flow waveform. There is an often a decrease in expiratory flow (upward deflection) immediately after the ventilator cycles off, this is due to the fact that the patient is still drawing in flow from the ventilator (their effort has not stopped) (fig 1). Additionally, if the effort continues long enough after the ventilator cycles off it may result in a double breath (fig 2).
A truly diagnostic approach is to view the esophageal pressure (Pes) waveform (fig 2), or electrical activity of the diaphragm (EaDi) to see inspiratory effort continuing after the ventilator breath has cycled off.
Figure 1 showing the esophageal tracing along with pressure and flow. Red box indicates the start and end of the patient effort, blue lines indicate the start and end of the ventilator breath.
Figure 2 showing a patient-triggered breath that resulted in a double breath. Waveforms are airway pressure (top blue), flow (middle green), and esophageal pressure (bottom red). Red arrows represent signs of patient effort, the yellow circle is to highlight there is no exhaled volume between the double breath, the blue arrows show differences in plateau pressure. The first blue arrow on the left points to a low pressure due to patient effort and controlled flow (volume control), the second blue arrow shows an elevated plateau due to breath-stacking, the last blue arrow is a passive breath with the patients’ baseline plateau.
The most common cause of short cycling is the mismatch between the neurological time of the patient and the cycling criteria of the ventilator breath. Methods to increase inspiratory time will depend on which mode the patient is on. Ways to increase inspiratory time in basic conventional modes are as follows:
|Volume Control||Increase tidal volume (may be harmful)
Decrease flow (this may lead to other asynchronies such as flow-starvation)
Increase inspiratory time (seconds) (may lead to other asynchronies such as flow-starvation)
|Pressure Control||Increase inspiratory time (seconds)|
|Pressure Support||Decrease cycle % criteria
Attempt to lower PEEP if it is high as this could affect respiratory system compliance which alters the slope of the inspiratory flow (see video below)
Patients may have an uncontrollable respiratory drive that you are trying to match by increasing inspiratory time, but the result may be a tidal volume that is not safe or appropriate. Determine if the drive is related to sedatives or neurological issues. Additionally, when patients have poor compliance or have excessive PEEP while receiving a mode like pressure support, where the flow cycles off based on % deterioration, the flow may decelerate faster than expected and cause the breath to cycle, in this scenario it may be worth decreasing PEEP (if medically appropriate) prior to adjusting cycle %. An example of these adjustments can be found in the video below.
Short cycling can often be confused with ineffective efforts because of the decrease in expiratory flow (upward deflection). However, when it occurs immediately after a patient-triggered breath it is likely short-cycling but can be confirmed by closely watching the patient for effort and timing with the ventilator, or by using an esophageal or Eadi catheter.
Other, non-conventional modes available are designed to more closely match the patients inspiratory time; these modes include neurally adjusted ventilatory assist (NAVA) and proportional assist ventilation (PAV). These modes are worth their own discussion, but in general, they respond and provide support to the patient that is proportional to their effort, including how long the breath lasts.
Short cycling is a common often missed form of patient-ventilator asynchrony. Small adjustments can correct the short cycling, but may not be able to correct the problem if the patient has an excessive inspiratory drive. Consider the patient condition at all times, and observe the resulting tidal volume delivered to determine the safety of allowing spontaneous breathing.
In the following video visual inspection of the patient and ventilator graphics indicated that the patient’s effort continued after the ventilator cycled off. As there was no longer a reason for the patient requiring elevated PEEP, it was reduced (which has the potential to negatively affect compliance when it is set too high), followed by lowering the % cycle criteria for Pressure Support (the setting which determines at which point the ventilator cycles off during inspiration). The result is a breath that is more synchronous with the patient effort.