Riding the Waves: Waveform Interpretation Part 2

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Picture of Danelle Howard
Danelle Howard
Registered Respiratory Therapist, cross-trained in the Pulmonary Lab, caring for critically ill patients one breath at a time. Professional interests: mechanical ventilation, capnography, and waveforms.
Picture of Sam Epstein • Illustrator
Sam Epstein • Illustrator

Aspiring Medical Student and current Critical Care RN. Enjoys everything outdoors but can also be found inside nerding out on her medical education artwork

The Pre-brief

In Waveform Interpretation Part Two, we will discuss patient-ventilator dyssynchrony.  Patient-ventilator synchrony is dependent on the ventilator responding to the patient’s respiratory effort and demands and how the patient responds to the breath delivered by the ventilator. Patient-ventilator dyssynchrony occurs when the patient’s demands are not met by the ventilator. Patient-ventilator dyssynchrony can occur due to 

  • Pressure
  • Flow
  • Mode
  • Timing 
  • Flow delivery 
  • Volume, 
  • Pressure demands
  • Diaphragmatic electrical activity

The patient population also plays a part in patient-ventilator dyssynchrony, for example, COPD patients who have dynamic hyperinflation.   Dyssynchrony can cause increased work of breathing, accessory muscle use, tachycardia, increased oxygen demands, and can cause the patient to cough, interrupting ventilation all of which can cause the patient to become distressed.  Dyssynchrony can cause alveolar overdistention, lung injury, and the need for excessive sedation, all of which lead to lengthened ventilator days, increased length of stay, and poor outcomes.

Newer modes PAV and NAVA may be worth looking into as they promote patient-ventilator synchrony and help in the weaning process. Synchrony occurs when the ventilator responds and provides flow, pressure, and volume in perfect timing with the patient.  Every clinician managing the ventilator should be able to recognize dyssynchrony on the pressure and flow waveform to achieve effective and safe ventilation of your patient.

Trigger dyssynchrony 

The first phase of a ventilator assisted breath is initiation.  It is in this stage when the clinician will observe trigger dyssynchrony.  The clinician will observe a negative deflection which will not be followed by a rise in positive pressure above baseline. This occurs when the ventilator fails to detect and immediately respond to the pressure or flow demands of the patient due to a few reasons.  If the trigger sensitivity setting is too insensitive, the ventilator will not sense sufficient effort and will not immediately deliver a ventilator breath when the patient demands it.  This can be fixed by appropriately increasing the sensitivity or switching from a pressure trigger to a flow trigger.  Trigger dyssynchrony also occurs from ineffective patient effort which may be from respiratory muscle weakness, decreased neural drive, initiating a breath against a closed inspiratory valve, and/or dynamic hyperinflation and autoPeep, thus causing the patient to expend excessive energy overcoming this larger pressure gradient enough to drop intrathoracic pressure for the ventilator to sense and initiate a breath.  Ineffective triggering can also occur due to patient-centered issues. An example is muscular weakness, which can occur due to the patient’s sleep and nutrition status. Sleep and nutrition indirectly affect lung strength and function. It should be noted that false triggers can occur due to cardiac oscillations, rainout in the vent circuit, cuff leaks, and/or circuit leaks.

Flow dyssynchrony 

The clinician can observe flow dyssynchrony in the second phase of a ventilator breath which is the demand for airflow.  Flow dyssynchrony, or “ flow starvation,” occurs  when the ventilator delivers a set inspiratory flow lower than the patient’s inspiratory demand.  The clinician will notice an upward concave in the flow waveform and a dip in the pressure waveform in this scenario.  This more frequently happens in a volume mode due to inappropriately set flow settings and/or when the tidal volume set is not meeting the patient’s desired minute volume. The patient pulls flow from the circuit dropping the pressure in the circuit.  This still can occur in a pressure mode due to inappropriate pressure rise settings. In this case, the clinician can fix this by increasing the flow rate to meet the patient’s demands, decreasing inspiratory time, increasing expiratory time,  or change to a pressure mode or flow variable mode.  Flow dyssynchrony can also occur when the flow rate is set higher than the patient demand resulting in an overshooting of inspiratory flow.  In this case, the clinician can reduce the flow rate, applied set pressure, and/or increase rise time.

Double triggering

When the patient’s neural inspiratory time is longer than the ventilator set inspiratory time, double triggering or “breath stacking” occurs.  Premature cycling due to dyssynchrony of inspiratory times between patient and ventilator can also result in double triggering. This is two consecutive inspirations with an expiration interval of less than half of the mean inspiratory time.  The clinician will also notice scalloping of the breath on the pressure-time waveform reflecting the negative pressure generated by the patient. When the ventilator ends inspiratory flow sooner than desired by the patient, the patient’s inspiratory muscles continue to contract causing the ventilator to sense a second effort resulting in double triggering resulting in high lung volumes and pressures. This causes the patient to take an additional breath since their demands for flow and/or volume has not been met. Double triggering occurs from a high patient respiratory drive and/or low clinician set tidal volume.   In this case, the clinician can try a mode that delivers maximal flow at the beginning of a breath, adjust the ramp of the flow curve, and/or increase set tidal volume. It is important to note that heavily sedated patients not triggering the ventilator can present reverse triggering on the waveforms. Reverse triggering is respiratory muscle contraction triggered by mechanical insufflation contracting the diaphragm causing a double cycle.  More on this topic at a later time.

Termination dyssynchrony 

The next phase of a ventilator-assisted breath is the end of inspiration and breath triggering. This phase of the ventilator-assisted breath is where the clinician will notice termination dyssynchrony also called cycle dyssynchrony.  Termination dyssynchrony can occur as a premature or a delayed termination.  


In premature termination, the exhalation valve opens too early causing the flow of air to stop before the patient stops inhaling.  This premature termination can result in low tidal volumes and can be corrected by the clinician by increasing the inspiratory time or increasing the tidal volume in volume control modes or by increasing inspiratory time in pressure modes. Premature cycling sometimes results in double triggering which can result in volutrauma.

Delayed termination occurs when the exhalation valve opens too late resulting in the ventilator inspiratory time cutting into the patient’s neural expiratory time.  The clinicians will notice this by the spike at the end of inspiratory breath.  The patient has finished inhaling before the ventilator is ready to cycle into exhalation resulting in an increase in the patient’s respiratory workload.    This can result in high delivered tidal volumes and/or dynamic hyperinflation seen in the next breath. Delayed termination could also result in ineffective triggering of the next breath.  This can be corrected by the clinician by increasing inspiratory flow in volume modes or decreasing inspiratory times in pressure modes.  Delayed cycling can also occur in pressure support mode due to cycling flow set too low, pressure support level set too high, and/or rise set too long. Lastly delayed cycling can also occur due to air-leaks.

Expiratory dyssynchrony 

Expiratory dyssynchrony will occur before the start of the next inspiration and can occur in all modes. Expiratory dyssynchrony occurs when the ventilator set expiratory time is not the same as the patient’s neural expiratory time (Te). Expiratory flow not returning to baseline before the start of the next breath will result in air trapping and auto-PEEP.  If the set rate is appropriate and the patient is not breathing above the set rate, then the clinician can increase expiratory time to correct air trapping. If the patient has a high respiratory drive, tidal volume can be decreased, or in some cases sedation may be needed.  Prolonged expiration may be required depending on the patient population and disease process. This can be seen in patients with COPD, ARDS, and obesity.   In their case, other interventions like bronchodilation may be necessary.

The Debrief

  • Patient-ventilator dyssynchrony can cause a poor outcome
  • Treat underlying issues like intrinsic PEEP, COPD, bronchospasm, etc
  • Appropriate mode, sensitivity, flow rate, volume, pressure, and timing should be adjusted and set 
  • PAV and NAVA are two new potential approaches to ventilator synchrony 

References

  1. Haro, C., Ochagavia, A., Lopez-Aguilar, J., & Fernandez-Gonzalo, S. (2018, December 05). Patient-ventilator asynchronies during mechanical ventilation: Current knowledge and research priorities.

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