Riding the Waves: Ventilator Waveform Interpretation

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Reading Time: 5 minutes
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.
Sam Epstein
Sam Epstein

Waveform Illustrator

The Pre-brief

Have you ever walked up to a ventilator and weren’t sure what you were looking at? Sure, it’s easy to write numbers down, but much harder to understand what you are looking at, what it means, and how to manipulate the ventilator to ventilate your patient safely and effectively.  Waveforms are an integral part of adequately treating patients.  Waveforms show real-time, breath to breath patient respiratory pathophysiology, which can aid in diagnosing and analyzing abnormal ventilator parameters, patient response to interventions, assess lung mechanics, evaluate patient compliance and synchrony, and achieve optimal and safe ventilation.  

There are three major waveform scalars: Pressure, flow, and volume.  These waveforms are displayed versus time.  The pressure scalar is the overall pressure generated and can assess patient lung mechanics such as response to respiratory medications.  The flow scalar assesses and identifies auto-PEEP, dyssynchrony, helps in setting optimal inspiratory times, and shows overall patient-ventilator interactions.  Note, however, that synchrony is best identified in the waveform of the non-controlled variable. The volume scalar assesses ventilator circuit related problems. 

Assist Control/Volume Control

The first picture you see is a normal pressure, flow, and volume scalar waveform in Assist Control/Volume Control mode.  A normal pressure scalar looks like a slope. The pressure will increase until the predetermined tidal volume (VT) is reached. Pressures are variable and are determined by the patient’s airway resistance, lung compliance, and the selected flow pattern. The normal flow scalar looks like a square. However, some ventilators will allow the clinician to change the flow pattern to an accelerating, decelerating, and/or sine flow pattern.  The normal volume scalar looks like a shark fin.   The volume of each breath uses a constant flow pattern.  Note, however, this pattern would change in a different flow pattern.

Pressure Control and PRVC

This picture is a normal Pressure Control (PC) and Pressure Regulated-Volume Control (PRVC) mode scalar waveform. In PC, the pressure is determined by the clinician and the pressure rises to the set level and then maintained at that level during inspiration.  Flow and volume vary depending on the patient’s airway resistance and lung compliance.  In PRVC the clinician is able to use dual controlled ventilation, combining both volume control and pressure control to deliver the desired VT. (Dr. Matt Siuba does a great job describing PRVC HERE) It uses breath to breath feedback on a breath to breath basis in order to adjust the pressure delivered.   Other than the startup breath in PRVC, both PC and PRVC modes have a  square pressure scalar with a decelerating variable inspiratory flow.

If you notice with the pressure waveform, it has an upward inspiration and a downward expiration that ends at the set PEEP level. The uppermost part of the waveform represents peak inspiratory pressure (PIP). With the flow waveform, anything above zero baseline represents positive flow, with the highest point being the peak inspiratory flow.  Anything below zero represents negative flow or expiration.  The lowest point represents peak expiratory flow.   

Gas trapping/Auto-PEEP:  

During passive exhalation, the lungs empty by elastic recoil.  In gas trapping/auto-PEEP, the lungs are not fully deflating before the next breath is initiated. On the pressure scalar the clinician will notice that the waveform rises above baseline when the clinician performs an expiratory hold during passive exhalation.  This measurement will read out total PEEP and/or auto-PEEP.  With the flow waveform, the decelerating expiratory waveform does not reach the baseline before the inspiratory flow of the next breath begins. On the volume scalar the expiratory portion does not return to baseline.  This prevents complete emptying of the lungs. 

The incomplete emptying of the lungs is due to dynamic hyperinflation, whether with or without intrinsic expiratory flow limitation. Reasons for this include COPD, asthma exacerbation, high respiratory rate set, high tidal volume set, and inspiratory time greater than the expiratory time. 

Possible ways to correct this problem are to: change ventilator parameters, reduce ventilator demand, reduce flow resistance for example, administer bronchodilators. (More on ventilating obstructive airway disease HERE)

Auto-triggering and leaks

Auto triggering of the ventilator is the inappropriate triggering of ventilation when the patient is not attempting to initiate a breath, by causing a decrease in airway pressure.  The respiratory rate will suddenly increase without patient input and the exhaled tidal volume and the minute ventilation will suddenly decrease.  This is shown on the scalar waveforms as rhythmic breaths without a pause.   On the pressure scalar, a decrease in peak inspiratory pressure will be evident, while on the flow scalar the PEF is decreased, and on the volume scalar the expiratory tidal volume doesn’t return to baseline.  The clinician will also note that the expiratory tidal volume is less than the inspiratory tidal volume.  This is usually seen with leaks in the ventilator circuit, a cuff leak, and/or a profound pneumothorax.

 Auto-triggering is sometimes caused by the sensitivity being set too high, a circuit leak, endotracheal cuff leak and/or an air leak due to a chest tube. Possible ways to fix this problem include minimizing leaks by checking the endotracheal tube cuff, and the ventilator circuit.  Another way to fix it is to adjust the trigger sensitivity.

Turbulence

At times condensation and/or secretions end up sloshing around in the ventilator circuit. Condensation, or “rain out,” ends up in the circuit due to ambient temperature changes. Turbulent scalar waveforms appear noisy and irregular.  You will notice this on both the pressure and the flow scalar waveforms. Other times you will notice this noisy pressure and flow scalar waveforms due to secretion build up in the patients’ lungs and sometimes during bed percussion. 

Be proactive and inspect both limbs of the ventilator circuit and drain the circuit if necessary. Keep in mind that you may have to change the circuit completely. Also note that if the circuit is no longer the problem, the problem may be the cassette if you are using a Servo.  If condensation and/or secretions slosh around in the circuit unnoticed for an amount of time, it could back up in the cassette causing the noisy appearing waveform, in which case the cassette would have to be changed out.  Make sure there is not a fan directed onto the temperature probe and make sure the room isn’t so cold that the ventilator circuit is cooling off. If this is the case and the problem persists you could always cover part of the circuit with a blanket or towel.  Sometimes the problem is a build-up of secretions in your patients’ lungs in which case you would then suction your ETT. 

Conclusion

Understanding waveforms helps clinicians recognize problems which in turn allows for enhanced ventilator effectiveness and optimized patient care.  Understanding waveforms minimizes ventilator-induced injury, decreases work of breathing, and decreases gas exchange alterations. This in turn decreases the need for sedation which will help to execute faster extubations and a shorter intensive care length of stay. 

The Debrief

  • Pressure, flow, and volume scalar waveforms are real-time breath to breath patient respiratory pathophysiology. 
  • Understanding how to read and interpret scalar waveforms helps clinicians optimize ventilation and patient synchrony while decreasing injury. 
  • Be aware of rain out to prevent artifact on your waveforms.

 

References

  1. Mellema, M. S. (2013 August). Ventilator Waveforms. Science Direct. https://doi.org/10.1053/j.tcam.2013.04.001

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