Mechanical Ventilation: Waveform Interpretation (Respiratory Therapy)
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Mechanical ventilators may incorporate software into the ventilator interface that allows a respiratory therapist (RT) to evaluate real-time patient-ventilator interactions. Graphics are displayed as different types of waveforms. The most common waveforms measure flow, pressure, and volume, and are graphed on a scale of time, called a scaler. Waveforms that use a scaler are useful in identifying the breath type, mode of ventilation, and auto-positive end-expiratory pressure (auto-PEEP). Waveforms that use loops or curves are helpful to monitor pulmonary compliance, airway resistance, and flow patterns.
Patient–ventilator asynchrony, also called dyssynchrony, may occur when the ventilator is not set appropriately to meet the ventilation demands or timing of the patient’s breath. Patient–ventilator asynchrony may be associated with patient discomfort, distress, and poor clinical outcomes, such as more ventilator, intensive care, or inpatient days. Asynchrony may happen in any mode of ventilation and during any phase of a breath.
Assessing the breath trigger is important because the timing of the ventilator breath may be normal (or in sync with the patient), early, or late. This is most often identified with the pressure and flow waveforms (Figure 1).undefined#ref1">1,3 Trigger asynchrony may also be seen on the pressure-volume flow loop, creating a “fish tail” (Figure 2)1 appearance at the beginning of the breath. False-breath trigger may be identified by excessive artifact in the pressure or flow waveforms and is usually caused by secretions or fluid in the circuit, endotracheal tube, or airways.2 A trigger may fail to deliver a ventilator breath if the expiratory flow waveform does not reach zero, which is a requirement of both flow and pressure triggering. The breath trigger can most often be adjusted with sensitivity setting.
Patient–ventilator asynchrony can occur when the flow (volume mode) or inspiratory time (pressure mode) does not meet the patient’s demand. The range of inspiratory time should be between 0.6 and 1 second, depending on the patient’s pulmonary mechanics.2 The flow and volume graphic should represent a smooth transition from inspiration to expiration. If the flow graphic shows a drop-off that does not go smoothly to baseline, it indicates an inspiratory time that is too short (Figure 3). If the volume graphic is flat at the top instead of rising to a peak and then declining, this indicates that the inspiratory time is too long for the patient. If the breath does not cycle to exhalation soon enough for the patient, it may lead to overdistention. A beak appearance at the top of the pressure-volume loop indicates pressure in excess of volume benefit or overdistention (Figure 4). The flow-time and pressure-time scalars show auto-PEEP (Figure 5) where the breath is not returning to baseline before the subsequent breath is given. The pressure-time and flow-volume loops may help RTs identify lung compliance and airway resistance changes. The flow-volume loop is most often used to determine whether lung function improves after the administration of a bronchodilator.
RTs must be able to assess waveform graphics to determine patient–ventilator synchrony. Using waveform analysis allows the RT to adjust the ventilator settings for a more comfortable experience while preventing ventilator-induced lung injury. It takes time and practice to acquire an understanding of graphics and how to use waveforms to assess patient-ventilator synchrony.
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Rationale: Condensate in the ventilator tubing may cause false-breath trigger along with additional waves in the graphics that may interfere with the interpretation of the waveforms.
Mireles-Cabodevila, E., Siuba, M.T., Chatburn, R.L. (2021). A taxonomy for patient-ventilator interactions and a method to read ventilator waveforms. Respiratory Care. doi:10.4187/respcare.09316 Epub ahead of print.
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