Mechanical Ventilation: Waveform Interpretation (Respiratory Therapy)

OVERVIEW

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.

SUPPLIES

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EDUCATION

• Provide developmentally and culturally appropriate education based on the desire for knowledge, readiness to learn, and overall neurologic and psychosocial state.
• Explain the procedure to the patient and family.
• Discuss the potential benefits of mechanical ventilation waveform interpretation.
• Explain what to expect while the patient is ventilated.
• Encourage questions and answer them as they arise.

ASSESSMENT AND PREPARATION

Assessment

1. Perform hand hygiene before patient contact. Don appropriate personal protective equipment (PPE) based on the patient’s need for isolation precautions or the risk of exposure to bodily fluids.
2. Introduce yourself to the patient.
3. Verify the correct patient using two identifiers.
4. Assess the need for continued mechanical ventilation.
   1. Acid-base imbalance
   2. Hemodynamic instability
   3. Increased work of breathing
   4. Respiratory insufficiency or failure (e.g., hypercapnia secondary to hypoventilation, hypoxia)
5. Assess the patient for any obvious signs of patient-ventilator asynchrony.
6. Assess the patient’s comfort level.

PROCEDURE

1. Perform hand hygiene and don gloves. Don additional PPE based on the patient’s need for isolation precautions or the risk of exposure to bodily fluids.
2. Verify the correct patient using two identifiers.
3. Explain the procedure and ensure that the patient agrees to treatment.
4. Ensure the ventilator tubing is free from excessive moisture (condensate).

   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.

5. Select the graphics screen to observe and evaluate the ventilator waveforms.
6. Interpret waveforms to detect the most common clinical findings (below) associated with mechanical ventilation. Use the following waveform selections:
   1. Leaks in the system
      1. Select the volume scaler.
      2. Select the flow-volume loop.
      3. Select the pressure-volume loop.
   2. Lung compliance changes or overdistention
      1. Select the pressure-volume loop (Figure 4).
      2. Select the flow-volume loop.
      3. Select the volume scaler.
   3. Airway obstruction
      1. Select the flow-volume loop.
      2. Select the pressure-volume loop.
      3. Select the pressure scaler.
   4. Inadvertent PEEP or auto-PEEP (Figure 5)
      1. Select the flow scaler.
      2. Select the pressure scaler.
   5. Trigger asynchrony (Figure 1) (Figure 2)
      1. Select the pressure scaler.
      2. Select the flow scaler.
      3. Select the pressure-volume loop.
   6. Flow asynchrony (Figure 3)
      1. Select the pressure scaler.
      2. Select the flow scaler.
   7. Cycling asynchrony
      1. Select the pressure scaler.
      2. Select the flow scaler.
7. Discard supplies, remove PPE, and perform hand hygiene.
8. Document the procedure in the patient’s record.

MONITORING AND CARE

1. Monitor the patient’s vital signs, including heart rate, respiratory rate, oxygen saturation, and blood pressure.
2. Monitor the mechanical ventilator settings and alarms to ensure they are working properly. Monitor for excessive moisture in the circuit.
3. Monitor the patient’s peak inspiratory pressure, peak flow rate, and inspiratory and expiratory volumes using the pressure, flow, and volume scalers.
4. Monitor for patient–ventilator synchrony or asynchrony.
5. Monitor the waveform graphics for the following:
   1. Auto-PEEP
   2. Optimal inspiratory flow rate (volume) or inspiratory time (pressure)
   3. System leaks
6. Observe the patient for signs and symptoms of pain. If pain is suspected, report it to the authorized practitioner.

EXPECTED OUTCOMES

• Patient–ventilator synchrony
• Accurate interpretation of waveform graphics
• Ventilator adjustments to correct asynchrony

UNEXPECTED OUTCOMES

• Respiratory muscle fatigue
• Auto-PEEP or intrinsic PEEP
• Inadequate peak flow rate
• Breath trigger delay
• System leaks
• Increased time spent on the ventilator

DOCUMENTATION

• Education
• Patient’s vital signs
• Mechanical ventilator setting changes made to correct waveform abnormalities
• MRB with mask at the bedside
• Suction set properly and functioning at the bedside
• Mechanical ventilator mode, set breath rate, volume or pressure settings, PEEP setting, peak flow rate settings, and FIO₂
• Measured peak inspiratory pressure, mean airway pressure and volumes (both inspiratory and expiratory), and auto-PEEP
• Waveform graphic abnormalities
• Unexpected outcomes and related interventions

REFERENCES

1. Forrette, T.L. (2020). Chapter 9: Ventilator graphics. In Cairo, J.M. (Ed.), Pilbeam’s mechanical ventilation: Physiological and clinical applications (7th ed., pp. 140-157). St. Louis: Elsevier.
2. Kacmarek, R.M. (2021). Chapter 48: Patient–ventilator interactions. In R.M. Kacmarek, J.K. Stoller, A.J. Heuer (Eds.), Egan’s fundamentals of respiratory care (11th ed., pp. 1053-1071). St. Louis: Elsevier.
3. Waugh, J.B., Harwood, R.J. (2023). Chapter 5: Common clinical findings initiation. In J.B. Waugh, R.J. Harwood (Eds.), Rapid interpretation of ventilator waveforms (3rd ed., pp. 83-100). Burlington, MA: Jones & Bartlett Learning.

ADDITIONAL READINGS

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.