Mechanical Ventilation: Waveform Interpretation (Respiratory Therapy)

ALERT

If a mechanical ventilator malfunction is suspected, remove the patient from the ventilator immediately and begin manual ventilation with a manual resuscitation bag (MRB).

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 the breath.^{undefined#ref1">1}

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).² Trigger asynchrony may also be seen on the pressure-volume flow loop, creating a “fish tail” (Figure 2)² 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.³ 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.³ 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 flow loop indicates pressure in excess of volume benefit (Figure 4). The flow-time and pressure-time waveforms 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.

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 mechanical ventilation before initiating ventilator support.
   1. Acid-base imbalance
   2. Adventitious breath sounds
   3. Altered level of consciousness
   4. Cyanosis
   5. Decreased oxygen saturation
   6. Hemodynamic stability
   7. Hypotension or hypertension
   8. Patient’s inability to maintain the airway
   9. Increased work of breathing
   10. Signs and symptoms of respiratory insufficiency or failure (e.g., hypercapnia secondary to hypoventilation, hypoxia)
5. Assess the waveforms for signs of ventilator asynchrony.
6. Assess the patient's comfort level.

Preparation

1. Before initiating mechanical ventilation, check the system microprocessor or ventilation system. Perform a short self-test as appropriate.
2. Ensure that the ventilator graphic interface is functioning properly using a test lung.
3. Ensure that an MRB with mask is at the bedside.
4. Ensure that suction is set properly and functioning at the bedside.

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. Select the graphics screen to observe and evaluate the waveforms.
5. Interpret waveforms to detect:
   1. Leaks in the system
   2. Lung compliance issues
   3. Correct inspiratory time
   4. Possible airway obstruction
   5. Lung overinflation or inadvertent PEEP
   6. Trigger asynchrony
   7. Flow asynchrony
   8. Cycling asynchrony
6. Discard supplies, remove PPE, and perform hand hygiene.
7. 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.
3. Carefully monitor the patient’s peak inspiratory pressure, mean airway pressure, and inspiratory and expiratory volumes.

   Rationale: Rising peak inspiratory pressure can indicate a change in compliance or the need for suctioning.

   Rationale: Patients receiving pressure ventilation can show an increase or decrease in exhaled volumes with a change in compliance. If compliance decreases, volumes decrease; if compliance increases, volumes increase.

   Rationale: Patients receiving volume ventilation can show an increase or decrease in pressure with changes in compliance. If compliance decreases, pressure increases; if compliance increases, pressure decreases.

4. Monitor the waveform graphics for auto-PEEP.
5. Monitor the waveform graphics for appropriate inspiratory flow rate or inspiratory times.
6. Monitor the waveform graphics for patient–ventilator synchrony or asynchrony.
7. Monitor the waveform graphics for excessive moisture in the circuit. Waveform graphics are smooth lines, so a wavy graphic can signify water in the circuit.
8. 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

• Patient–ventilator asynchrony
• Respiratory muscle fatigue
• Increased time spent on the ventilator
• Auto-PEEP or intrinsic PEEP
• Misinterpretation of waveform graphics

DOCUMENTATION

• Education
• Patient's vital signs
• Mechanical ventilator performing properly and receiving power from a source that has backup power
• MRB with mask at the bedside
• Suction set properly and functioning at the bedside
• Peak inspiratory pressure, mean airway pressure and volumes (both inspiratory and expiratory)
• Mechanical ventilator mode, set breath rate, volume or pressure settings, PEEP setting, flow settings, and FIO₂
• Waveform graphic abnormalities
• Unexpected outcomes and related interventions

HOME CARE CONSIDERATIONS

• Home health ventilators are available, but most do not have waveform graphics capabilities.

REFERENCES

1. Bruni, A. and others. (2019). Patient-ventilator asynchrony in adult critically ill patients. Minerva Anestesiologica, 85(6), 676-688. doi:10.23736/S0375-9393.19.13436-0 (Level VII)
2. Forrette, T.L. (2020). Chapter 9: Ventilator graphics. In Cairo, J.M. (Ed.), Pilbeam’s mechanical ventilation: Physiological and clinical applications (7th ed., pp. 140-555). St. Louis: Elsevier.
3. 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.

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.

Elsevier Skills Levels of Evidence

• Level I - Systematic review of all relevant randomized controlled trials
• Level II - At least one well-designed randomized controlled trial
• Level III - Well-designed controlled trials without randomization
• Level IV - Well-designed case-controlled or cohort studies
• Level V - Descriptive or qualitative studies
• Level VI - Single descriptive or qualitative study
• Level VII - Authority opinion or expert committee reports