Mechanical Ventilation: Lung Compliance and Airway Resistance Measurement (Respiratory Therapy)

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OVERVIEW

Clinical measurements should be done to monitor lung compliance and airway resistance in patients on a ventilator to check breathing status, see if the treatment is working, and adjust the treatment if needed. Lung compliance is a measure of how easily the lungs and chest wall can stretch (elasticity). Conditions such as acute respiratory distress syndrome (ARDS), pulmonary edema, pneumothorax, atelectasis, pneumonia, and obesity can make the lungs stiffer and harder to inflate (decreased compliance).^{undefined#ref2">2} Emphysema may increase lung compliance because of the loss of lung elasticity; however, the loss of lung elasticity makes it more difficult for the lungs to recoil or exhale the air. Airway resistance (R_aw) is a measure of how easily air can move through the airways.¹ Obstructions such as bronchospasms, secretions, and an endotracheal (ET) tube can make it harder for air to move through the airways.² The respiratory therapist (RT) should regularly check patients on a ventilator for changes in R_aw and static compliance (C_S) to catch early signs of pathophysiologic changes that may lead to ventilator-induced lung injury (VILI) and to optimize patient-ventilator synchrony and comfort. If the patient’s ventilator is set on a volume mode, increases in peak inspiratory pressure (PIP) and plateau pressure (P_plat) may be signs of gradual or acute compliance changes. Monitoring PIP, P_plat, and the difference between the two can help the RT identify lung problems quickly (Figure 1). If the patient’s ventilator is set on a pressure mode, decreases in tidal volume (VT) may be signs of gradual or acute compliance changes.

Dynamic compliance (C_D) is measured when air flow is moving in the ventilator circuit and patient airways. C_D measurements include R_aw. C_S is a more accurate measure of pressure inside the lungs because it is measured when all air flow is stopped in the ventilator circuit and the patient’s airways to allow a pressure equilibration within the system. During either pressure or volume ventilation, the pressure seen on the pressure-time waveform is an indicator of P_plat when gas flow has been stopped at end-inspiration during the breath pause.¹ Most critical care ventilators can accurately measure and display the PIP, flow rate, positive end-expiratory pressure (PEEP), tidal volume delivered (VT_D), and tidal volume exhaled (VT_E). The ventilator uses these values, along with values from maneuvers done by the RT, to calculate C_D, C_S, and R_aw. These values can be found on the ventilator screen, can be calculated manually by the RT, or can be found by using a computer program in the electronic health record.

C_D is calculated by using numerical data (PIP, PEEP, and VT_D) from the ventilator, without any additional maneuvers required, using the formula: C_D = VT_D ÷ (PIP – PEEP).

C_S is calculated by using the VT_E measurement from the ventilator along with these two steps done by the RT to stop the air flow in the ventilator circuit:

• Performing a pause (hold) at the end of the breath to get a P_plat.
• Performing a pause (hold) at the end of a breath out to get the total positive end-expiratory pressure (PEEP_TOT), which is set PEEP plus any extrinsic or auto-PEEP.

C_S calculations should use the formula: C_S = VT_E ÷ (P_plat – PEEP_TOT). To get an accurate C_S measurement, the patient and ventilator must be in sync and the patient should not be making any effort to breathe.¹ The normal C_S in a patient who is not on a ventilator is 70 to 100 ml/cm H₂O.¹ The lower the value, the lower the compliance, which means the lungs are stiffer, sicker, and more difficult to ventilate.¹ If the C_S is less than 25 ml/cm H₂O in an intubated patient, the work of breathing is very high.¹

R_aw is calculated after the pause at the end of the breath and is obtained using the formula: R_aw = (PIP – P_plat) ÷ flow in liters per second (L/sec). The normal value in a nonintubated patient is 0.6 to 2.4 cm H₂O/L/sec.¹ The R_aw increases to approximately 5 to 10 cm H₂O/L/sec or higher (R_aw increases as ET tube size decreases).²

SUPPLIES

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EDUCATION

• Give developmentally and culturally appropriate education based on the desire for knowledge, readiness to learn, preferred learning style, and overall neurologic and psychosocial state.
• Explain the procedure to the patient and family.
• Encourage questions and answer them as they arise.

ASSESSMENT AND PREPARATION

Assessment

1. Clean hands and don appropriate personal protective equipment (PPE) based on the risk of exposure to bodily fluids or infection precautions.
2. Observe the pressure-time waveform.
3. Assess the patient to ensure a decreased level of agitation. Request sedatives as needed.

   Compliance and resistance measurements may be difficult to obtain in a patient who is breathing rapidly or is agitated.

PROCEDURE

1. Clean hands and don appropriate PPE based on the risk of exposure to bodily fluids or infection precautions.
2. Suction the patient’s airway.
3. Allow breathing and patient comfort to return to baseline before obtaining lung mechanics.
4. Obtain C_D measurement: C_D = VT_D ÷ (PIP – PEEP) (Box 1).
5. Obtain C_S measurement: C_S = VT_E ÷ (P_plat – PEEPTOT) (Box 2).
6. Obtain R_aw measurement: R_aw = (PIP – P_plat) ÷ flow (L/sec) (Box 3).
7. Compare manually calculated values with the automatic values found on the ventilator, if available.
8. Remove PPE and clean hands.
9. Document the C_D, C_S, and R_aw in the patient’s record.

EXPECTED OUTCOMES

• Accurate assessment of C_D, C_S, and R_aw

UNEXPECTED OUTCOMES

• Patient-ventilator asynchrony
• VILI

DOCUMENTATION

• VT_D
• VT_E
• PIP
• P_plat
• Flow rate
• Set PEEP
• PEEP_TOT
• C_D
• C_S
• R_aw
• Changes in C_D, C_S, and R_aw
• Unexpected outcomes and related interventions
• Education

REFERENCES

1. Cairo, J.M. (2024). Chapter 8: Initial patient assessment. In Pilbeam’s mechanical ventilation: Physiological and clinical applications (8th ed., pp. 124-146). St. Louis: Elsevier.
2. Piraino, T. (2025). Chapter 52: Monitoring the patient in the intensive care unit. In J.K. Stoller and others (Eds.), Egan’s fundamentals of respiratory care (13th ed., pp. 1145-1179). St. Louis: Elsevier.

ADDITIONAL READINGS

Marini, J.J., Rocco, P.R.M., Gattinoni, L. (2020). Static and dynamic contributors to ventilator-induced lung injury in clinical practice. Pressure, energy, and power. American Journal of Respiratory and Critical Care Medicine, 201(7), 767-774. doi:10.1164/rccm.201908-1545CI

Scott, J.B. and others. (2020). Chapter 9: Critical care patient assessment and monitoring part II: Monitoring and care. In D.C. Shelledy, J.I. Peters (Eds.), Mechanical ventilation (3rd ed., pp. 449-526). Burlington, MA: Jones & Bartlett Learning.

Clinical Review: Jennifer Elenbaas, MA, BS, RRT, AE-C

Published: September 2024