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

ALERT

To increase cooperation and decrease anxiety, explain to the patient that there will be a test involving changing the ventilator settings (breath hold).

OVERVIEW

A patient’s pulmonary system should be monitored for early signs of pathophysiologic changes that may lead to ventilator-induced lung injury (VILI). Respiratory mechanics, also called lung mechanics, should be performed frequently on mechanically ventilated patients because changes in respiratory system compliance and airway resistance (R_aw) may occur abruptly, or they may be observed more slowly over time.^{undefined#ref2">2} Pulmonary system compliance and R_aw monitoring may help the respiratory therapist (RT) with mechanical ventilation management by optimizing patient synchrony and comfort while minimizing the risk of VILI.

Compliance is the ability of the lungs to stretch in response to increased air volume or how easily the lungs or chest wall expand during inspiration. Compliance is reported as the volume change per unit of pressure change (ml/cm H₂O or L/cm H₂O).³ Normal compliance in a nonmechanically ventilated patient is 60 to 100 ml/cm H₂O.³ Lung compliance is more difficult to measure when a patient is intubated receiving mechanical ventilation so the most common measurements for mechanically ventilated patients are dynamic compliance (C_D) and static compliance (C_S). Compliance may be affected by conditions that change lung or thoracic compliance, as well as conditions that change the elasticity of the lungs. Some common conditions that may affect compliance include, but are not limited to, pneumonia, acute respiratory distress syndrome, atelectasis, ascites, obesity, chronic obstructive pulmonary disease (COPD), or air-trapping.² When compliance decreases, it takes more driving pressure to inflate the lungs making the ventilatory workload greater.

C_D measures both lung compliance and R_aw. C_D is not an accurate assessment of how compliant the lungs are because it measures airflow through the airway and the patient-ventilator circuit.¹ The patient’s lung and chest wall elastic recoil, R_aw, endotracheal (ET) tube, and the patient-ventilator circuit all influence the C_D.¹

C_S is a more precise indicator of lung compliance because it is measured when no airflow is in the airway or the patient-ventilator circuit. C_S is usually calculated by the mechanical ventilator, but there are two manual maneuvers that the RT must perform, including an end-inspiratory breath pause (hold) and an end-expiratory breath pause (hold) to obtain the plateau pressure (Pplat) and total positive end-expiratory pressure (PEEP_TOT).² Obtaining an accurate assessment of C_S requires patient-ventilator synchrony and the absence of patient effort.¹ Monitoring C_S helps maintain lung-protective ventilation by ensuring that the driving pressure remains low and the Pplat is kept less than 30 ml/cm H₂O.¹

The normal R_aw in a nonmechanically ventilated patient is 0.6 to 2.4 cm H₂O/L/sec.¹ The normal R_aw in an intubated, mechanically ventilated patient is 5 to 10 cm H₂O/L/sec.² R_aw incorporates airflow and pressure measurements between the airway opening and the alveoli.² Secretions, bronchospasm, cuff leak, airway compression, and ET tube kinking or plugging may increase R_aw.

Lung problems may also be identified by monitoring the changes in peak inspiratory pressure (PIP), Pplat, and the difference between the PIP and Pplat (Figure 1). If PIP and Pplat are both increasing with no change in the delivered tidal volume (VT) and the difference between them is constant, then the C_S is decreasing. If the PIP increases along with the difference in C_D and C_S, then R_aw is increasing. If the C_S decreases or R_aw increases, then C_D is decreasing.¹

Most critical care mechanical ventilators have the capability to accurately measure and display PIP, PEEP, flow rate, and VT. The mechanical ventilator uses these measured values, along with a manual inspiratory breath hold and an expiratory breath hold, to calculate C_S and R_aw. Some ventilators may display the values using graphic waveforms.

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.
• 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. Review the patient’s record for the practitioner’s order, diagnosis, chest radiograph interpretation, precautions, and possible contraindications to performing the procedure.
5. Assess the patient’s vital signs and oxygen saturation.
6. If the patient’s ventilator is set on a volume mode, assess PIP for gradual or acute compliance changes.
7. If the patient’s ventilator is set on a pressure mode, assess the VT for gradual or acute compliance changes.

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. Assess the patient’s breath sounds.
5. Suction the patient’s airway.
6. Allow breathing and patient comfort to return to baseline before obtaining lung mechanics.
7. Measure C_D.
   1. Obtain PIP; PEEP or PEEP_TOT; and delivered, exhaled, or corrected VT.
   2. Calculate C_D where VT is the volume delivered by the ventilator: C_D = VT ÷ (PIP – PEEP).
8. Measure the Pplat.
   1. At the end of inspiration, start an inspiratory pause (hold) for 0.5 to 1 second¹ until a Pplat is reached.

      Rationale: Activation of the inspiratory pause (hold) on most ventilators automatically closes the inspiratory and expiratory valves at end inspiration ceasing airflow. The pressure falls from PIP to a lower Pplat.

      Keep Pplat below 30 cm H₂O to avoid VILI.^1,3

   2. Observe the Pplat displayed digitally on a graphic screen or on a pressure manometer and record it.
9. Measure C_S and R_aw.
   1. Obtain PIP; PEEP or PEEP_TOT; flow rate; and delivered, exhaled, or corrected VT.
   2. Calculate C_S, where VT is the volume delivered by the ventilator: C_S = VT ÷ (Pplat – PEEP) (Figure 1).
   3. Calculate R_aw: R_aw = (PIP – Pplat) ÷ Flow (L/sec).
10. Remove PPE and perform hand hygiene.
11. Document the procedure in the patient’s record.

MONITORING AND CARE

1. Monitor ventilator parameters.
   1. PIP
   2. Pplat
   3. PEEP
   4. Flow rate
   5. VT delivered, exhaled, or corrected by the ventilator
2. Monitor the patient for PEEP_TOT or auto-PEEP.

   Rationale: Gas trapped in alveoli may cause auto-PEEP.

3. Monitor heart rate, breath sounds, and oxygen saturation.
4. Monitor the patient’s overall comfort.
5. Observe the patient for signs and symptoms of pain. If pain is suspected, report it to the authorized practitioner.

EXPECTED OUTCOMES

• Accurate assessment of C_D, C_S, and R_aw

UNEXPECTED OUTCOMES

• Patient-ventilator asynchrony
• VILI

DOCUMENTATION

• VT delivered, exhaled, or corrected by the ventilator
• PIP
• Pplat
• Flow rate
• Set PEEP
• PEEP_TOT (Set PEEP plus auto-PEEP)
• C_D
• C_S
• R_aw
• Changes in C_D, C_S, and R_aw
• Unexpected outcomes and related interventions
• Education

OLDER ADULT CONSIDERATIONS

• Assess older adult patients for hearing problems, which may prevent clear understanding of the procedure, including inspiratory pause (hold) instructions.

REFERENCES

1. Cairo, J.M. (2020). Chapter 8: Initial patient assessment. In Pilbeam’s mechanical ventilation: Physiological and clinical applications (7th ed., pp. 117-139). St. Louis: Elsevier.
2. Piraino, T. (2021). Chapter 52: Monitoring the patient in the intensive care unit. In R.M. Kacmarek, J.K. Stoller, A.J. Heuer (Eds.), Egan’s fundamentals of respiratory care (12th ed., pp. 1146-1183). St. Louis: Elsevier.
3. 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.

ADDITIONAL READINGS

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