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To increase cooperation and decrease anxiety, explain to the patient that there will be a test involving changing the ventilator settings (breath hold).
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 (Raw) may occur abruptly, or they may be observed more slowly over time.undefined#ref2">2 Pulmonary system compliance and Raw 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 expands 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 (CD) and static compliance (CS). 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.
CD measures both lung compliance and Raw. CD is not an accurate assessment of how compliant the lungs are because it measures airflow through the airway as well as the ventilator circuit.1 The patient’s lung and chest wall elastic recoil, Raw, endotracheal (ET) tube, and the patient-ventilator circuit all influence the CD.1
CS is a more precise indicator of lung compliance because it is measured when no airflow is in the airway or the patient-ventilator circuit. CS 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 (PEEPTOT).2 Obtaining an accurate assessment of CS requires patient-ventilator synchrony and the absence of patient effort.1 Monitoring CS 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 Raw in a nonmechanically ventilated patient is 0.6 to 2.4 cm H2O/L/sec.1 The normal Raw in an intubated, mechanically ventilated patient is 5 to 10 cm H2O/L/sec.2 Raw 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 Raw.
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 CS is decreasing. If the PIP increases along with the difference in CD and CS, then Raw is increasing. If the CS decreases or Raw increases, then CD 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 CS and Raw. Some ventilators may display the values using graphic waveforms.
Rationale: Activation of the inspiratory pause (hold) on most ventilators automatically closes the inspiratory and expiratory valves at end inspiration. The pressure falls from PIP to a lower Pplat.
Keep Pplat below 30 cm H2O to avoid VILI.1,3
Rationale: Gas trapped in alveoli at end expiration may cause auto-PEEP.
Marini, J.J. and others. (2020). Static and dynamic contributors to ventilator-induced lung injury in clinical practice. American Journal of Respiratory Critical Care Medicine, 201(7), 767-774. doi:10.1164/rccm.201908-1545CI
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