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PADRONIZAR COMPETÊNCIAS PARA O CUIDADO CONSISTENTE
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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 (Raw) 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 Raw and static compliance (CS) 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 (Pplat) may be signs of gradual or acute compliance changes. Monitoring PIP, Pplat, 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 (CD) is measured when air flow is moving in the ventilator circuit and patient airways. CD measurements include Raw. CS 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 Pplat 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 (VTD), and tidal volume exhaled (VTE). The ventilator uses these values, along with values from maneuvers done by the RT, to calculate CD, CS, and Raw. 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.
CD is calculated by using numerical data (PIP, PEEP, and VTD) from the ventilator, without any additional maneuvers required, using the formula: CD = VTD ÷ (PIP – PEEP).
CS is calculated by using the VTE measurement from the ventilator along with these two steps done by the RT to stop the air flow in the ventilator circuit:
CS calculations should use the formula: CS = VTE ÷ (Pplat – PEEPTOT). To get an accurate CS measurement, the patient and ventilator must be in sync and the patient should not be making any effort to breathe.1 The normal CS 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 CS is less than 25 ml/cm H2O in an intubated patient, the work of breathing is very high.1
Raw is calculated after the pause at the end of the breath and is obtained using the formula: Raw = (PIP – Pplat) ÷ 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 Raw increases to approximately 5 to 10 cm H2O/L/sec or higher (Raw increases as ET tube size decreases).2
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Compliance and resistance measurements may be difficult to obtain in a patient who is breathing rapidly or is agitated.
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
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