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Ventilator Management of 70-Year-Old Man Utilizing an Airway Pressure/Lung Volume Loop* FREE TO VIEW

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Correspondence to: Rolf D. Hubmayr, MD, FCCP, Pulmonary/Critical Care, Mayo Clinic, 200 First Street, SW, Rochester, MN 55905-0001; e-mail: rhubmayr@mayo.edu

From the ACCP-SEEK program, reprinted with permission. Items are selected by Department Editors Richard S. Irwin, MD, FCCP, and John G. Weg, MD, FCCP. For additional information about the ACCP-SEEK program, phone 1-847-498-1400.

Chest. 2001;120(5):1723-1724. doi:10.1378/chest.120.5.1723
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You have been asked to assist in the ventilatory management of a 70-year-old man with ARDS complicating urosepsis. He weighs 70 kg, he is deeply sedated, and he has been paralyzed with a nondepolarizing agent. Figure 1 shows an airway pressure/lung volume loop recorded while the patient was receiving volume preset mechanical ventilation with constant inspiratory flow of 0.6 L/s. Based on Figure 1, which of the following statements is correct?

A. Positive end-expiratory pressure (PEEP) should be raised to 18 cm H2O

B. Some units of the lung are being inflated close to total lung capacity

C. The deflation compliance of this patient’s lungs is 0.3 L/cm H2O

D. The area between the inflation and deflation limbs reflects lung hysteresis and is determined by recruitment and surface tension phenomena

E. The vital capacity of this patient is probably < 0.5 L

Answer: B. Some units of the lung are being inflated close to total lung capacity.

The key to choosing the correct answer is the appreciation that Figure 1 does not show a static pressure/volume curve, and that only the rightward inflation limb contains meaningful information about the dynamic mechanical properties of the relaxed respiratory system. The shape of the inflation curve, with its upper inflection to the right, reflects the fact that the lungs stiffen at high volumes. In a nonuniformly expanded lung, this stiffening implies that recruited lung units are being inflated to close to their total capacity. Whereas this may be of theoretical concern, there are no published outcomes data that establish that inflating the lungs to volumes above the upper inflection point is damaging or that it predisposes to ventilator-induced lung injury apart from positive end-expiratory pressure (PEEP) and tidal volume settings.

Whereas most authorities believe it is important to apply PEEP above the lower inflection point of the static inflation airway pressure/lung volume curve, recall that the relationship shown in Figure 1 was obtained while the patient received mechanical ventilation with flows exceeding 0.1 L/s. Under dynamic conditions such as this, the initial step change in pressure reflects inadvertent PEEP as well as the resistive properties of the intubated respiratory system above and beyond some recruitment threshold. Therefore, it would be inappropriate to adjust PEEP on the basis of pressure volume curves recorded while the patient received mechanical ventilation at conventional flow settings.

The leftward deflation limb shown in Figure 1 contains little if any information about the lungs. This is because, during expiration, airway pressure is determined only by expiratory flow and the resistive properties of the ventilation tubing and PEEP valves. For this very reason, one cannot compute deflation compliance unless the endotracheal tube is intermittently occluded, so that airway pressure can equilibrate with alveolar pressure. For the same reason, one cannot compute respiratory system hysteresis from plots like the one shown.

Because recruitable lung units approach total lung capacity at end-inflation, the set tidal volume of 0.4 L more or less defines the inspiratory capacity of this patient’s respiratory system. It does not define vital capacity, however, because lung inflation is not initiated from residual volume.

There is considerable controversy about the necessity to measure static pressure volume curves of the relaxed respiratory system for the purpose of finding a lung-protective ventilator setting. Resolution of this controversy awaits the results of better outcomes research that specifically addresses the benefits of PEEP and PEEP settings unrelated to the choice of tidal volume and peak inflation pressure.

  1. Jonson B, Richard JC, Straus C. et al. Pressure-volume curves and compliance in acute lung injury: evidence of recruitment above the lower inflection point. Am J Respir Crit Care Med 1999; 159:1172–1178

  2. Hubmayr RD, Gay PC, Tayyab M. Respiratory system mechanics in ventilated patients: techniques and indications. Mayo Clin Proc 1987; 62:358–368




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