Portable Chest Radiographs Identify Mechanical Ventilator-associated Hyperinflation

¹ From the Department of Internal Medicine (Pulmonary and Critical Care Medicine), and Public Health Sciences, Winston-Salem, N.C.
² From the Department of Radiology, and Public Health Sciences, Winston-Salem, N.C.
³ From The Bowman Gray School of Medicine of Wake Forest University, Winston-Salem, N.C.

Objective: Portable chest radiographs (CRs) are obtained routinely in mechanically ventilated patients, but little is known about relationships between CR findings and ventilator parameters. It is unclear whether radiographically apparent hyperinflation correlates with tidal volume (V_T), body weight (BW), V_T/kg, or levels of peak airway pressure (PAP), positive end-expiratory pressure (PEEP), or pressure support (PS).

Design: Prospective comparison of CR and ventilator parameters in 62 mechanically ventilated patients in surgical and medical intensive care units of a university hospital.

Patient selection: All mechanically ventilated adults with portable CR on four separate dates.

Methods: Chest radiographs were classified by subjective assessments as hyperinflated (H+) or nonhyperinflated (H), independent of knowledge of patients or their mechanical ventilation. Chest radiographs were reclassified independently as H+, H, or indeterminate by a radiologist using objective criteria. Ventilator parameters recorded at the time of the CR were obtained and compared.

Results: Patients with CRs classified subjectively as H+ compared with patients with CRs classified as H had a larger V_T/kg (12.0±0.4 ml/kg [mean±SEM] vs 10.2±0.4; p=0.004), lower BW(70.8±2.9 kg vs 81.5±3.8; p=0.03), higher PEEP (6.5±0.5 cm H₂O vs 5.0±0.4, p=0.01), and higher PAP(38.2±2.1 cm H₂O vs. 33.4±1.8; p=0.06). Using objective CR classifications, patients with H+ CRs had a V_T/kg of 12.6±0.4, larger than in the indeterminate (11.1±0.8) and H(9.9±0.3) groups (p<0.001). The BW differed among objectively classified groups (66.5±2.7 H+, 68.9±5.1 indeterminate, and 85.2±3.7 H; p<0.001), but other ventilator parameters did not correlate univariately with the degree of inflation on CR. Multivariate analysis showed that higher V_T was predictive of H+ after adjusting for BW in subjective (p=0.076) and objective (p=0.017) classifications. PEEP (p=0.004) and older age (p=0.021) were also associated with H+ in multivariate analysis. Four of 25 (16 percent) patients with objectively H+ CRs developed barotrauma, while no patient with H CR had this complication (p=0.037).

Conclusions: In mechanically ventilated patients, hyperinflation seen on portable CR is associated with higher V_T, V_T/kg, and lower BW, and may help predict subsequent barotrauma.

Key Words: barotrauma • hyperinflation • mechanical ventilation

Submitted on July 30, 1993
Accepted on February 15, 1994

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