The Science Journal of the American Association for Respiratory Care

2004 OPEN FORUM Abstracts

Spontaneous breathing mechanics of patients with acute lung injury (ALI) and acute respiratory distress syndrome (ARDS).

RH Kallet MS RRT, JA Alonso RRT, AR Campbell MD, R Dicker MD, RC Mackersie MD. Departments of Surgery and Anesthesia, University of California, San Francisco at San Francisco General Hospital.

Background: There is little information describing the spontaneous breathing pattern and mechanics of ALI/ARDS patients. Work of breathing (WOB) is elevated in ALI/ARDS and is believed to result from an increased chest elastance. A rapid-shallow breathing pattern is predicted when the elastic recoil properties of the chest are elevated (1) and this often presages weaning failure during spontaneous breathing trials (2). We measured work-related and breathing variables during a spontaneous breathing trial to document these abnormalities.

Methods:
Data was collected from 31 patients with ALI/ARDS. Patients underwent a brief 1-2 min trial on continuous positive airway pressure (CPAP) with a mean pressure of 5.4 ± 3.9 cm H2O. Measurements were made with a pulmonary mechanics monitor (BICORE CP-100) incorporating Campbell Diagram software. An average of 10-15 breaths was used for analysis. Data are reported as mean and 95% confidence interval (CI).

Results:
WOB was highly elevated; elastic WOB accounted for 57% of the total WOB. Despite a near-normal MIF, most work-related measures approached the fatigue-inducing range and were manifested by a markedly elevated frequency-to-tidal volume ratio (f/VT).

  Mean 95% CI
WOB total (Joules/L) 1.60 1.33-1.88
WOB elastic (Joules/L) 0.88 0.71-1.04
WOB resistive (Joules/L) 0.65 0.52-0.78
Power (Joules/min) 12.39 9.47-15.32
Inspiratory Time (sec) 0.80 0.73-0.87
Inspiratory Duty-Cycle 0.38 0.34-0.41
Peak Flow Rate (L/min) 36 31-41
Tidal Volume (VT) in mL 278 233-324
Respiratory Frequency (f) 29 27-32
f/VT 128 102-155
Pressure-time product (cmH2O·sec·min) 286 241-331
Pressure-time index 0.13 0.11-0.15
Maximum Inspiratory Force (cm H2O) 49 42-55
Tidal Inspiratory Pressure/MIF 0.37 0.32-0.42
Respiratory system compliance (mL/cmH2O)† 30 26-35

†measured during passive ventilation prior to CPAP trial.

Conclusion: Elevated f/VT ratio during brief CPAP trials occurs with markedly reduced compliance and increased WOB that primarily is related to the increased elastic workload.

1. Otis AB, Fenn WO, Rahn H. J Appl Physiol 1950; 2:592.

2. Yang KL, Tobin MJ. N Engl J Med. 1991;324:1445

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