The Science Journal of the American Association for Respiratory Care

2004 OPEN FORUM Abstracts

rapid-shallow breathing in 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: A rapid-shallow breathing pattern is predicted when the elastic recoil properties of the chest are elevated (1). This often presages weaning failure during a spontaneous breathing trial (2). As ALI/ARDS patients typically have increased lung and chest wall elastance, We determined if the frequency-to-tidal volume ratio (f/VT) is associated with specific measures of chest mechanics and breathing effort during a spontaneous breathing trial.

Methods:
Data were collected from 31 ALI/ARDS patients who underwent brief (1-2 min) trials 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. Eight variables were grouped into 6 multivariate, logistical regression models to assess their association with f/VT. The models were designed to account for various combinations of 1.) maximal inspiratory pressure (Pmax), 2.) respiratory system compliance (Crs), 3.) elastic and resistive work of breathing (WOBe, WOBr), 4.) mechanical indices of inspiratory muscle oxygen consumption [inspiratory muscle power output (W), and pressure-time product (PTP)], 5.) indices of inspiratory muscle fatigue [pressure-time index (PTI),and inspiratory pressure/maximal inspiratory pressure (Pi/Pmax)], and 6.) Central respiratory drive [inspiratory pressure in 0.1sec (P0.1), peak inspiratory flow rate (PFR) and mean inspiratory flow rate (MFR)].

Results: Models with predominantly pressure-related variables appear to have a higher predictive value in assessing f/VT changes than WOB and W measurements. Measures of central drive contribute strongly to the predictive value of the models.

Model Variables grouped in the model R2 p
1 Pmax + Crs+ P0.1 + MFR +PTI +PTP +Pi/Pmax + WOBe 0.74 0.001
2 Pi/Pmax + PTI + W + PTP + P0.1 + MFR + WOBe 0.74 0.003
3 Pmax + Pi/Pmax + PTI + PTP + WOBe + P0.1 + MFR 0.68 0.0011
4 Pmax + Crs + Pi/Pmax + PTI + W + PTP + P0.1 + WOBe 0.66 0.0065
5 Pmax + Crs + P0.1+ PFR + W + PTI + Pi/Pmax + WOBr 0.62 0.012
6 Pmax + Crs + W + WOBe + WOBr + PTP + PFR + P0.1 0.51 0.08


BOLD denotes p < 0.05 for individual variables.

Conclusion: Changes in f/VT ratio during brief CPAP trials in ALI/ARDS patients appear to be strongly influenced by changes in central respiratory drive and mechanical indices of respiratory muscle oxygen consumption and fatigue. Variables related to the elastic work load (WOBe and Crs) appear to have less influence.

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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