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