2004 OPEN FORUM Abstracts
Spontaneous breathing mechanics of patients with acute lung injury (ALI) and acute respiratory distress syndrome (ARDS).
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).
|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|
|Inspiratory Time (sec)||0.80||0.73-0.87|
|Peak Flow Rate (L/min)||36||31-41|
|Tidal Volume (VT) in mL||278||233-324|
|Respiratory Frequency (f)||29||27-32|
|Pressure-time product (cmH2O·sec·min)||286||241-331|
|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|
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