Subjective and objective measurement of tidal volume in critically ill patients.

Subjective assessment of the respiratory rate and the adequacy of tidal ventilation are the oldest and most widely practiced forms of respiratory monitoring in critically ill patients. Surprisingly, this method of assessment has itself never been evaluated in patients. The estimation of tidal volume in nine patients was performed by full-time intensive care unit (ICU) personnel and compared to the objective measurement of using a calibrated and validated respiratory inductive plethysmograph (RIP). Linear correlation analysis was performed. Poor correlation between clinical estimation by ICU personnel and actual measurement by RIP was revealed in all cases. A widespread and potentially dangerous tendency to overestimate tidal volume was noted. These data suggest that subjective assessment of tidal volume is inaccurate in critically ill patients and that the development of techniques of continuous, nonobtrusive and objective monitoring are to be encouraged.

The pattern of breathing during successful and unsuccessful trials of weaning from mechanical ventilation.

We prospectively examined the pattern of breathing in patients being weaned from mechanical ventilation: one group (n = 10) underwent a successful weaning trial and were extubated, whereas another group (n = 7) developed respiratory failure and required the reinstitution of mechanical ventilation. During the period of ventilator support, minute ventilation (VI), tidal volume (VT), and respiratory frequency (f) were similar in the 2 groups. After discontinuation of the ventilator, VI remained similar in the 2 groups, but VT was lower and f was higher in the patients who failed the trial compared with those who were successful, 194 +/- 23 and 398 +/- 56 ml (p less than 0.001), respectively, and 32.3 +/- 2.3 and 20.9 +/- 2.8 breaths/min (p less than 0.001), respectively. The failure group displayed a significant increase in PaCO2 (p less than 0.005) during spontaneous breathing, without a concomitant increase in the alveolar-arterial PO2 difference. Eighty-one percent of the variance in PaCO2 was accounted for by the pattern of rapid, shallow breathing. During weaning, resting respiratory drive (reflected by mean inspiratory flow, VT/TI) and fractional inspiratory time (TI/Ttot) were similar in the 2 groups. The patients in the failure group showed significant increases in VT/TI, 265 +/- 27 to 328 +/- 32 ml/s (p less than 0.01), and VI, 5.82 +/- 0.53 to 7.32 +/- 0.52 L/min (p less than 0.01), from the beginning to the end of the weaning trial; VT and f showed no further change.(ABSTRACT TRUNCATED AT 250 WORDS)