Determination of Ventilatory Reserve in Mechanically Ventilated Patients: A Comparison of Techniques

Abstract

BACKGROUND: The vital capacity (VC) has commonly been used as an indicator of ventilatory reserve in mechanically ventilated patients. However, because patient cooperation is required for a valid VC, the reliability of VC measurements is often questioned. We evaluated an alternate technique for the measurement of ventilatory reserve in intubated patients. METHODS: In consecutive ICU patients, we randomly compared ventilatory-reserve determinations by Method A: the standard VC (the maximal expired volume following maximal inspiration) and Method B: the volume inspired and expired immediately following a maximal inspiratory pressure (MIP) measurement. A Wright respirometer was used to measure volumes during both methods. Three measurements were made with Method A, and the largest was recorded. With Method B, we measured the inspired volume (IV) and expired volume (EV) on the first breath after a 20-second MIP measurement made with one-way valves. Sets of determinations were made at least 1 hour apart. Usable data were obtained on 64 patients, 30 women and 34 men, age 67 ± 13 years. RESULTS: Of the 64 patients, 39 were able to cooperate with Method A and 25 could not. In the cooperative group, the Method-A VC (982 ± 479 mL) was significantly greater than either the Method-B IV (841 ± 436 mL) (p = 0.001) or EV (900 ± 472 mL) (p = 0.029). However, in the uncooperative group, the Method-A VC (485 ± 332 mL) was significantly lower than the Method-B EV (610 ± 444 mL) (p = 0.029). In addition, for the total group, there was a moderate correlation (0.57) between the Method-A VC and the MIP, with VC predicted from the MIP by the equation: VC = (MIP) (17.6) + 84.2, although this accounted for only 32.5% of the variance in VC. CONCLUSIONS: In intubated patients unable to perform the VC maneuver, the ventilatory reserve determined by the expired volume after an MIP measurement was larger than the standard VC. In cooperative patients, the addition of 80 mL to the expired volume after an MIP measurement resulted in a volume equal to the VC. Furthermore, VC was moderately correlated to MIP; however, only about one third of the variance in VC was predicted by MIP alone. We do not recommend prediction of VC based solely on MIP. However, in the total group, each cm H₂O of MIP was equal to 20 mL of VC.