Inspiratory flow effects on mechanically ventilated patients: lung volume, inhomogeneity, and arterial oxygenation

A simple method to estimate functional residual capacity in mechanically ventilated patients

OBJECTIVE

The aim of the present study was to evaluate a simplified method for FRC measurement.

DESIGN

Accuracy and precision of the method were assessed in a physical lung model; reproducibility was tested in 10 mechanically ventilated patients. In each patient FRC was measured at three PEEP levels.

SETTING

Post-operative intensive care unit in a university hospital.

MEASUREMENTS AND RESULTS

Gas flow, CO2 concentration, and O2 concentration were measured during in- and expiration by pneumotachography, a mainstream capnometer and a sidestream O2-analyser. For FRC-measurement inspiratory O2 concentration was changed by 30%. FRC was determined as mean value of a N2 washout and N2 washin procedure. Evaluation of this method in a lung model shows a good correlation between FRC set in the lung model and FRC measured (FRC measured = 1.028*FRG model + 22.92 ml; r2 = 0.957; n = 30). The mean difference was 4.4% of FRC-reference (range -8.4% to +21.7%). Duplicate determinations in 10 mechanically ventilated patients differed by an average of -2.7% (range -30.1% to +27.3%).

CONCLUSION

Our results suggest that the proposed method can be used in daily clinical work.

Effects of inspiratory flow rate alterations on gas exchange during mechanical ventilation in normal lungs. Efficiency of end-inspiratory pause

The influence of inspiratory flow rate (TI), without changing respiratory frequency, tidal volume, and FIO2, was investigated in 11 normal lungs in patients undergoing mechanical ventilation because of central respiratory failure due to stable coma. The patients were anesthetized and paralyzed. They first received a conventional ventilation (TI = 25 percent, pause = 10 percent) and then, were submitted to four different TI values, randomly administered without any end-inspiratory pause (EIP) (TI = 20 percent; TI = 33 percent; TI = 50 percent; TI = 67 percent). In the middle and at the end of the procedure, a return to basal conditions was introduced. At each ventilator setting, the following were obtained: respiratory flow (Pneumotachograph Fleish No. 2), airway pressure, FRC changes (inductive plethysmography), arterial and mixed venous blood gases, hemodynamic data, and VA/Q ratios distribution using multiple inert gases technique. EIP suppression provides a significant increase in VA/Q mismatch (until TI = 50 percent) and in shunt effect (between 3 and 9 percent of cardiac output [QT]). The absence of simultaneous PaO2 change is due to increasing PVO2 linked to a higher QT. The shorter the TI, the higher the PaCO2 connected with a relative alveolar hypoventilation. However, increasing TI without EIP significantly decreases ventilation distribution inequalities. This improvement is concomitant with a rise in FRC (FRC67-FRC20 = 0.340 +/- 0.450, p < 0.05) without any change in other variables or auto-PEEP production. In summary, in subjects with very slight mechanical lung impairment (peak inspiratory pressure = 20.5 +/- 5.3 cm H2O at TI = 20 percent and 15.2 +/- 3.3 cm H2O at TI = 67 percent), this study confirms the deleterious effect of EIP suppression and TI decrease. One can compensate for this effect of EIP absence by increasing TI as soon as it reaches TI = 67 percent, ie, inverse ratio ventilation.