Flow resistance in mechanically ventilated patients with severe neurological injury

Respiratory mechanics in brain-damaged patients

OBJECTIVE

To assess respiratory mechanics on the 1st and 5th days of mechanical ventilation in a cohort of brain-damaged patients on positive end-expiratory pressure (PEEP) of 8 cmH(2)O or zero PEEP (ZEEP).

DESIGN AND SETTING

Physiological study with randomized control trial design in a multidisciplinary intensive care unit of a university hospital.

PATIENTS AND MEASUREMENTS

Twenty-one consecutive mechanically ventilated patients with severe brain damage and no acute lung injury were randomly assigned to be ventilated with ZEEP (n = 10) or with 8 cmH(2)O of PEEP (n = 11). Respiratory mechanics and arterial blood gases were assessed on days 1 and day 5 of mechanical ventilation.

RESULTS

In the ZEEP group on day 1 static elastance and minimal resistance were above normal limits (18.9 +/- 3.8 cmH(2)O/l and 5.6 +/- 2.2 cmH(2)O/l per second, respectively); on day 5 static elastance and iso-CO(2) minimal resistance values were higher than on day 1 (21.2 +/- 4.1 cmH(2)O/l; 7.0 +/- 1.9 cmH(2)O/l per second, respectively). In the PEEP group these parameters did not change significantly. One of the ten patients on ZEEP developed acute lung injury. On day 5 there was a significant decrease in PaO(2)/FIO(2) in both groups.

CONCLUSIONS

On day 1 of mechanical ventilation patients with brain damage exhibit abnormal respiratory mechanics. After 5 days of mechanical ventilation on ZEEP static elastance and minimal resistance increased significantly, perhaps reflecting "low lung volume" injury. Both could be prevented by administration of moderate levels of PEEP.

Intrinsic positive end-expiratory pressure in mechanically ventilated patients with and without tidal expiratory flow limitation

OBJECTIVE

To assess static intrinsic positive end-expiratory pressure (PEEPi,st) and expiratory flow limitation (FL) in 32 consecutive mechanically ventilated patients with acute respiratory failure (ARF), using a commercial ventilator with an incorporated device that allows the application of a negative expiratory pressure (NEP).

DESIGN

Prospective clinical study.

SETTING

Multidisciplinary intensive care unit of a university hospital.

PATIENTS

Thirty-two consecutive ventilated patients with ARF of various etiologies.

INTERVENTIONS

Evaluation of respiratory mechanics, PEEPi,st, and FL from pressure, flow, and volume traces provided by the ventilator.

MEASUREMENTS

Peak airway pressure, PEEPi,st, dynamic elastance, and interrupter resistance were measured in relaxed patients in a supine position. Comparison of tidal flow-volume curves before and during the application of an NEP of 5 cm H2O was used to assess tidal expiratory FL.

RESULTS

Twelve of 32 patients studied exhibited tidal expiratory FL, which was detected by the absence of increase in expiratory flow despite application of an NEP over the entire or part of the baseline expiratory flow-volume curve. All patients exhibited PEEPi,st, which amounted to 1.2 +/- 0.9 cm H2O (mean +/- SD) in the 20 non-FL patients and 7.1 +/- 2.8 cm H2O in the 12 FL patients (p < 0.00001). The majority of patients with ARF resulting from underlying lung disease (11 of 13) had FL and a PEEPi,st > 4 cm H2O, whereas in patients with ARF of extrapulmonary origin, PEEPi,st was always < 4 cm H2O and only one grossly obese patient exhibited FL. Based on multiple regression analysis, in non-FL patients, PEEPi,st correlated significantly only with minute ventilation, whereas in FL patients PEEPi,st correlated significantly with peak airway pressure.

CONCLUSIONS

Because all the patients exhibited PEEPi,st and 12 of 32 patients (38%) also had FL, the authors conclude that the assessment of these variables at the bedside could provide useful information concerning respiratory mechanics in mechanically ventilated patients.

Flow and volume dependence of respiratory mechanics in anesthetized children

With the use of constant flow, end-inspiratory airway occlusion, respiratory system resistance (Rrs) can be partitioned into a flow resistive component (Rint) and an additional component (deltaR), reflecting viscoelasticity and time constant inequality. Similarly, respiratory system elastance (Edyn) can be partitioned into static elastance (Est) and elastance due to viscoelasticity and time constant inequality (deltaE). We measured Rrs and Edyn and their subdivisions (Rint and deltaR, Est and deltaE, respectively) and studied their flow and volume dependence in eight otherwise healthy children (median age 3.6 y; range 1.9-5.2 y) undergoing general anesthesia for oral rehabilitation. With a constant inspiratory flow (VI) of approximately 15 mL/s/kg and tidal volume of 12 mL/kg, the mean values of Rrs, Rint, and deltaR were: 0.20, 0.11, and 0.10 cmH2O/mL/s.kg. Under the same conditions, the mean Est and deltaE were: 1.04 and 0.12 cmH2O/mL/kg. With increasing VI and under constant VT, deltaR decreased (p < 0.001) progressively. Rint also decreased paradoxically (p < 0.001). Hence, Rrs decreased (p < 0.001) with increasing VI. Est decreased (p < 0.001) with increasing VI, whereas delta E increased (p < 0.005). With increasing VT and under constant VI, Rint decreased (p < 0.001) and deltaR tended to increase (p = 0.058); Rrs did not change. With increasing VT under constant VI, both Est and deltaE decreased (p < 0.001 and p = 0.001, respectively). Thus, in contrast to the findings in adults, Rint and Est decreased in children with increasing flow and under constant tidal volume, probably reflecting decreased functional residual capacity in anesthetized children, compared with adults. The flow and volume dependence of deltaR and deltaE were similar to those in adults, whereas Rrs did not necessarily follow the direction of changes of deltaR.