Ventilator Settings

Tuxen and Lane [13] studied the effects of ventilatory pattern on the degree of hyperinflation, airway pressures and hemodynamics in patients with severe airflow obstruction. These authors observed that end-inspiratory lung volume was increased by dynamic hyperinflation as much as 3.6± 0.4 l above the apneic FRC depending on ventilator settings. When Vt was increased and/or when expiratory time was decreased either by an increase in rate (and hence minute ventilation) or by a decrease in inspiratory flow (at constant minute ventilation), dynamic hyper-inflationworsened. Pulmonaryhyperinflationwas associated with increased alveolar, central venous and, esophageal pressures as well as with systemic hypotension.

These authors demonstrated that, at constant minute ventilation, mechanically ventilated airflow obstructed patients exhibited the lowest degree of dynamic hyperinflation when ventilation was performed at high inspiratory airflows and long expiratory time [13]. Above all, imposed mean expiratory flow was the main determinant of hyperinflation. In a related study, Connors and coworkers [16] observed that higher airflow rates improved gas exchange in patients mechanically ventilated due to COPD, presumably because of decreased dynamic hyperinflation. Georgopoulos et al. [17] studied a group of passively ventilated COPD patients at constant respiratory rate and VT. The authors also documented that shortening expiratory time (by decreasing inspiratory flow or by adding an end-inspiratory pause) had a major influence on respiratory system mechanics, gas exchange and hemodynamics. These findings were explained by significant increases in auto-PEEP and trapped gas volume above the passive FRC when expiratory time was shortened.

At the present time, and according to the physiological data available, a general recommendation can be made when initiating volume controlled mechanical ventilation in COPD patients: set a moderate FiO2, usually 0.4 suffices to improve hypoxemia. Arterial oxygen saturation about 90% is acceptable in these individuals. Initiate ventilation with a respiratory rate of 15/min, Vt about 8 ml/kg and constant inspiratory flow 60-90 l/min. Inspiratory to expiratory ratio should be set at 1/3 or shorter (1/4, 1/5). Readjust these parameters once basic respiratory variables (resistance and compliance) have been measured. Provide enough ventilation to keep a normal pH, not a normal PaCO2.

External PEEP can be added to counterbalance auto-PEEP due to expiratory flow limitation. However, when patients are passively ventilated, the total impedance of the respiratory system, including the elastic extraload due to auto-PEEP, is overcome by the ventilator. In passively ventilated subjects under volume controlled mechanical ventilation, the ventilator will generate more or less airway pressure depending on the pressure needed to overcome total elastance and the pressure needed to overcome total resistance. An important notion is that external PEEP is needed to counterbalance auto-PEEP when patients have spontaneous inspiratory efforts. In this scenario, external PEEP will counterbalance the elastic mechanical load induced by auto-PEEP, thus decreasing inspiratory muscle effort [18]. It has to be taken into account that external PEEP does nothing with regards the degree of dynamic hyperinflation, either in passively ventilated patients or in patients with spontaneous inspiratory efforts. The amount of dynamic hyperinflation is the same, regardless of the external PEEP levels. Adjust the trigger at maximal sensitivity and check there is no auto-triggering. Although trigger variable is useless in passively ventilated patients, a proper trigger adjustment will insure that patients are not burdened when spontaneous inspiratory activity resumes. The same holds true for external PEEP.

Was this article helpful?

0 0

Post a comment