Hemodynamic and ventilatory effects associated with increasing inverse inspiratory-expiratory ventilation.
BACKGROUND
Increasing the percentage of inspiratory time during mechanical ventilation (i.e., inverse inspiratory-expiratory (I:E) ventilation) is frequently used to improve oxygenation in patients with acute respiratory distress syndrome; however, an optimal I:E ratio is unknown.


METHODS
To assess for an optimal I:E ratio, hemodynamic, ventilatory, and oxygenation parameters were determined in eight adult trauma patients with acute respiratory distress syndrome supported with pressure-control ventilation. An indwelling pulmonary artery catheter facilitated the extensive measurements as I:E ratios were randomly changed between 1:1 and 3:1. Measurements were determined 30 minutes after each change in the I:E ratio.


RESULTS
Increasing the percentage of inspiratory time resulted in a progressive increase in arterial oxygenation (p < 0.05) in conjunction with elevations in mean airway pressure (p < 0.05) and a decrease in alveolar-arterial oxygen difference (p < 0.05). Furthermore, progressive reversal of the I:E ratio significantly diminished alveolar ventilation (p < 0.01), with worsening dynamic compliance (p < 0.01). There were no demonstrable changes in hemodynamics.


CONCLUSIONS
These findings demonstrate the effectiveness of increasing inspiratory time to improve oxygenation, yet to the detriment of ventilation. This suggests that within the parameter confines of this study, the preferential I:E ratio is a balance between oxygen demands and ventilatory requirements.
Department of Surgery, Medical College of Virginia, Richmond, USA.
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D000328:Adult; D001784:Blood Gas Analysis; D006439:Hemodynamics; D006801:Humans; D008170:Lung Compliance; D009104:Multiple Trauma / Q000150:complications; D010100:Oxygen / Q000097:blood; D011175:Positive-Pressure Respiration / Q000009:adverse effects / Q000379:methods*; D011659:Pulmonary Gas Exchange; D012128:Respiratory Distress Syndrome / Q000097:blood / Q000209:etiology / Q000503:physiopathology* / Q000628:therapy*; D015656:Respiratory Mechanics / Q000502:physiology*; D013997:Time Factors
D010100:Oxygen