Effects of inspiratory pause on CO2 elimination and arterial PCO2 in acute lung injury

Abstract

A high respiratory rate associated with the use of small tidal volumes, recommended for acute lung injury (ALI), shortens time for gas diffusion in the alveoli. This may decrease CO(2) elimination. We hypothesized that a postinspiratory pause could enhance CO(2) elimination and reduce Pa(CO(2)) by reducing dead space in ALI. In 15 mechanically ventilated patients with ALI and hypercapnia, a 20% postinspiratory pause (Tp20) was applied during a period of 30 min between two ventilation periods without postinspiratory pause (Tp0). Other parameters were kept unchanged. The single breath test for CO(2) was recorded every 5 min to measure tidal CO(2) elimination (VtCO(2)), airway dead space (V(Daw)), and slope of the alveolar plateau. Pa(O(2)), Pa(CO(2)), and physiological and alveolar dead space (V(Dphys), V(Dalv)) were determined at the end of each 30-min period. The postinspiratory pause, 0.7 +/- 0.2 s, induced on average <0.5 cmH(2)O of intrinsic positive end-expiratory pressure (PEEP). During Tp20, VtCO(2) increased immediately by 28 +/- 10% (14 +/- 5 ml per breath compared with 11 +/- 4 for Tp0) and then decreased without reaching the initial value within 30 min. The addition of a postinspiratory pause significantly decreased V(Daw) by 14% and V(Dphys) by 11% with no change in V(Dalv). During Tp20, the slope of the alveolar plateau initially fell to 65 +/- 10% of baseline value and continued to decrease. Tp20 induced a 10 +/- 3% decrease in Pa(CO(2)) at 30 min (from 55 +/- 10 to 49 +/- 9 mmHg, P < 0.001) with no significant variation in Pa(O(2)). Postinspiratory pause has a significant influence on CO(2) elimination when small tidal volumes are used during mechanical ventilation for ALI.

Fig. 1

The single breath test for carbon dioxide (SBT-CO₂) depicts partial pressure of CO₂ at airway opening and Pa_CO2 (horizontal dotted line) versus volume. Area A within the loop represents the amount of CO₂ eliminated during the breath (VtCO₂). Vertical interrupted line indicates airway dead space distal to the CO₂ sensor (V_Daw). Area C reflects CO₂ re-inspired from y-piece and tubing and represents proximal airway dead space (V_Daw,prox). Area B between the alveolar plateau and Pa_CO2 illustrates alveolar dead space. The alveolar plateau was characterized by the equation PCO₂ = b + m(lnV). Its slope was determined at its mid-point.

Fig. 2

Representative tracings from single breaths of airway flow, pressure (Paw) and partial pressure of CO₂ at the y-piece in mainstream CO₂ analyzer, PCO₂. Thin black lines represent Tp0 and thicker grey lines Tp20.

Fig. 3

Diamonds represent mean values for the 15 patients at 5 minutes interval throughout the study. At 35 min the knob on the ventilator panel for setting Tp was reset from 0 to 20 % to start Tp20. At 70 minutes Tp0_late was started by turning the knob back to 0 %. Times for resetting are indicated with vertical interrupted lines. Data at 35 and 70 minutes represent the first breaths during Tp20 and Tp0_late. Panel A. Tidal elimination of CO₂ (VtCO₂) against time. For the 15 patients, diamonds and thin lines represent percent of mean value during Tp0_init ±2 SE. Panel B. Airway dead space (V_Daw) against time in each subject, patients ventilated with heated humidifiers (continuous lines) or heat and moisture exchangers (broken lines). Diamonds shows mean V_Daw. Panel C. Slope of the alveolar plateau against time. For the 15 patients diamonds and thin lines represent percent of mean value during Tp0_init ±2 SE.

Fig. 4

Single breath test for CO₂ from a representative patient (n° 2) with and without pause recorded at the end of initial period of zero pause (Tp0_init, continuous line) and after 10 and 30 min at Tp20, (dotted and interrupted lines, respectively). At Tp0_init, airway dead space (V_Daw, vertical lines) was larger and the slope of the alveolar plateau steeper, while the area within the loop representing VtCO₂ was smaller.

Fig. 5

Individual values and mean ± SE of Pa_CO2 after mechanical ventilation with or without a post-inspiratory pause of 20% (Tp0_init, Tp20, and Tp0_late).