How does positive end-expiratory pressure decrease pulmonary CO2 elimination in anesthetized patients?

doi:10.1016/S0034-5687(99)00087-0

Respiration Physiology

Volume 118, Issues 2–3, 1 December 1999, Pages 227-236

Respiration Physiolo...

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Abstract

In anesthetized, mechanically ventilated patients, 10 cm H₂O positive end-expiratory pressure (PEEP10) immediately decreased the CO₂ volume exhaled per breath (V_CO₂,br) by 96%, as exhaled tidal volume (Vt) decreased to expand functional residual capacity during the first 8 breaths after PEEP10 began. Then, the sustained decrease in V_CO₂,br for over 10 min was due to the 19% decrease in cardiac output (Q̇t, decreased CO₂ delivery from tissues to lung) and to the decrease in alveolar ventilation (V̇a). In turn, decreased V̇a resulted from decreased Vt (loss of inspired volume into the compressible volume of the ventilating circuit) and possibly from increased physiological dead space, due to the potential for new high alveolar ventilation-to-perfusion (V̇a/Q̇) lung regions. V_CO₂,br increased and recovered to baseline by 20 min of PEEP10 ventilation because Q̇t increased to augment the CO₂ delivery to the lung and alveolar P_CO₂ increased (increased mixed venous P_CO₂ and tissue CO₂ retention) to increase V_CO₂,br while alveolar Vt remained depressed. End-tidal P_CO₂ (Pet_CO₂) progressively increased during PEEP10 and did not detect the decrease in V_CO₂,br during PEEP10 ventilation because Pet_CO₂ does not account for exhaled volume.

Introduction

In a previous study of non-steady state CO₂ kinetics in anesthetized, open-thorax dogs (Breen and Mazumdar, 1996), CO₂ volume exhaled per breath (V_CO₂,br) decreased by 54% after 1 min of positive end-expiratory pressure (PEEP) (11 cm H₂O). Three major mechanisms explained the decrease in V_CO₂,br during PEEP. First, expired tidal volume (Vt) decreased at onset of PEEP (as thoracic gas volume increased), which decreased V_CO₂,br. Second, alveolar P_CO₂ (Pa_CO₂) decreased during PEEP, which decreased V_CO₂,br. The decrease in Pa_CO₂ resulted from the decrease in venous return and CO₂ delivery to the lung, in turn due to the increased central venous pressure during PEEP. Note that the dilution of functional residual capacity (FRC) with fresh gas at onset of PEEP, which decreased Pa_CO₂, was offset by the decreased V_CO₂,br due to decreased exhaled Vt, which increased Pa_CO₂. Third, wasted ventilation decreased V_CO₂,br. During PEEP, increased physiological dead space (Vd_phy) resulted from an increase in anatomical dead space (Vd_ana) and appearance of new high alveolar ventilation-to-perfusion (V̇a/Q̇) lung units.

Other authors describe adverse effects of PEEP during steady state which could decrease V_CO₂,br, including decreased delivered Vt (Elliot et al., 1989), generation of high V̇a/Q̇ lung units (Dueck et al., 1977, Coffey et al., 1983, Nieman et al., 1988), increased dead space (Suter et al., 1975, Coffey et al., 1983, Vigil and Clevenger, 1996) with CO₂ retention (Dueck et al., 1977, Pesenti et al., 1985, Tokics et al., 1987), and decreased cardiac output (Q̇t) (Berglund et al., 1994, Huemer et al., 1994, Dambrosio et al., 1996).

Since PEEP is a common intervention used by anesthesiologists in the operating room, we sought to investigate how PEEP affects the non-steady state elimination of CO₂ in patients under anesthesia. It was hypothesized that the application of PEEP in mechanically ventilated patients would decrease V_CO₂,br, similar to the experimental study (Breen and Mazumdar, 1996). However, in patients, the intact thorax could blunt the increase in FRC and Vd_ana and the intact pleural pressure gradient could limit the generation of high V̇a/Q̇ lung units. During clinical anesthesia, CO₂ kinetics monitoring is generally restricted to measurement of end-tidal P_CO₂ (Pet_CO₂), which may not reflect V_CO₂,br because Pet_CO₂ does not contain exhaled volume information.

To test these hypotheses, serial breath-by-breath measurements of V_CO₂,br were conducted in anesthetized patients for 20 min after applying 10 cm H₂O PEEP during mechanical ventilation. Q̇t was measured non-invasively using a new esophageal Doppler cardiac output monitor, which allowed Q̇t measurements in this study on healthy patients without the risks of pulmonary artery catheterization. Simultaneous measurements of other cardio-respiratory variables facilitated the elucidation of mechanisms underlying the non-steady state effects of PEEP on V_CO₂,br.

Section snippets

General preparations

After the Institutional Review Board approval, informed consent was obtained from five adult patients (Table 1) to conduct this study in the period after induction of general anesthesia and before the scheduled surgery began. After placement of ASA standard monitors and pre-oxygenation, patients were induced with intravenous sodium thiopental or propofol and maintained with isoflurane. Muscle relaxation was achieved with succinylcholine at induction followed by non-depolarizing neuromuscular

Results

The breath-by-breath changes for about 20 respiratory cycles after the application of PEEP10 are depicted in Fig. 2. Compared to baseline, V_CO₂,br (Panel A) decreased by 96% at the first breath, with significant recovery by about breath 8. These changes in V_CO₂,br were mirrored by Vt_E (Panel E) which decreased by 912 ml to 265±69 ml by the first breath after PEEP10. Consequently, FRC (Panel G) increased by 779±228 ml with the first inspiration after PEEP10; the total increase in FRC was

Discussion

These are first clinical data, to the authors’ knowledge, of non-steady state breath-by-breath changes in CO₂ kinetics after application of PEEP. In anesthetized, mechanically ventilated patients, the application of 10 cm H₂O positive end-expiratory pressure (PEEP10) results in an immediate 96% decrease in the CO₂ volume exhaled per breath (V_CO₂,br) by the first exhalation. Thereafter, V_CO₂,br increased with each breath, but recovered to the baseline value only by 20 min of PEEP10. How does

Acknowledgements

This work was supported by the National Heart, Lung, and Blood Institute Grant HL-42637. We thank Bradley Jacobsen for expert technical assistance and Deltex Medical for provision of the esophageal Doppler cardiac output monitor.

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How does positive end-expiratory pressure decrease pulmonary CO2 elimination in anesthetized patients?

Abstract

Introduction

Section snippets

General preparations

Results

Discussion

Acknowledgements

Respir. Physiol.

Chest

J. Thorac. Cardiovasc. Surg.

PEEP-induced tricuspid regurgitation

Intensive Care Med.

Compression volume during mechanical ventilation: comparison of ventilators and tubing circuits

Crit. Care Med.

Echocardiographic analysis of cardiac function during high PEEP ventilation

Intensive Care Med.

Comparison of end-tidal PCO2 and average alveolar expired PCO2 during positive end-expiratory pressure [letter response]

Anesth. Analg.

Bymixer provides on-line calibration of measurement of CO2 volume exhaled per breath

Ann. Biomed. Eng.

Constant flow ventilation in anesthetized patients: efficacy and safety

Anesth. Analg.

Simple computer measurement of pulmonary VCO2 per breath

J. Appl. Physiol.

Comparison of end-tidal PCO2 and average alveolar expired PCO2 during positive end-expiratory pressure

Anesth. Analg.

Measurement of pulmonary CO2 elimination must exclude inspired CO2 measured at the capnometer sampling site

J. Clin. Monit.

Mechanisms of physiological dead space response to PEEP after acute oleic acid lung injury

J. Appl. Physiol.

Wasted ventilation measured in vitro with eight anesthetic circuits with and without inline humidification

Anesthesiology

Right ventricular myocardial function in ARF patients; PEEP as a challenge for the right heart

Intensive Care Med.

How does positive end-expiratory pressure decrease pulmonary CO₂ elimination in anesthetized patients?

Comparison of end-tidal P_CO₂ and average alveolar expired P_CO₂ during positive end-expiratory pressure [letter response]

Bymixer provides on-line calibration of measurement of CO₂ volume exhaled per breath

Simple computer measurement of pulmonary V_CO₂ per breath

Comparison of end-tidal P_CO₂ and average alveolar expired P_CO₂ during positive end-expiratory pressure

Measurement of pulmonary CO₂ elimination must exclude inspired CO₂ measured at the capnometer sampling site