Abstract
Purpose: Carbogen has long been under investigation as an adjuvant to radiotherapy of tumors. A major factor confounding its evaluation is its inconsistency in raising blood partial pressure of CO2 (pCO2). We investigated whether a new partial rebreathing method would provide better control of pCO2 than carbogen.
Methods and materials: We compared the efficacy of each method in 10 healthy volunteers. Volunteers breathed 1.5, 3 and 5% carbogen in 5-min stages via the usual non-rebreathing circuit. All the volunteers then breathed 100% O2 through a commercial sequential gas delivery (SGD) circuit modified by attaching a reservoir to its exhalation port. Hypercarbia was induced by step reductions in oxygen flow to the SGD circuit. We monitored minute ventilation and end-tidal pCO2 (ETpCO2) as a surrogate for its arterial value.
Results: Inhalation of 1.5 and 3% carbogen did not increase ETpCO2 from baseline (40 ± 1.5 mmHg); 5% carbogen increased ETpCO2 to 45 ± 1.6 mmHg (p < 0.001). With the SGD circuit, reducing O2 flow to 4.3 ± 0.7 l/min increased ETpCO2 in all subjects from 41 ± 2.0 mmHg (baseline) to 46 ± 2.1 mmHg (p < 0.001). Voluntary hyperventilation reduced ETpCO2 with 5% carbogen but not with SGD (p = 0.379).
Conclusions: We confirm previous observations that carbogen inhalation does not result in a predictable rise in ETpCO2 and suggest that a precise and stable target ETpCO2 can instead be induced by simply controlling O2 flow into a modified SGD circuit. We hoped that the reliable control of pCO2 will enable studies that address first, the efficacy of raising ETpCO2 on specific tumor blood flow, and eventually, its benefit as an adjuvant to radiotherapy.
Abbreviations | ||
SGD | = | sequential gas delivery |
pCO2 | = | partial pressure of carbon dioxide |
pO2 | = | partial pressure of oxygen |
ETpCO2 | = | end-tidal partial pressure of CO2 |
ETpO2 | = | end-tidal partial pressure of O2 |
Hypercarbia | = | increased carbon dioxide concentration in the blood |
Hyperoxia | = | increased oxygen concentration in the blood |
End-tidal FO2 | = | fractional concentration of end-tidal oxygen |
Abbreviations | ||
SGD | = | sequential gas delivery |
pCO2 | = | partial pressure of carbon dioxide |
pO2 | = | partial pressure of oxygen |
ETpCO2 | = | end-tidal partial pressure of CO2 |
ETpO2 | = | end-tidal partial pressure of O2 |
Hypercarbia | = | increased carbon dioxide concentration in the blood |
Hyperoxia | = | increased oxygen concentration in the blood |
End-tidal FO2 | = | fractional concentration of end-tidal oxygen |