Coronary circulation

Since the 1970's, many authors have reported that providing oxygen at high pressures causes a decrease in coronary blood flow (with a mean figure of 20 to 30 % for 1-2 ata FiO2 = 1) (Daniell & Bagwell54, Podlesch & Herpfer55, Winter56). This was combined with a decrease in myocardial VO2 of around 20 % and a decrease in lactate extraction.

However, this decrease in coronary blood flow has to be interpreted in the light of a decrease in cardiac output. It is well known the coronary blood flow is closely adjusted to the myocardial VO2, since the myocardium cannot increase its capacity for oxygen extraction which is already maximal. Hutter57 has shown how myocardial VO 2 could be approached by the product of systolic arterial pressure by heart rate (SAP*HR). On rats in marked hyperoxia (5 ata, FiO2 = 1), Brego10 reported a 59% decrease in myocardial blood flow at 5 minutes, eventually stabilising at 51% after 60 minutes. In contrast, the external work of the left ventricle was only reduced by 13% after 5 minutes but more importantly, it had returned to normal after 60 minutes. These authors concluded that from the beginning and throughout the exposure to HBO, the coronary output became increasingly inadequate for the amount of ventricular work. This would lead to myocardial ischemia. Yet, the authors were not able to indicate any proof of such hypoxia.

Savitt35'58 again studied this problem by means of an experimental model in dogs. They showed that at 3 ata FiO2 = 1, coronary blood flow decreased by 17 + 10 % myocardial VO2 decreased by 11 + 6 % whereas cardiac output decreased by 24 %. Myocardial function, assessed by systolic work remained the same: end diastolic volume curves and the myocardial energetic state as determined by the relationship between myocardial VO2 and total mechanical energy expenditure (i.e. systolic work of the left ventricle + myocardial internal work) were unchanged. Thus coronary blood flow did not appear inadequate for cardiac output in hyperbaric hyperoxia.

To summarize, there is a decrease in myocardial blood flow in HBO, but this decrease is related to the decrease in myocardial work induced by a decrease in cardiac output.

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