Dosage is important when using oxygen for patients with myocardial infarction

Abstract

In questioning the use of oxygen supplementation for patients with myocardial infarction, Beasley and colleagues1 failed to discuss the importance of the plasma tension and the relevant science (JRSM 2007;100:130-133). Cardiac output is tailored to oxygen demand and vascular tone relates to the plasma oxygen tension because it determines the oxygen saturation of haemoglobin. The calibre of vessels is determined by the degree of binding of the vasodilator nitric oxide (NO) by oxygenated haemoglobin in forming S-nitrosohaemoglobin.2 In the presence of a coronary artery occlusion, upstream vasoconstriction by oxygen reduces blood flow, limiting the delivery of erythrocytes and hence oxygen availability. However, this only applies over the small range of inspired oxygen concentrations used in oxygen supplementation, because the vasoconstriction is limited by the sites available for NO binding by oxyhaemoglobin. Using 100% oxygen under hyperbaric conditions, plasma oxygen levels may be increased to more than compensate for the loss of oxygen transport by erythrocytes. Boerema et al.3 demonstrated this experimentally in 1960, showing that life is possible without blood when breathing oxygen under hyperbaric conditions. Higher plasma oxygen concentrations greatly increase the concentration gradient for the transport of oxygen to tissues; at two atmospheres absolute breathing 100% oxygen the concentration gradient is increased ten-fold. This explains the reduction of myocardial infarct size demonstrated experimentally using hyperbaric oxygenation. Beasley et al. do not reference the controlled trials of oxygen treatment under hyperbaric conditions. A controlled study of hyperbaric oxygen in myocardial infarction published in 19734 demonstrated benefit, especially in cardiogenic shock, and the paper includes ECG tracings showing abnormalities improving as the patients were pressurized. More recently, a controlled trial has shown the benefit of combining hyperbaric oxygen treatment with thrombolysis.5