Physiologic Effects Of Hyperbaric Oxygen On Hemodynamics And Microcirculation

Daniel Mathieu, Raphael Favory, François Collet, Jean-Christophe Linke, Francis Wattel

Service d'Urgence Respiratoire, de Réanimation Médicale et de Médecine Hyperbare, Hôpital Calmette, Centre Hospitalier Régional Universitaire, Lille, France

Abstract: Hyperbaric oxygen (HBO) in the pressure range used for therapeutic purposes induces haemodynamic changes both at the macro- and micro circulatory levels. In normal subjects, systemic haemodynamic changes mainly concern the heart rate (an effect related to both the barometric and the oxygen pressures). Arterial blood pressure tends to increase slightly. Cardiac output is maintained or moderately decreased due to compensating responses which may fail in patients with pre-existing cardiac failure. At the microcirculatory level, HBO causes vasoconstriction with a decrease in microcirculatory blood flow but with no decrease of oxygen pressure in the tissues. This hyperoxic vasoconstriction does not appear in previously hypoxic areas where the microcirculatory blood flow remains unchanged. Consequently, oxygen pressure in the tissues increase to levels close to normal. The reappearance of cyclic vasomotion seems to indicate an improvement in the local metabolic condition. The haemodynamic and microcirculatory effects of HBO appear to be effective in compensating ischemic conditions, particularly in cases of heterogeneous microcirculatory hypoperfusion

Keywords: haemodynamics, hyperbaric bradycardia, hyperoxic bradycardia, hyperoxic hypertension, cardiac output, regional circulation, cerebral blood flow, coronary blood flow, renal blood flow, hepatosplanchnic blood flow, muscle blood flow, microcirculation, capillary blood flow, hyperoxic vasoconstriction, vasomotion

Hyperbaric oxygen therapy (HBO) is a therapeutic modality in which oxygen is administered to a patient through his respiratory system at a pressure above atmospheric pressure. The objective is to obtain an increase in pressure of oxygen in the tissues, either to compensate for a deficiency in oxygen supply, or to recruit special effects of oxygen at pressure above normal.

However, delivery of high pressures of oxygen are not exclusively beneficial; toxic effects do occur but are fortunately rare. For the most part, pressures and durations of oxygen exposure used in clinical practice are well off from the limits of toxicity. However, high pressures of oxygen do cause important haemodynamic and circulatory alterations, the most well-known being bradycardia and hyperoxic vasoconstriction. These alterations which have been well demonstrated for healthy subjects, tend to protect tissues from the consequences of hyperoxia.

A number of studies have been published on this subject, with apparently conflicting results. Differences in experimental protocols, populations studied and situations compared make a global synthesis difficult. Here we propose a synthesis of the works available in the literature and our own research in this field.

In the chain of events that brings oxygen from the ambient atmosphere to where it can be used in the cells, there are three basic stages which have long been identified by physiologists :

- Respiration, which involves the entry of oxygen into the body and its distribution in the blood,

- Circulation, which involves oxygen delivery from the small circulation to the capillaries in the tissues,

- Cell oxygenation, the stage where oxygen is brought to the cells where it is to be used.

The period in which this circulation takes place must also be seen as a multi-level process :

- first of all, there is a global macrocirculatory level to consider, where circulation is compared to a circuit of pipes (arteries and veins), pumps (right and left ventricles) and supply reservoirs (lungs and tissues). The effects of HBO on the macrocirculatory level have often been studied by physiologists and clinicians ; here we shall present the effects on central haemodynamics,

- a second level in the study of circulation involves blood flow distribution to each organ and even to each part of an organ. This will include a study on the effects of HBO on regional blood flow,

- the third level involves microcirculation where oxygen is brought to the locations where it is to be consumed.

At each level, high pressures of oxygen produce varying effects. In particular, the final effect varies depending on the condition - normal or pathological - of the body, organ or tissue.

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