What is the effect of hypoxia on cardio-cerebrovascular system?
Hypoxia has a series of effects on physiological function and metabolism, including compensatory response and injury.
The influence of hypoxia on the body mainly depends on the degree of hypoxia, duration, occurrence rate, tissue tolerance, hemoglobin concentration, metabolic rate and central nervous function. The amount of oxygen stored in the body is very small, about 400ml of oxygen in the functional residual gas of the lungs, about 850ml of oxygen bound with hemoglobin in the blood, a total of 1250ml.
According to the calculation of the basic metabolic oxygen consumption of the body 250ml/min, the oxygen storage in the body takes only 5 minutes. It is generally considered that PaO2 below 30mmHg (4kPa) is a dangerous level, which has obvious damage to tissues. If PaO2 is lower than 20mmHg (2.67kPa), it can immediately threaten life, and irreversible damage will occur in the cerebral cortex, and the acute hypoxia limit time that the body can tolerate is usually less than 5 minutes (Figure 4-3). In general, mild and moderate hypoxia (30 ~ 60mmHg), the body reaction is mainly compensated, and severe hypoxia (< 30mmHg), the body reaction is mainly damaged.
Tolerance to hypoxia varies among tissues. The brain tissue is shortest, the vascular smooth muscle is longer, and the hair and fingernails are longer.
Effects on the cardiovascular system
- Compensatory adaptive response
The cardiovascular system is very sensitive to hypoxia. The main compensatory reactions induced by hypoxia are increased heart rate, increased cardiac output, coronary vasodilation, pulmonary hypertension caused by contraction of pulmonary arterioles and redistribution of blood flow. When oxygen saturation decreased to 80%, heart rate increased by 10% and cardiac output increased. When SaO2 decreased to 72%, heart rate increased by 30% and cardiac output increased by more than 20%. During acute hypoxia, renal vessels were severely constricted and blood flow decreased significantly.
- Traumatic reaction
When the arterial oxygen saturation is less than 50%, myocardial hypoxia, insufficient energy production, acidosis and changes in cardiomyocytes ions lead to decreased myocardial systolic and diastolic function, slow heart rate, and even various arrhythmias (sinus bradycardia, prephase contraction, conduction block, and even ventricular fibrillation).
Effects on the central nervous system
- Compensatory adaptive response
The central nervous system is very sensitive to hypoxia, and its response appears relatively early and sometimes more dangerous. Brain oxygen consumption accounts for 20% to 25% of the total oxygen consumption of the body, and the reserves of oxygen, glycogen and ATP in the brain are also less than other tissues and organs, so the tolerance of brain tissue cells during hypoxia is poor. When PaO2 is lower than 50mmHg (6.7kPa), cerebrovascular dilation and cerebral blood flow increase, which is also a compensatory adaptive response, and mild hypoxia may only lead to headache, irritability, and inattention. With the aggravation of hypoxia, irritability, trance, delirium and other symptoms may appear.
- Traumatic reaction
When PaO2 is lower than 30mmHg (3.99kPa), the patients are often confused or unconscious. If PaO2 is lower than 20mmHg (2.67kPa), irreversible brain cell damage occurs. Hypoxic edema and hypoxic injury of brain cells are the main pathophysiological basis of central nervous system dysfunction.
For seniors only. In case of emergency, please follow doctor's advice
