The function of the cardiovascular system is to circulate blood. When the cardiovascular system does not provide enough flow to meet demand then the system is insufficient. The delivery of oxygen is the most critical function of the circulatory system and the first to be noticed if flow is insufficient. The heart as a pump is a significant part of the system but the flow of blood is also dependent on the resistance in the vascular system.
Circulatory Insufficiency produces a feeling of breathlessness as the cardiovascular system fails to deliver enough oxygen to the tissues. This causes the tissues especially the highly metabolic tissues to develop anaerobic metabolism producing lactic acid. The acidemia that results causes hyperventilation as a respiratory compensation for the metabolic production of lactic acid. The appearance of this functional disturbance mimics respiratory disease. A person cannot perceive or identify the lack of blood flow. One does not say I do not feel enough blood flow and oxygen to the tissues of my body. Instead the lack of oxygen from inadequate perfusion gives the same sensation as in hypoxia from respiratory disease. The individual will complain of shortness of breath and be hyperventilating to compensate for the acidemia that develops.
The term heart failure is strongly associated with the appearance of fluid retention and edema. The point is frequently made that heart failure is a syndrome. It is not defined by measurement of any parameter. It might be said that cardiac function is impaired but that is not the way the syndrome is defined.
Circulatory Insufficiency is the functional concept of insufficient blood flow to meet demand. The heart as a pump is a significant part of the functioning of the cardiovascular system. The vascular system is also a significant part. High resistance in the vascular system and intense vasoconstriction can limit the flow of blood and is sometimes a part of insufficiency. Circulatory Insufficiency can also develop temporarily in states of high demand in normal individuals such as athletes. When the demand for blood and oxygen exceeds the supply and the tissues are starved of blood and oxygen the circulatory system is insufficient regardless to weather the flow of blood is withing normal or near normal parameters.
Acute Heart Failure is similar to Circulatory Insufficiency with an emphasis on the heart as a pump. When the heart suffers an acute insult such as a large infarction of myocardial tissue the flow of blood may be compromised to become insufficient to meet demand. This generally causes acidemia and shortness of breath. There may also be symptoms referable to the acute heart illness. In the case of acute myocardial infarction there may also be symptoms associated with the infarction of the myocardium such as chest pain.
Circulatory Insufficiency becomes significant when the demand for oxygen exceeds that which can be delivered by the circulatory system. This can occur in normal individuals and is not necessarily pathological. An athlete exercising at or near the limit of their capability will quickly exceed the capacity of their cardiovascular system to deliver oxygen. This can only occur for a limited time but is useful for bursts of strenuous activity. While this is occurring aerobic metabolism will be supplemented by anaerobic metabolism which causes a limited accumulation of lactic acid and a decrease in the pH of the blood. This is not a pathological condition as it is an adaptation and a normal part of strenuous activity.
Life depends on the circulation of blood. It can be inadequate only temporarily. The hallmark of Circulatory Insufficiency is the accumulation of lactic acid from anaerobic metabolism which results from inadequate oxygen delivery. Anaerobic metabolism is a short term solution to the problem of inadequate supply or excessive demand. The accumulation of lactic acid cannot continue indefinitely. The acidemia of the blood that occurs in high oxygen demand can be easily reduced and eliminated simply by resting. For this reason anaerobic metabolism in an exercising individual is an entirely different situation than in disease states where the imbalance between supply and demand for oxygen delivery may be difficult to reverse. It is true that short bursts of intense exercise cause anaerobic metabolism to produce lactate and a decrease in blood pH, an acidemia. At the same time increased frequency and depth of respiration will lower CO2 levels and reduce the effect of anaerobic metabolism and limit the acidemia that develops.
The amount of blood flow, that is cardiac output cannot be measured in any convenient way. Measurement requires an invasive catheter in the central circulation or sophisticated imaging. Edema of the lungs is a secondary result of the less effective circulation of blood and easily observed. Edema can be identified by physical signs and a plain film chest X-ray. To rely on what is easily seen leads to the common misunderstanding. Edema of the lungs is only a part of the reason for dyspnea. The proximate cause of the dyspnea comes from the tissues being starved of oxygen due to a deficiency in the delivery of oxygen. This is a cardiovascular disease. The most observable of evidence, the edema is misleading.
While blood provides may nutrients, hormones, and other substances to include any medications, etc. it is oxygen which critically requires adequate flow. Even the removal of CO2 is not so dependent on flow as oxygen is. Oxygen requirements are dramatically influenced by activity. An athlete exercising at the limit of their ability will be limited by the delivery of oxygen which can easily be exceeded by intense metabolic demand.
Cardiovascular insufficiency causes a feeling of shortness of breath as the tissues of the body cannot distinguish between tissue hypoxia due to a lack of blood flow and tissue hypoxia due to the lack of oxygen due to pulmonary disease. People afflicted with cardiovascular insufficiency will not say “my blood is not circulating adequately” which would be correct. They will complain of shortness of breath which suggests something it is not, a pulmonary abnormality when in reality it is an inadequate circulation of blood.
Heart Failure is generally thought of a the result of diseases of the myocardium such as ischemia or other specifically cardiac causes such as valvular heart disease, arrhythmia etc. Circulatory Insufficiency is considered an inadequacy of the system more often the result of inappropriate levels of systemic vascular resistance which leads to inadequate perfusion.
In Circulatory Insufficiency the emphasis is on supply not meeting demand. In fact the supply may be normal or near normal. The same physiology develops in athletes that exercise strenuously. Such individuals will be breathing heavily and if tests are done will be shown to have acidemia from metabolic acidosis and hyperventilation for respiratory compensation. In any case the system has not failed.
Inadequate blood flow and oxygen delivery to meet demand is difficult to demonstrate. The flow of blood or cardiac output cannot be measured except by sophisticated technology and or invasive methods. The classic method is a Fick calculation. This requires a closed respiratory system and a pulmonary artery catheter. This is awkward, expensive and invasive. Even an indirect Fick method which utilizes estimates of tissue oxygen consumption and requires a central venous catheter. There are other sophisticated approaches to determining flow such as dynamic MRI and advanced ultrasound techniques. These are not feasible in most situations. Consequently blood flow measurements are generally impractical.
Blood flow is dependent on blood pressure but equally dependent on the resistance to flow in the reverse manner. If resistance were a constant known value we could make a direct correlation. Resistance to flow is variable. The variations are not necessarily predictable. Resistance is often assumed to be normal or constant. This is done for the sake of simplicity but sometimes is wildly inaccurate.
Only the highly metabolic tissues particularly muscle are starved of oxygen when the level of oxygenation is adequate but when flow is low. The skin has low metabolism and therefore oxygen deprivation is not apparent and cyanosis of the skin will not appear. Other metabolic tissues such as liver and kidney are not visible even as they might be starved of oxygen due to low perfusion.