The function of the Cardiovascular System is to circulate blood. Circulatory Insufficiency means that the flow of blood does not meet the demand for it. This is a statement of function. It is dynamic. The imbalance between demand and supply can change quickly and can improve or resolve quickly. This is not the same as Heart Failure, which is a diagnosis representing a disease state. It is sometime stated that the reason for heart failure is circulatory insufficiency but these terms are different. Heart Failure is a syndrome with signs and symptoms mostly the result of fluid retention. Fluid retention is slow to develop or improve. It cannot be used to determine blood flow especially acute changes. Circulatory Insufficiency and Heart Failure have entirely different meanings. The terms have entirely different uses.
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.
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 metabolic production of lactic acid produces an acidemia. This results in hyperventilation as a respiratory compensation. 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 lack of oxygen is at the tissue level not the bloodstream. The individual will complain of shortness of breath and be hyperventilating to compensate for the acidemia.
When circulatory Insufficiency develops abruptly we may say that it is Acute Circulatory Insufficiency. This can develop when the resistance to blood flow rises abruptly. It may also develop abruptly as the result of an acute cardiac event. As the resistance of blood flow and the output of the heart both contribute to blood flow it can be difficult to tell the difference between cardiac causes and vascular causes of acute changes.
It is often thought that the function of the heart as a pump determines the flow of blood but it should be emphasized that the circulation of blood involves both the blood vessels and the heart. The term heart failure reinforces the misconception that the sufficiency of blood circulation is solely dependent on heart function.
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. Heart Failure can be graded by severity but this is from exercise intolerance and not the degree of flow limitation.
Cardiovascular Insufficiency is the concept of insufficient flow whether or not it can be easily demonstrated. We can use Heart Failure as a diagnosis which allow for billing insurance companies. The flow of blood is difficult to determine and so it is too difficult to demonstrate circulatory insufficiency in most cases to the satisfaction of insurance companies. This term is not used as a diagnosis.
The demand on the cardiovascular system is quite variable with respect for the need for oxygen. The blood vessels play a role. The flow of blood can be augmented by reducing resistance to blood flow and flow can be diminished by increasing resistance. Increasing demand can be met by increasing the force of the heart as a pump but it is useful to keep in mind that this is the cardiovascular system and not just the function of the heart alone.
The circulation of blood allow many substances to be transported including nutrients, hormones, and also medications. The most sensitive and time critical are oxygen, carbon dioxide and glucose. The most critical of these is oxygen as the lack of it is quickly apparent especially in organs that are highly metabolic. This includes the muscle tissue but also internal organs including liver, kidney and neural tissue. The requirements of the muscle tissue is the most variable as exercise increases dramatically the need for oxygen by muscle tissue.
The tissues depend on the respiratory system to add oxygen to the blood and the cardiovascular system to deliver the blood. It is fairly easy to determine the oxygen content of the blood and it generally can be monitored continuously. The measurement of blood flow and oxygen delivery to tissue is considerably more difficult especially when tissues are not the skin or mucous membranes which are directly visible.
With exertion the supply of oxygen may approach the limit of what the cardiovascular system can deliver. Fatigue and sometimes a feeling of dyspnea occurs. With intense exertion anaerobic metabolism which does not require oxygen may supplement aerobic metabolism. This is most often a natural process that supplements energy for times of increased need. It is not life threatening most of the time as it is easily limited simply by rest as in a middle distance runner crossing the finish line or otherwise limiting the intensity of activity.
It is important that Cardiovascular Insufficiency is a process in healthy people as well as in those that have disease. The demand for blood can easily overwhelm the supply with exertion in healthy persons as the demands of the cardiovascular system vary greatly with activity. Running, swimming, wrestling etc increase the demand much above baseline. When this happens a part of the energy expenditure will come from anaerobic metabolism This mechanism for supplementing energy needs has evolved over millions of years and is safe so long as resting is possible. If illness is causing the increased demand and not exertion, the situation is serious if not grave.
When the demand exceeds the supply of blood the effects are regional. The muscle tissues are the most affected by high demand. These suffer a relative lack of oxygen and carry out the production of energy with oxygen requiring metabolism but also with anaerobic metabolism. The increased demand for oxygen can be dramatic, notably in athletes engaged in high intensity sports like sprinting.
Other tissues have lower metabolic requirements at baseline and are not significantly affected by exertion. Bone, neural tissue, etc. will not suffer from this metabolic imbalance. Importantly the skin is not a highly metabolic tissue and will not suffer the relative lack of oxygen from the imbalance of supply and demand. For this reason cardiovascular insufficiency does not lead to cyanosis even as an oxygen debt is accumulating. Runners develop an oxygen debt very rapidly but do not develop significantly lowered oxygen levels in the blood and do not appear cyanotic.
Cardiovascular Insufficiency becomes pathological when it can’t be stopped. Anaerobic metabolism produces energy without using oxygen but the process is temporary. It produces lactic acid which accumulates with high levels of activity. Bicarbonate buffer in the blood and increasing tendency toward acidemia. The athlete cane easily reduce or resolve this issue it anytime by resting.
Often there is a mix of both limited supply and some measure of excessive demand. When demand exceeds supply for whatever reason, anaerobic metabolism will develop.
When Cardiovascular Insufficiency develops lactic acid is produced and begins to accumulate. This happens in health and disease. The total amount of lactic acid begins to rise and an acidemia develops. The process of metabolic acidosis from this anaerobic metabolism leads to falling bicarbonate levels. A sensation of shortness of breath develops as a result of this cardiovascular insufficiency. This is notably seen in runners after a race particularly the middle distance races like 800 meters or 1600 meters At the end of strenuous excise the acidosis that had developed begins to resolve. This happens because the demand for oxygen is reduced not because the supply is increased.
Anaerobic metabolism, energy production without oxygen, in humans and indeed in all mammals produces lactic acid. There are other types of anaerobic metabolism most notably fermentation which occurs in yeast and other microorganisms which produces ethanol. In humans anaerobic metabolism produces lactic acid. Some authors refer to anaerobic metabolism as anaerobic respiration. This is an unfortunate term as respiration refers to breathing, the function of which is to oxygenate the blood. Anaerobic refers to the absence of oxygen. Respiration or respiratory refers to breathing. In humans or mammals or indeed any organism with lungs, anaerobic respiration does not make sense. In bacteria and other organisms that do not have lungs the term anaerobic oxidation would be better that anaerobic respiration. The best term for an anaerobic process that produces ethanol is fermentation. Trying to combine this different terminology under the term anaerobic respiration is a poor choice.
The term anaerobic exercise is also misleading. During high intensity exercise there is a mix of anaerobic and aerobic energy production. The term anaerobic exercise would imply that there is high intensity exercise where aerobic metabolism ceases and anaerobic metabolism provides all the energy molecules.
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 defigned. Cardiovascular Insufficiency is the concept of insufficient flow weather it can be easily demonstrated or not.
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. Cardiovascular 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 Cardiovascular 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, just that it has reached its limit of performance.
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 which is not predictable. Resistance is often assumed to be normal or constant. This is done for the sake of simplicity but sometimes is 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.
In addition to acidemia and breathlessness as the disease progresses edema of the lungs often develops. This gives the mistaken impression that the illness is a respiratory and not a cardiovascular illness. If the edema could somehow be removed we would still have a cardiovascular illness that is the root cause of this disorder. The acidosis would still be present and elevated pulmonary venous pressure that accompanies the edema would still be present.