The perception of breathlessness is felt if the body and its various tissues, liver, kidney, muscles are not supplied with enough oxygen to meet the demand for it. Supplying the oxygen to these tissues is a two step process. First the respiratory system, airways and lungs add oxygen to the bloodstream, The second step is that the circulatory system has deliver to the oxygenated blood to the tissue. The delivery and utilization of oxygen has a considerably more complicated science to understand. The oxygen actually has to be delivered to the mitochondria where metabolism utilizing oxygen is carried out. It is commonly thought incorrectly that if oxygenation of the blood is adequate then tissue hypoxia will not occur and the sensation of breathlessness or shortness of breath will not occur. The tissue of the liver, kidney, muscle, etc. cannot perceive the difference between low oxygen saturation of the blood and limitations of blood flow producing tissue hypoxia. There is the same perception of breathlessness or shortness of breath in either case.
Measuring the oxygen content of the blood is easy. The saturation of oxygen by hemoglobin in the red blood cells is easily measured and can be done by a sensor on the skin. The values are continuously displayed. When oxygenation falls below approximately 85%, cyanosis of the skin is visible. Cyanosis can be seen without instruments although it can be quantified with a pulse oximeter that reads oxygen saturation from the skins surface. Low oxygen saturation of the arterial blood is caused by some abnormality of the respiratory system, the lungs together with the airways.*
It is much more difficult to determine the flow of blood to body tissues than to assess the oxygenation of blood with oxygen saturation. It is commonly assumed that if blood pressure is adequate then blood flow will be adequate. This is fundamentally incorrect but in a range of situations blood pressure is taken to be directly reflective of blood flow. If this the best that can be done, is and it almost always is, adequate blood pressure means adequate the blood flow and low blood pressure means low blood flow. This is scientifically and fundamentally incorrect. Blood pressure “is not shock” is often stated while reasoning and assuming that low blood pressure is shock. The behavior of clinicians is frequently based on the belief of this fallacy. This foolish behavior is so often done and so often is accidentally, fortuitously or serendipitously correct it is accepted to be the nature of hemodynamics. This may fail in a spectacular way and the embarrassment is difficult to admit. Artificial Intelligence will aggravate this problem because it is believed so widely.
The delivery of oxygen in adequate amounts to various tissues depends on the demand of the particular tissue. Some tissues, bone, fat and skin have low metabolism compared to internal organs, liver, kidney, glandular tissue etc. The muscle tissue has the highest need which varies greatly depending on physical exertion.
It is difficult to determine if the delivery of oxygen is effective in providing enough oxygen for the demand of the tissue. Even though we can see and feel skin, Skin has low metabolic demand and usually does not suffer from the lack of blood flow. The flow of blood to the skin may be low but the metabolic requirements are also low. Supply may keep up with demand in tissues such as skin with low metabolism. Cyanosis is primarily reflective of the oxygenation of blood rather than the flow. Skin may appear pale. The oxygenation of skin that is pale may still be adequate if the supply of oxygen keeps up with the generally low metabolic demand.
In other tissue particularly muscle especially with high levels of exertion the delivery of oxygen may may inadequate and the balance of demand and supply may suffer. Hypoxia of the tissue is the result even if the level of oxygen in the arterial blood is adequate.
Perfusion versus Oxygen Saturation with normal physiology.
The amount of oxygen available at the tissue level is dependent on the saturation of blood with oxygen which normally is near 100 per cent and the perfusion or flow of blood to the tissue is generally adequate. With exercise or physical exertion the oxygenation of blood generally remains near 100% but the tissues with high metabolic demand, principally muscle tissue may begin to produce lactic acid. The develops with energy is generated from metabolism that does not require oxygen, anaerobic metabolism. A person may say after finishing a race that “I need to catch my breath” reflecting that they are feeling breathless but also knowing that with rest this feeling will dissipate. Breathlessness in this situation is understood as a temporary feeling.
Perfusion versus Oxygen Saturation in Illness.
A person that develops anaerobic metabolism from illness will have the same breathless feeling but won’t anticipate it resolving.
Reducing the level of exertion is not possible if the person is already already at rest. The person that develops anaerobic metabolism from illness is likely to be in serious trouble. The usual causes of anaerobic metabolism aside from exertion are hemorrhagic shock, infection with sepsis and poisoning with toxins such as cyanide.
*Prior to the widespread us of pulse oximetry direct arterial sampling of blood was common. This gives additional information not available when pulse optometry is used to determine oxygen saturation. Other important tests on arterial blood include testing for the dissolved gases, oxygen and carbon dioxide as well as measurement of pH, ion concentrations like sodium, potassium and particularly important, bicarbonate ion. Some information is lost when oxygen saturation is substituted for the direct measurement of oxygen, PaO2. It is so much easier to measure oxygen saturation, the importance of the other tests this difference is ignored sometimes with significant loss of understanding and perhaps causing significant errors.