The Sensation of breathlessness has two parts to it, oxygen tension and acidosis generally caused by rising CO2.
With altitude ventilation increased in response to the lower oxygen tension, PaO2. From this we know that oxygen tension is one of the factors driving ventilation.
Even though PaO2 drives ventilation it is the level of dissolved CO2 and the pH of the venous blood returning to the heart that is the major determinant of ventilation. Ventilation is the movement of air in and out of the lungs so that the air in the alveoli are exposed to the pulmonary capillaries so that gas exchange can take place.
Metabolic acidosis increases ventilation in compensation for reduced pH. The compensatory hyperventilation reduces the dissolved CO2 and increases the pH back toward normal. In diabetic ketoacidosis even though ventilation increases this is not accompanied by the sensation of shortness of breath. The person with diabetic ketoacidosis will be breathing deeply and at increased rate. They feel severe generalized malaise but do not sense breathlessness and will not complain of it. When the metabolic acidosis is caused by the buildup of lactic acid from anaerobic metabolism the ventilation also increases but this is typically also accompanied but a sensation of breathlessness. Even though both diabetic ketoacidosis and lactic acidosis produce compensatory hyperventilation, it is only lactic acidosis that produces a sensation of breathlessness. This is the result of not only the acidosis but also the tissue hypoxia.
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 pulse ox sensor on the skin.* The values can be continuously displayed. When oxygenation falls below approximately 85%, cyanosis of the skin is visible. Low oxygen saturation of the arterial blood is caused by some abnormality of the respiratory system, the lungs together with the airways. Low oxygen content in the inspired air or low barometric pressure can also cause low oxygen tension in the arterial blood.
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 tissue blood flow will be adequate. This is fundamentally incorrect but in most clinical 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 equates directly to low blood flow. This is scientifically and fundamentally incorrect. Blood pressure “is not shock” is often stated while reasoning and applying treatments that assume that low blood pressure is shock. The behavior of clinicians is almost always based on the assumption of this fallacy. This is so often done and so often is accidentally, fortuitously or serendipitously correct it is accepted to be the nature of hemodynamics. Clinicians generally have no choice but to use blood pressure to assess blood flow because they would be ridiculed if they went against the flow of thought.
Artificial Intelligence and the problem of Blood Flow.
The idea that blood pressure is the same as blood flow is pervasive and most of the time an unavoidable practical necessity. Most clinicians work with the idea that blood pressure equals perfusion. Artificial Intelligence is maligned for a variety of reasons but in cases where the prevailing opinion is incorrect it will aggravate the problem. Artificial intelligence rationalizes that garbage is not garbage when most everybody believes in garbage.
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 and the imbalance of supply and demand. The flow of blood to the skin may be low but the metabolic requirements are also low. Cyanosis is primarily reflective of the oxygenation of blood rather than the flow. Poor flow to the skin generally produces pale skin not cyanosis. Skin that is pale may still not shift from aerobic metabolism to anaerobic metabolism if the supply of oxygen keeps up with the generally low metabolic demand.
In muscle tissue especially with high levels of exertion the delivery of oxygen is likely to be inadequate when exertion is intense. The balance of demand and supply will suffer. Anaerobic metabolism in 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 muscle tissue with high metabolic demand may begin to produce lactic acid. The muscle produces energy in a process 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 always understood as a temporary feeling.
Perfusion versus Oxygen Saturation in Illness.
A person that develops anaerobic metabolism and lactic acidosis from an illness will have the same breathless feeling as an athlete 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, feels breathless and does not understand why. The usual causes of anaerobic metabolism aside from exertion are hemorrhagic shock, infection with sepsis and overdose with such substances as methamphetamine.
*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.