Blood pressure in Circulatory Insufficiency can be either high, low or normal depending on the systemic resistance to blood flow. Although blood pressure can be easily measured the measurement of systemic vascular resistance is much more difficult. Generally this measurement requires a catheter to be placed in the pulmonary artery. This is commonly done in ICU settings but not in clinic or emergency department settings. The concept of systemic vascular resistance is a useful one. This is the resistance to blood flow that the heart pumps against.
The flow of blood is dependent on the pressure driving it vs the resistance it encounters. The functional problem when the resistance to the flow of blood is very high is that the flow of blood may be impaired. Most of the time the high resistance is overcome by high blood pressure. In the short term the resistance may be so high that even very high blood pressure will not overcome it and blood flow and perfusion will diminish. The flow of blood and oxygen may fall below what the demand for oxygen requires even when the blood pressure is very high.
Symptoms due to reduced blood flow and cardiac output can develop when systemic vascular resistance increases generally due to drugs or sympathetic tone. This mechanism of shock is not what is usually thought of when the idea of inadequate perfusion is considered. Most of the time inadequate perfusion is attributed to loss of cardiac function. Spikes in vascular resistance can develop from the use of drugs with vasoconstrictor properties such as methamphetamine. Other causes of increased vascular tone such as emotional or physical stress can also cause a sudden increase in vascular resistance due to sympathetic outflow. Stress and vasoconstrictor drugs can combine together to produce a greater effect. In cases where blood flow, cardiac output is significantly impaired, anaerobic metabolism can develop leading to an acidemia from the production of lactic acid. The symptoms are a feeling of breathlessness and hyperventilation. It appears as if the individual has a breathing disorder.
It is also commonly thought that if the oxygen level in the arterial blood is adequate then the tissue will be adequately supplied. Oxygen also has to be delivered. Elevated blood pressure is a clue that the resistance to blood flow is elevated. It is however, blood pressure and total peripheral resistance are not the same. Resistance and blood pressure are related by this equation, CO = BP/SVR. The poor circulation of blood due to elevated resistance will reduce the delivery of oxygen. This is perceived as dyspnea when combined with an acidemia produced by the production of lactic acid.
Acute Circulatory Insufficiency can occur with low blood pressure. This typically occurs when there is an acute insult to the heart such as a large myocardial infarction. In this case the resistance may be normal or below normal. Circulatory Insufficiency may occur because the blood pressure is low and even as the resistance is not elevated insufficient flow occurs. Acute Circulatory Insufficiency can be identified when anaerobic metabolism develops leading to an acidemia from the production of lactic acid. This is sometimes identified as pump failure. The cardiovascular nature of this syndrome is easier to appreciate than when blood pressure is elevated.