When a rise in PCO2 increases cerebral circulation a little too much, bad things happen

Manic Depressive Insanity; and the Cushing Reflex. Are they the same?

Increase in blood volume

  • Increased cerebral blood flow during hypercapnia [retention of PCO2 in the blood] will eventually cause increased intracranial pressure (ICP). Eventually, the Cushing Response will be triggered to help mitigate possible injury. https://www.ncbi.nlm.nih.gov/books/NBK482119/

The Cushing reflex is a physiological response  to acute elevations of intracranial pressure (ICP) resulting in the Cushing triad of widened pulse pressure (increasing systolic, decreasing diastolic), bradycardia, and irregular respirations. https://www.ncbi.nlm.nih.gov/books/NBK549801/

The physical signs of Mania resemble the second stage of the Cushing reflex; see Chapter 3 of Manic Depressive Insanity by Emile Kraepelin, Bodily Signs. [the archived chapter is available online and I give the link in earlier blogs, if you are interested.] Hypertension continues to be present, the heart rate is intermittently too slow [bradycardia] and breathing rate is to fast with irregular and periodic respirations. Long-lasting, unmedicated attacks, of Mania [if they do not resolve on their own-which they often do] may cause cardiovascular collapse and death, as can the Cushing’s Reflex. . [month long attacks happened in Kraepelin’s day, and some patients- young and strong as they were, did die.].

 In the first stage of the Cushing reflex, blood pressure and heart rate rise in response to sympathetic activation to overcome increases in ICP.[3] This sympathetic response allows for brain perfusion as long as the ICP is not too high to overcome. For the brain to remain adequately perfused, mean arterial pressure (MAP) must be maintained higher than ICP. The depressive insanity, Kraepelin describes resembles the first stage of Cushings; chronic respiratory depression [no yoga, no drugs], very high blood pressure and heart rate with vasoconstriction [pale cold hands, feet, lips] and strangely mild hypothermia [perhaps due to respiratory depression?-who knows].

In the second stage of the Cushing reflex, hypertension continues to be present, but the patient becomes bradycardic rather than tachycardic. This finding surprised Kraepelin. He expected to find a high heart rate in the constantly moving or speaking manic patient. Perhaps we have got it wrong as well. Perhaps the manic behaviour of patients occurs to raise a failing heart.

There are differing opinions on the mechanisms leading to this stage of the Cushing reflex. We still do not understand. We are still learning….


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