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The situation:

“太阳病发汗，汗出不解，其人仍发热，

心下悸，目眩，身瞤动，振振欲擗地也，

真武汤主之。”

There’s an exterior disease, and somehow sweating doesn’t help get rid of it. Whether just due to the loss of fluids, or if just part of the original disease process, the guy starts palpitating (likely to be tachycardia), gets dizzy, the body goes into spasms, and he looks like he wants to fall to the ground.

Well, the second part of it looks like symptoms of low blood pressure: palpitations, dizziness and almost lossing consciousness. This could be shock, probably due to dehydration from over-perspiration.

I could imagine the doctor trying his best to ‘resolve the exterior’ of his patient, believing that he could sweat out the bad guys. And trying too hard, the upshot of which being a dehydrated patient.

I believe that this prescription is a vasopressor and that 附子 is the main ingredient:

Firstly, I think 附子 is the main herb in the prescription, and that its properties are congruous with that of the prescription itself. 温阳利水 is the broad term used, however I would not interpret 利水 is a diuretic function as biomedicine understands it. 附子 is able to reverse the vicious cycle that hypovolemic shock is, where the blood vessel walls are further compromised by a messed up system, causing further fluid loss and further lowering blood pressure. 附子 is able to help maintain a high-enough arterial blood pressure - via a combination strengthening the heart, constricting the appropriate blood vessels, and dilating the ones around the capillary bed.

Comparison with other formulae:

• With 五苓散: The cause is very similar, although the guy who could use 五苓散 is certainly not as badly hypovolemic as the guy who needs 真武汤.
• With 附子汤: The herbs are the same except that in 附子汤, 人参 replaces 生姜 in 真武汤. Also there’s double the amount of 附子 and 白术. So we see that with more 附子、白术、人参, 附子汤 is certainly more potent.