Final answer:
Blood pressure measurement should be consistent, typically with the arm at heart level. Variances in readings can occur if the arm is held above the head or if measurements are taken on the leg while standing, due to the influence of hydrostatic pressure changes. Mercury manometers are preferred for their compact size and readability.
Step-by-step explanation:
The position for measuring arterial blood pressure should remain consistent to maintain accuracy. Generally, for the first 12 hours, blood pressure measurement should be taken with the arm at heart level to avoid hydrostatic pressure changes impacting the reading. For subsequent days, if the position changes, such as raising the arm above the shoulder or placing the cuff on the upper leg while standing, the measured pressure can vary due to the weight of the fluid column that is added or subtracted from the arterial system pressure.
When the measurement is taken with the arm raised above the shoulder, the heart has to pump against gravity, causing a temporary decrease in the blood pressure measured. Conversely, if the cuff is on the upper leg while standing, the blood pressure could be measured as higher, considering the additional pressure exerted by the weight of the blood in the leg's vasculature. Mercury manometers are used to measure blood pressure because mercury's density allows a relatively small column to measure the pressures in mmHg, making the device more compact and easier to read than water manometers.