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Having been diagnosed with high blood pressure (BP) at the age of 70, I was often puzzled by the considerable variations between systolic values produced by different types of monitors (all claimed to be calibrated). The same monitors consistently produced only marginal differences between diastolic values.
This led me to question the meaning of the systolic readings and how well are they understood.
Since pressure is force per unit area, and force is defined as the change of momentum (a product of mass and velocity) with time, the shorter the pumping impulse produced by the heart, the higher the peak pressure. In fact, the theoretical pressure at the very start of the pumping action, when the blood just starts to flow, is infinite!
What is measured at the upper arm or wrist is also greatly influenced by the state of the arteries. The stiffer these are the higher the pressure will be. This is due to the fact that stiff artery walls will absorb much less energy than supple ones.
And then there is the third major factor – the sensitivity / response time of the measuring instrument.. This can probably explain why, using a number of different makes / types of BP monitors, within a short period and under the same conditions, significantly different systolic BP readings can be produced.
However, what matters most is the doubt over the interpretation of systolic BP readings. How many of us are given powerful drugs aimed at lowering pressures produced by nothing more than well functioning hearts.
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