Sunday, August 26, 2007
Measuring The Blood Pressure
'Normal' or 'acceptable' blood pressure varies with age, state of health and clinical situation. At birth, a typical blood pressure is 80/50 mmHg. It rises steadily throughout childhood, so that in a young adult it might be 120/80 mmHg. As we get older, blood pressure continues to rise and a rule of thumb is that normal systolic pressure is age in years + 100. Blood pressure is lower in late pregnancy and during sleep.
From this, you can see that a systolic pressure of 160mmHg for an elderly man or 90 mmHg for a pregnant woman may be quite normal.
From this, you can see that a systolic pressure of 160mmHg for an elderly man or 90 mmHg for a pregnant woman may be quite normal.
Automatic non-invasive blood pressure measurement (Electronic Sphygmomanometer)
Automatic devices which essentially apply the same principle as the oscillotonometer have been produced (e.g. the 'Dinamap' made by Critikon). They require a supply of electricity. A single cuff is applied to the patients arm, and the machine inflates it to a level assumed to be greater than systolic pressure. The cuff is deflated gradually. A sensor then measures the tiny oscillations in the pressure of the cuff caused by the pulse. Systolic is taken to be when the pulsations start, mean pressure is when they are maximal, and diastolic is when they disappear. They can produce fairly accurate readings and free the hands of the anaesthetist for other tasks. There are important sources of inaccuracy, however. Such devices tend to over-read at low blood pressure, and under-read very high blood pressure. The cuff should be an appropriate size. The patient should be still during measurement. The technique relies heavily on a constant pulse volume, so in a patient with an irregular heart beat (especially atrial fibrillation) readings can be inaccurate. Sometimes an automatic blood pressure measuring device inflates and deflates repeatedly "hunting" without displaying the blood pressure successfully. If the pulse is palpated as the cuff is being inflated and deflated the blood pressure may be estimated by palpation and reading the cuff pressure on the display.
Automatic devices which essentially apply the same principle as the oscillotonometer have been produced (e.g. the 'Dinamap' made by Critikon). They require a supply of electricity. A single cuff is applied to the patients arm, and the machine inflates it to a level assumed to be greater than systolic pressure. The cuff is deflated gradually. A sensor then measures the tiny oscillations in the pressure of the cuff caused by the pulse. Systolic is taken to be when the pulsations start, mean pressure is when they are maximal, and diastolic is when they disappear. They can produce fairly accurate readings and free the hands of the anaesthetist for other tasks. There are important sources of inaccuracy, however. Such devices tend to over-read at low blood pressure, and under-read very high blood pressure. The cuff should be an appropriate size. The patient should be still during measurement. The technique relies heavily on a constant pulse volume, so in a patient with an irregular heart beat (especially atrial fibrillation) readings can be inaccurate. Sometimes an automatic blood pressure measuring device inflates and deflates repeatedly "hunting" without displaying the blood pressure successfully. If the pulse is palpated as the cuff is being inflated and deflated the blood pressure may be estimated by palpation and reading the cuff pressure on the display.
# posted by Dr. Ulman @ 9:12 AM 
