Biological variability
Circadian rhythm
Blood pressure is, on average, at its lowest when sleep is deepest (about 3 a.m.) and gradually rises to a peak at about 10 a.m. before subsequently decreasing again, and at the end of the afternoon or in the early evening a second peak is reached, after which a gradual decrease occurs during the rest of the evening and night.
Breathing rhythm
Normally systolic blood pressure during inspiration is about 5 mmHg higher than during expiration.
Environmental factors
Blood pressure can rise as a consequence of loud noises, strong light, cold or pain sensations.
Blood pressure can fall as a consequence of warmth and relaxation (vasodilatation).
Posture
Lying
Blood pressure in the lying position is virtually the same as that in the sitting position. However, it does take 10 minutes to reach a new haemodynamic equilibrium if a lying position is adopted after a sitting position.
Standing
Several seconds after standing up, a decrease in systolic blood pressure of 5-20 mmHg occurs as a consequence of a brief reduction in the venous blood supply to the heart (and therefore the heart minute volume as well). Within 30 seconds baroreceptors and the sympathetic nervous system compensate for this decrease in blood pressure. Blood pressure increases as a result of this, diastolically more than systolically, to values above the original sitting value. Therefore during a measurement in the standing position, higher values are normally found in diastole.
In some patients, however, a decrease in blood pressure occurs in a standing position. This is termed orthostatic hypertension if a systolic blood pressure decrease of more than 30 mmHg occurs compared to the sitting or lying position. This phenomenon can occur if there is a dysfunction of the sympathetic nervous system. Examples are: diabetic autonomous neuropathy, pheochromocytoma (catecholamine-producing adrenal medulla tumour), cerebral pathology (reflex interruption), medicinal treatment with antihypertensives (especially beta-blockers and vasodilators, especially in the elderly).
Recent activities of the patient
Psychological stress and a full bladder. These factors can all lead to increased blood pressure.
White-coat response
Increase in the blood pressure due solely to the presence of a doctor. This rise occurs mainly during the first visit and if the patient does not feel at their ease, but it cannot be accurately predicted. The effect can, however, be 20 mmHg systolically and 10 mmHg diastolically.
Factors related to the examiner
- incorrect technical execution
- inadequate hearing, visual acuity or manual dexterity
- results rounded off to the nearest 5 mmHg
- prejudice of the examiner with respect to result
- influenced by patient’s emotions.
Defective/inadequate measuring equipment
- incorrect cuff
- leakage (porous tube)
- blockage (cut off)
- calibration problem.