Blood pressure represents the lateral force exerted by circulating blood against the walls of the arteries. This force is generated primarily by the pumping action of the heart and is modulated by the volume of blood, the elasticity of vessel walls, and the resistance offered by the peripheral vascular bed.
Flow through the circulatory system depends directly on the pressure gradient between two points and inversely on vascular resistance. When smooth muscle in the arterioles contracts, vessel diameter decreases, causing a sharp rise in resistance and a corresponding increase in systemic blood pressure. Conversely, vasodilation reduces resistance and lowers pressure. Midwives must understand these basic fluid dynamics because conditions like pre-eclampsia directly alter peripheral resistance, severely distorting blood pressure readings and requiring careful interpretation at the antenatal booking clinic.
Under normal physiological conditions, blood flows through unconstricted blood vessels in a smooth, streamlined pattern known as laminar flow. In laminar flow, the fluid particles move parallel to the vessel walls, with the fastest velocity in the center of the lumen and minimal friction against the endothelium. Because this flow is completely silent, a healthy artery makes no audible sound when auscultated with a stethoscope.
When a blood pressure cuff is inflated and partially compresses an artery, it disrupts this smooth pathway. The constriction forces the blood to move in chaotic, swirling eddies known as turbulent flow. This turbulence creates audible vibrations against the vessel wall, forming the physical basis for detecting blood pressure using a stethoscope and sphygmomanometer.
The auscultatory technique relies on listening to Korotkoff sounds as a pneumatic cuff is gradually deflated. As the cuff pressure drops below the peak systolic pressure, blood begins to spurt through the compressed vessel during the peak of the cardiac cycle, generating specific auditory phases:
Automated blood pressure machines do not use a stethoscope or listen for Korotkoff sounds. Instead, they operate on the oscillometric technique. An electronic monitor detects tiny pressure pulses, or oscillations, transmitted to the cuff from the arterial wall as the cuff slowly deflates from an inflated state.
When the artery is fully compressed, the pulsations are very small. As the cuff pressure drops and blood begins to force its way through during systole, the oscillations suddenly increase in amplitude, reaching a maximum peak at the mean arterial pressure. As the cuff continues to deflate past the diastolic pressure, the oscillations decrease sharply. A microchip inside the device uses a mathematical algorithm to calculate the systolic, mean, and diastolic pressures based on these changing wave amplitudes. While convenient in busy labor wards, midwives should remember that these devices can struggle to give accurate readings if the pregnant woman is experiencing severe shivering or cardiac arrhythmias.
Getting an accurate blood pressure reading depends heavily on using the correct equipment size. The inflatable bladder inside the cuff must be properly matched to the circumference of the patient's arm.
| Cuff Size Issue | Effect on Blood Pressure Reading | Clinical Consequence |
|---|---|---|
| Cuff too small | Artificially high readings | May lead to a false diagnosis of hypertension |
| Cuff too large | Artificially low readings | May miss dangerous hypertension like pre-eclampsia |
To ensure proper fit, the bladder width should cover approximately 40% of the upper arm circumference, and the length should wrap around at least 80% to 100% of the arm. Midwives must always inspect the equipment and select an extra-large cuff for women with higher body mass indexes, rather than forcing a standard cuff onto a conical arm.
Even with correct equipment, many technical and physiological mistakes can distort blood pressure readings during antenatal and labour ward checks.