L6 - Blood Pressure

Functions

• Provide blood $\to$ oxygen and energy
• Monitoring $\to$ pathology and physiology (cardiovascular system)

Systole

1. Two ventricles contracts
2. Pushes blood to vessels
3. Blood goes to organs and tissues
4. Blood pressure increases

Diastole

1. Heart muscle relaxes
2. Heart chambers fill with blood
3. Blood pressure decreases

Blood Pressure Reading

• $90/60\mathrm{mmHg}\to$ Hypotension
• $120/80\mathrm{mmHg}\to$ Normal
• $140/90\mathrm{mmHg}\to$ High blood pressure

We need at least 2 readings to determine

Pressure Gradient

$$R=\frac{8\eta L}{\pi r^4}$$

$R$	Resistance
$r$	Vessel radius
$\eta$	Blood viscosity
$L$	Length of the system

Blood Volume Flowrate

$$\begin{array}{rl}F& =\frac{\Delta P}{R}\\ & =\frac{\Delta P\pi r^4}{8\eta L}\end{array}$$

$F$	Blood Volume Flowrate ($L/min$ or $mL/min$)
$\Delta P$	Pressure gradient
$R$	Resistance

Definitions

Systolic Pressure (SBP)

Maximum pressure reached during systole

Diastolic Pressure (DBP)

Minimum pressure just before the beginning of systole

Mean Arterial Pressure (MAP)

Time average arterial pressure during a single cardiac cycle

Pulse Pressure (PP)

$PP=SBP-DBP$

Indirect measurements

1. An occlusive cuff is placed on arm
2. The cuff will be inflated to $P_{cuff}>SBP$
3. It will then be deflated

Longer cuff is preferred.
Usually measured at brachial artery, because it is close to the heart and convenient.

Palpatory (Riva-Rocci)

Cuff pressure must be greater than the highest blood pressure ($P_{cuff}>BP$) to completely constrict the artery.

At $P_{cuff}=BP$, blood can start flowing and pulse will be felt.

Pros

• Simple
• Works in noisy environment

Cons

• Can only measure SBP
• Not accurate for infants and hypotensive patients

Auscultatory (Korotkoff sound)

1. Stethoscope is held just below the cuff
2. As the cuff gradually deflate, blood flow is re-established and accompanied with the first Korotkoff sound $\to$ SBP
3. Keep deflating until the Korotkoff sound disappear $\to$ DBP

Turbulence is caused by sudden flow of blood, thus sound can be heard (SBP).
When the flow is smooth, the sound disappear (DBP).

Accuracy of SBP: +/- 4 mmHg
Accuracy of DBP: +/- 2 mmHg
The difference is caused by signal to noise ratio.

Pros

• Simple
• Not much equipments

Cons

• Environment cannot be noisy
• Results differ from observers
• Mechanical error
• Not accurate for infants and hypotensive patients

Oscillometric

1. Use pressure sensor (piezoelectric) instead of stethoscope, producing oscillometric reading.
2. Monitor the pulsatile changes in pressure
3. Deflate $\to$ blood flow increase $\to$ pressure pulses’ amplitudes increase
4. SBP and DBP are estimated from the amplitudes of the oscillation

MAP is the point with highest oscillation occurs

Pros

• Simple and reliable
• Reliable even for hypotension

Cons

• Large variance in blood pressures can be caused by different algorithms

Ultrasonic

Tonometry

Direct measurements

• Extravascular Sensor
• Intravascular Sensor