Arterial pressure

abdullah

1. ARTERIAL PRESSURE The mean arterial pressure (MAP) is defined as the average arterial pressure during a single cardiac cycle . As blood is pumped out of left ventricle in to the arteries pressure is generated . The MAP is determined by Cardiac Output (CO).Systemic Vesicular Resistance (SVR) and Central Venous Pressure (CVP). MAP= (CO*SVR ) + CVP (As CVP is near 0 mm Hg relationship can be simplified as below) MAP = (CO*SVR)

2. In practise the mean arterial pressure (MAP) is derived from a patients Systolic Blood Pressure (DBP ) Normal(MAP)is between 65 mm and 110 mm of Hg MAP=DP+1/3 Pulse Pressure ( PP=SP-DP) CLINICAL SIGNIFICANCE MAP is better indicator of tissue perfusion than SBP Hence MAP is calculated for the management of patients with acute conditions when there is a concern for appropriate organ perfusion.

3. Pulse wave velocity (PWV) is a measure of arterial stiffness. The properties of the arterial wall, thickness and the arterial lumen diameter are the major factors influencing (PWV). Velocity of pulse wave is independent and much higher than the velocity of blood flow max velocity of blood flow is only 50 cm/s. The rate of travel of the pulse wave is about 4 meters sec in aorta and its branches. 8 meters /sec in the large arteries . 16 meters /sec in the small arteries . Aortic pulse wave velocity is a measure of aortic stiffness. It is measured with ultrasound Aortic pulse wave velocity is the predictor of cardiovascular mortality in the hypertensive subjects Velocity of circulation of pulse waves

4. Cardiac cycle causes flow & pressure waves Relationship of pressure to flow is not simple resistance but mechanical impedance which is time dependent resistance is time dependent Mechanical impedance consists of Viscous impedance that opposes flow. Compliant impedance that opposes volume. Inertial impedance that opposes change of flow. Because of these resisive compliant and inertial properties the pressure and flow waves are different .

5. Cardiac cycle causes flow & pressure waves With increasing distance from the heart in the large arteries the pressure curves become steeper and the peak narrower and an important secondary pressure oscillation appears during diastole . As the pressure wave travels down the large arteries it gets distorted but not damped . MAP falls slightly with increasing distance from the heart.

6. Distortion of pressure waves The pressure wave in the aorta is more complex than a simple sine wave. The complex waveform can be thought of the algebric sum of many individual sine cosine waves each with its own amplitude frequency and phase. The precise mathematical method by which complex waveforms are broken down into simpler components is called Fourier analysis .

7. Cardiac cycle causes flow & pressure waves In terminal arteries and arterioles damping of pressure waves predominates over distortion because Increase in number of parallel vessels increases aggregate compliance damping the pressure waves. Having a small radius increases resistance In capillaries damping is so severe that pulsations do not occur and blood flow is continuous

8. Pulse wave harmonics One of the most useful tools of arterial dynamics is the Fourier analysis of the complex pulse waves in to their simple sinusoidal harmonic components. These harmonics are the simple functions described by relatively simple mathematics so that calculations on them are feasible . Principle of superposition is applied in the arteries . Summation of these harmonics leads in to a new complex waves. Digital computer is used to carry out the Fourier analysis .

9. Evaluation of pulse wave morphology by Fourier’s harmonic analysis Evaluation of pulse waves on five segments of lower limb in a recumbent healthy subject by means of Fourier's analysis show following phenomenon :: constant and marked accentuation of the amplitude of the second harmonic. :: distinct diminution of the amplitudes of further higher harmonics. :: relative augmentation of amplitude in a distal direction as a manifestation of mounting impedance towards periphery.

10. Distortion of pressure waves Vessels become more stiffer towards the periphery increasing wave velocity especially for higher frequency components. Pressure waves bounce off the end of the arterial tree and reflect back up the vessels. When we summate these various effect on the sine wave and cosine wave components downstream the reconstructed pressure wave has a different (distorted) shape than the original wave.

11. Circulation time The time required for the blood to flow between two given points . It measures the average linear velocity of blood. The time taken for the blood to pass through a given circuit of the vascular system, for Ex; the pulmonary or systemic circulation, from one arm to another, from one arm to tongue, or from arm to lung; it is measured by the injection into an arm vein of a substance, such as sodium dehydrochlorate,ether,fluorescein , histamine, or a radium salt which can be detected when it arrives at another point in the vascular system.

12. Factors influencing circulation time Increased cardiac output. Exercise. Excitement. Adrenaline. Rise of BMR-all these factors increase the velocity of blood and decrease C.T. C.T is important in diagnosis of right and left sided failure, in differentiating bronchial and cardiac asthma , myxoedema etc.