hemodynamics

HEMODYNAMICS

STROKE VOLUME = (END DIASTOLIC VOLUME) - (END SYSTOLIC VOLUME)

• Cardiac Index is Cardiac Output normalized for body mass.

CARDIAC OUTPUT = (STROKE VOLUME) x (HEART RATE)

PULSE PRESSURE = (SYSTOLIC PRESSURE) - (DIASTOLIC PRESSURE)

MEAN ARTERIAL PRESSURE = (CO) x (TPR) = (HR) x (SV) x (TPR)

• PERIPHERAL RESISTANCE UNITS (PRU): Units of mm Hg / mL / sec.
  • Or, it is TPR as above, where CO is expressed in mL/sec.

RESISTANCE alpha VISCOSITY

• For the lungs, this resistance is called Pulmonary Vascular Resistance, and the flow is equal to Cardiac Output.

General Trends in Circulation:

• Pressure drop is greatest at the level of the arterioles.
• Velocity of blood is slowest at the capillaries, because they have the largest total cross-sectional area, given the number of capillaries.
• Turbulence: The higher the velocity of blood flow, the greater the likelihood of turbulence.
  • Turbulence is most likely in large arteries. Never in capillaries and rarely in venous system.
• Arterial Elasticity (The Windkessel Effect): Arterial Elasticity accounts for a smaller pulse pressure.
  • It relieves a little pressure during systole, since it can give a little.
  • It maintains flow during diastole, since it can flex back.
  • Thus, atherosclerosis ------> Larger Pulse Pressure.
• THE BASIS OF STEADY BLOOD FLOW: Systole -vs- Diastole

• Systole: More blood is pumped into the arterial tree then flows out of the arterial tree, so arterial pressure rises.
  • Hence volume in arterial tree goes up ------> pressure in arterial tree goes up to systolic pressure.
  • During systole, about half of the blood is stored in the arterial tree, and the other half is pushed into the capillary beds.
• Diastole: Blood continues to leave the arterial system and no new blood enters it, so blood pressure goes back down.
  • During Diastole, more arterial blood flows into the capillary beds, providing capillaries with continuous blood flow whether in systole or diastole.

MEASURING BLOOD PRESSURE / SPHYGMOMANOMETER:

• SYSTOLIC PRESSURE: The first sound you hear -- a rush of blood flowing through the squeezed artery.
  • This happens the instant that the cuff pressure is reduced enough to let arterial blood squirt through during systole.
• DIASTOLIC PRESSURE: The last sound you hear -- blood is no longer stopped by the cuff-pressure during diastole.
• Phases:
  • Phase I (snapping):
  • Phase II (murmur): In hypertensive people, an auscultatory gap can occur during Phase II.
  • Phase III( thumping):
  • Phase IV (muffling): The beginning of this muffling is sometimes taken as the high end of diastole.
    • Some people think the muffling sound is a better indicator of diastolic pressure for children.
• Estimations:
  • SYSTOLIC PRESSURE is underestimated by auscultation -- you can't hear the sound "quick enough" to record the measurement.
  • DIASTOLIC PRESSURE is overestimated by auscultation.
  • Thus PULSE PRESSURE can be underestimated by auscultation by a significant amount.

FLOW, VISCOSITY, TURBULENCE, RESISTANCE:

• TURBULENCE: Turbulence is directly related to velocity of fluid. The higher the velocity, the more likely there is to be turbulence.
  • Reynold's Equation tells us the critical velocity at which turbulence will occur. We can derive three relationships from that equation:
    • Turbulence alpha Flow: The higher the flow, the higher the likelihood of turbulence.
    • Turbulence alpha (1 / viscosity): The lower the viscosity, the higher the likelihood of turbulence.
    • Turbulence alpha (1 / diameter): The narrower the radius of the vessel, the higher the likelihood of turbulence.
  • Turbulence is indicative of a larger pressure drop (larger DeltaP) across a region of vessel. Thus turbulence occurs when there is an atherosclerotic plaque.
• VISCOSITY: Relation between viscosity and turbulence:
  • Viscosity of blood is most closely related to hematocrit.
    • 20% of blood viscosity if from plasma; 80% is from blood cells.
  • ANEMIA: Lower hematocrit ------> Lower viscosity of blood ------> Higher blood flow ------> Higher likelihood of turbulence.
• FLOW: Relation between flow and radius = flow is inversely proportional to r⁴.
• RESISTANCE: The resistance to any organ is greater than the sum of all resistances!
  • That's true because the vessels are wired in parallel, and the sum of resistances in parallel is less than its individual parts.
  • Systemic Resistance (TPR) is much greater than Pulmonary Resistance.
  • Pulmonary Resistance = Delta Pulmonary Pressures / CO.

BRUIT: Turbulent flow is detected as a bruit which can be heard by the stethoscope.

• Innocent Ejection Murmur: Children can have high velocity of blood flow without there being any pathology. Bruits are not uncommon.
• Bruits with Anemia: Anemic patients can also have innocent bruits, for two reasons:
  • Lower hematocrit ------> lower blood viscosity ------> higher likelihood of turbulence.
  • Anemics tend to compensate their low hematocrit with a higher cardiac output.
• Atherosclerotic Plaque: Turbulence can be heard downstream from the plaque.
  • Upstream from Plaque: Greater resistance ------> a strong pulse pressure.
  • Downstream from Plaque: A bruit can be heard.

STANDING BLOOD PRESSURE: Mean Arterial Pressure goes down when standing, because of lower venous return.

• Stand up ------> Venous Pressure in feet goes up ------> capillary hydrostatic pressure goes up ------> fluid flows out of arterial tree and into tissues ------> venous pooling in the feet ------> venous return decreases ------> CO decreases ------> lower MABP.
  • Venous pressure goes up in feet because of gravity -- DeltaP = gh
• Skeletal Muscle Pump: Tonic contraction of leg muscles while standing aids venous return, because the veins have valves, so blood is squeezed in only one direction.
  • Thus prolonged standing can lead to incompetent valves in the veins in the legs.

BLOOD PRESSURE AND THE RESPIRATORY CYCLE:

• INSPIRATION: Systemic blood pressure goes down and pulmonary blood pressure goes up.
  • The Diaphragm moving down has two effects:
    • It increases the volume of thoracic airspace and so it decreases intrathoracic pressure.
    • Also the abdominal space becomes smaller, so it increases intra-abdominal pressure.
  • The combination of above two effects results in an increased pressure gradient for venous return from the IVC ------> increased venous return ------> More blood to right atrium and more blood to pulmonary circulation ------> less respective blood in left heart and less CO.
  • Thus overall result is the following:
    • Lower systemic pressure.
    • Higher pulmonary pressure.
    • Larger Blood Volume in pulmonary circulation.
  • The change in MABP from inspiration normally does not exceed 10 mm Hg.
• EXPIRATION: Has the exact opposite effect.
  • Pulmonary pressure decreases.
  • Systemic pressure increases.

CENTRAL VENOUS PRESSURE: The pressure going into the right atrium.

• Anything that decreases venous compliance (i.e. sympathetic tone) will increase venous return ------> Higher CVP.
• ESTIMATING CENTRAL VENOUS PRESSURE: You estimate in cm of water.
  • It is approximately equal to the distance from the end of the distended part (which you can see) to the sternal angle, plus 5, then convert it into mm Hg.

PRESSURES IN PERIPHERY -vs- AORTA:

• Mean Arterial Pressure is slightly higher in the Aorta than in, for example, the radial artery.
• But, Pulse Pressure is greater in the periphery, i.e. the systolic is higher and the diastolic is lower.
  • This effect in the periphery is due to constructive interference of reflected waves.

COMPLIANCE: The degree to which a pressure change leads to a corresponding change in volume. Or, Compliance = DeltaV / DeltaP, or the slope of a pressure-volume curve.

• VENOUS COMPLIANCE is about twenty times more than arterial compliance, therefore veins can hold a larger volume of fluid at lower pressure.
  • Arterial Compliance is about 1 mL / mm Hg
  • Venous Compliance is about 20 mL / mm Hg
• EFFECTS OF COMPLIANCE on Blood Pressure:
  • Higher Venous Compliance ------> higher capacitance in veins ------> less venous return ------> lower CVP.
  • Lower Venous Compliance (sympathetic influence) ------> lower capacitance veins ------> more venous return via the one-way valves ------> higher CVP.
  • Lower Arterial Compliance results in a higher pulse pressure.
    • AGE: Arteries in old people have lower compliance. Thus old people have higher pulse pressures.
• Pressure-Volume Curve: The analysis of old -vs- young can be done on the P/V curve.
  • The slope of the curve is compliance.

• Pressure is on the X-Axis. Volume is on the Y-Axis.
• Is you plot systolic and diastolic pressure, and look at the corresponding Y-Values, you can calculate the following:
  • The difference on the Y-axis (i.e. the volumes corresponding to systolic and diastolic pressures) is stroke volume.
  • The difference on the X-axis is pulse pressure.

MODULATION OF MEAN ARTERIAL PRESSURE: Under a lot of circumstances, it doesn't change, even when stroke volume and/or pulse pressures do change.

• EFFECT OF STROKE VOLUME: All other factors held constant, a high stroke volume results in a higher pulse pressure, i.e. higher systolic and lower diastolic, but MABP remains constant.
  • PULSE PRESSURE IS USUALLY DIRECTLY RELATED TO STROKE VOLUME
• EFFECT OF EXERCISE:
  • Increased CO and Stroke Volume
  • Compensatory lower vascular resistance (TPR)
  • Once again MABP doesn't change (within limits).
• HIGH SYSTOLIC PRESSURE: Tends to occur with higher stroke volume. The more fluid you pump in one beat, the higher the systolic pressure.
• HIGHER DIASTOLIC PRESSURE: CORRELATES WITH HIGH TPR.