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Medical Engineering > Cardiovascular Engineering > Flashcards

Cardiovascular Engineering Flashcards

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USMLE® Step 1 flashcards
1
Q

Describe the four main components of the cardiovascular system.

A

Blood (working fluid): 95% RBCs for gas transport. WBCs for immune response.

Heart (2 positive displacement pumps in series): Left side pumps to most of body for O2 and nutrient deliver. Right side pumps to lungs.

Arteries & Veins (2 networks of large diameter tubing): Systemic circulation supplies body, pulmonary supplies to lungs.

Capillaries: Small diameter tubing for diffusion.

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2
Q

What are the three main functions of the cardiovascular system?

A
  1. Deliver oxygen, nutrients and hormones.
  2. Remove waste.
  3. Regulate temperature.
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3
Q

Briefly describe the three layers found in blood vessels.

A

From inner to outer:

Intima - Endothelial cells + elastic membrane. Barrier allowing certain molecules to pass from blood into wall.

Media - Muscle cells, collagen, elastin surrounded by elastic membrane. Relative amounts of collagen (high stiffness) and elastin (low stiffness) determine stiffness.

Adventita: High elastin content, fibroblasts and own supply of blood as thickness too great for diffusion in large vessels.

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4
Q

Name the four main blood vessels entering/leaving the heart.

A
  1. Vena cava (from body into RA, de-O2)
  2. Pulmonary artery (from RV to lungs, de-O2)
  3. Pulmonary vein (from lungs to LA, O2)
  4. Aorta (from LV to body, O2)
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5
Q

What are the two types of valve found in the heart?

A

Atrioventricular (separate atria & ventricles)

Semi-lunar (separate ventricles from main arteries)

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6
Q

Describe the mechanics of a heart valve through the cycle of a semi-lunar valve.

A
  1. Ventricle contracts. Pressure builds causing valve to open due to positive pressure gradient.
  2. Ventricle continues contracting, forcing blood out. Ventricular pressure begins to drop.
  3. Once ventricular pressure lower than aortic, flow begins to reverse, closing valve.
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7
Q

What is isovolumetric pressure?

A

The pressure that would be generated by the ventricle if the aortic valve remained closed.

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8
Q

Define systolic and diastolic pressure.

A

Systolic - maximum pressure in the aorta (contraction).

Diastolic - Minimum pressure in the arteries (relaxation).

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9
Q

What is the effect of arterial disease on cardiovascular mechanics?

A

Atherosclerosis - fatty deposits build up in inner layer of artery.

Increases resistance. Systolic and diastolic pressure increase whilst flow decreases.

Increased ventricular pressure.

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10
Q

What is the effect of age on cardiovascular mechanics?

A

Degradation of elastic, arteries become stiffer reducing compliance.

Increase in systolic pressure, decrease in diastolic.

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11
Q

Outline the effects of exercise on each part the cardiovascular system.

A

Heart - Increased heart rate and cardiac output. Maximum ventricular pressure during systole increases. EDV and stroke volume increases.

Arteries - Constrict, directing away from organs. Blood pressure increases.

Veins - Contract, free up blood volume.

Capillaries - Capillaries in working muscles dilate. Increases blood flow and rate of diffusion.

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12
Q

What triggers the effects on the cardiovascular system observed during excercise?

A

Discharge of sympathetic nervous system. Stimulates entire circulation.

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13
Q

What are the main stages during systole (contraction)?

A
  • Atria contract, small pressure rise pushing blood into ventricles.
  • A-V valves close. Large rise as ventricles contract.
  • At sufficient pressure aortic valve opens. Ventricle still contracting but volume decreasing.
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14
Q

What are the main stages during diastole (relaxation)?

A
  • Ventricles relax, pressure decreases.
  • Reverse pressure gradient closes semi-lunar valves.
  • Relaxation becomes isovolumetric until pressure becomes lower than atria (cycle then repeats).
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15
Q

Define end-diastolic volume (EDV), end-systolic volume (ESV) and stroke volume.

A

EDV - Maximum volume of ventricle at end of diastole (relaxation) (110-120ml).

ESV - Minimum volume remaining in ventricle at end of systole (40-50ml).

Stroke Volume - Volume emptied from ventricle during cycle (EDV - ESV).

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16
Q

Define stroke work and minute work.

A

Stroke work - energy converted to work done pumping blood during each heart beat.

Minute work - stroke work * heart rate.

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17
Q

Define ejection fraction.

A

Fraction of EDV ejected in a heart beat.

Ejection fraction = stroke volume / EDV.

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18
Q

How can stroke work be found from a P-V loop.

A

Stroke work = Area

Approx. = (EDV - ESV) * (systolic pressure - diastolic)

19
Q

What two lines bound the heart P-V loop?

A

End systolic P-V relationship (ESPVR)

End diastolic P-V relationship (EDPVR)

20
Q

What is the effect of preload on the heart P-V loop?

A

Increases width by increasing EDV.

21
Q

What is the effect of afterload on the heart P-V loop?

A

Increased height. Reduced width due to increased ESV.

22
Q

What is the effect of increase inotropy of the heart P-V loop?

A

More forceful contractions. Increased ESPVR gradient.

23
Q

Define preload and afterload when describing ventricular pressure.

A

Preload - Filling pressure. Pressure at inlet.

Afterload - Outlet (aorta) pressure.

24
Q

What is the effect of exercise on the heart P-V loop?

A
  • Increased inotropy (increased ESPVR gradient)
  • Increased cardiac output - increased preload.
  • Increased arterial pressure - increased afterload.
25
Q

Outline the common sequence of events concerning heart failure.

A
  1. Atherosclerosis plaque blocks artery.
  2. Heart attack due to lack of blood flow.
  3. Cells die from lack of O2.
  4. Scar tissue forms.
  5. Scar tissue cannot contract. Heart remodels & enlarges to compensate.
  6. Heart becomes inefficient so insufficient blood delivered to body.
26
Q

What are the main treatments for heart failure?

A
  • Medication: Vasodilators + diuretics.
  • Pacemaker
  • Heart transplant
  • Ventricular Assist Device (VAD) or Total Artificial Heart (TAH)
27
Q

Describe the key elements of cardiac muscle and cardiac muscle cells.

A

Cardiac Muscle - Arranged helically twisting motion in contraction.

Cardiomyocytes - Cardiac muscle cells. Consist of sarcomeres and mitochondria. Corrugated membrane separates cells allowing rapid diffusion of ions.

Sarcomere - Contraction portion of cell. Interleaved thick (myosin) and thin (actin) protein filaments. Bind at sufficient ion concentrations, pulling together for contraction.

28
Q

What is the ‘action potential’?

A

The cyclic transfer of ions across the cell membrane, producing a potential difference between the inside and outside of the cell.

29
Q

Outline the key stage in the action potential.

A
  1. Inside of cell negatively charged - ‘polarized’.
  2. Na+ ions move through Na+ channels. Increase membrane potential (less -ve).
  3. K+ (out) and Ca2+ (in) channels open. Potential plateaus close to 0 - ‘depolarized’. (Contraction occurs).
  4. Ca2+ closes. K+ continues to leave. Decrease in potential.
  5. Ion concentrations restored by Na+/K+ pumps and Na+/Ca2+ exchanger.
30
Q

How transmembrane potential cause the action potential of neighbouring cells to be initiated?

A
  • As cells depolarize, potential difference along tissue, depolarization spreads.
  • Once all depolarized no p.d.
  • Cells on starting side repolarize, again causing a p.d. and spread of repolarization.
  • Eventually all tissue if repolarized and cycle can repeat.
31
Q

Where does contraction of the heart start?

A

Sinoatrial node.

Myocytes in region have higher natural frequency. Action potential triggers neighbouring cells.

32
Q

Outline the pathway for electrical signals in the heart.

A
  1. Sinoatrial node.
  2. Spreads throughout RA into atrioventricular node.
  3. Reaches bundle of His.
  4. Spreads into ventricles.
  5. In ventricles travels down ventricular septum and into outer walls.
33
Q

How is an ECG measured?

A

Three electrodes positioned on body surrounding the heart to from triangle.

Right arm (RA), left arm (LA) and left leg (LL).

Potential difference between electrodes (lead) gives ECG.

Left ventricle dominates. Predominant cardiac vector 60 degrees to horizontal.

34
Q

Describe the main phases of the ECG signal.

A

P: signal beings in sinoatrial node.
Q: ventricular septum depolarises, left -> right.
R ventricle walls are depolarised.
S: small regions of ventricles activated.
T ventricles repolarize.

35
Q

What are the two methods of measuring blood flow and velocity?

A
  • Doppler Ultrasound

- Phase Contrast MRI

36
Q

State the doppler equation used to measure blood velocity from doppler ultrasound.

A

v = (c * fd) / (2fecos(alpha))

fd = doppler shift, fe = ultrasound frequency

37
Q

Outline the stages for measuring blood flow using phase contrast (PC) MRI.

A

MRI
1. Hydrogen atoms have randomly oriented spins.

  1. Align to applied magnetic field. Either in same direction (lower energy) or opposite (higher energy).
  2. RF pulse excites spins which are aligned with field into higher energy state - flip.
  3. Eventually flip back, releasing RF waves which can be detected.

PCMRI
-Apply magnetic field gradient to alter phase of spins directly after RF pulse.

-Opposite gradient then applied. Phase of spins which have moved between two gradients depends on velocity.

38
Q

What are two major causes of increases vascular resistance?

A
  1. Atherosclerosis - fatty deposits, reduced section.
  2. Kidney disease - deposits in renal arteries, kidney receives reduced flow. Kidney stimulated to increase blood volume by retaining water.
39
Q

What are two causes of increased blood volume?

A
  1. Kidney disease - deposits in renal arteries, kidney receives reduced flow. Kidney stimulated to increase blood volume by retaining water.
  2. Salt. Excess salt causes kidneys to retain water.
40
Q

How is a pressure cuff used to measure blood pressure?

A
  • Exerts pressure to stop flow in artery.
  • Pressure slowly released. Readings heard when characteristic sounds heard.
  • Whooshing once fallen below systolic. Sound stops once below diastolic.
41
Q

How are pressure catheters used to measure blood pressure?

A
  • Thin, saline filled tube inserted.
  • Saline conducts pressure along tube to diaphragm.
  • Strain gauge measures motion of membrane to record pressure.
42
Q

Briefly describe the rheology of blood.

A
  • Blood cells suspended in plasma. Gives complex rheology.
  • At low shear rate, RBCs aggregate (rouleaux).
  • At high shear rate rouleaux break up, reduced viscosity.
43
Q

State the equation describing the velocity profile and flow rate of blood in a long, straight pipe.

A 
v = (dP)*(R^2 - r^2)/4Lu
Q = (dP*pi*R^4)/8Lu

u = viscosity

Medical Engineering (5 decks)

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