Cardiac Physiology

Question 1

aerobic requirements of the heart

Answer 1

1. cardiac tissue is metabolically very active
2. cardiac energy needs can increase 9x from rest to heavy exercise
3. O2 extraction from the blood remains fairly constant regardless of work load
4. Blood flow increases from 80 ml to 400 ml/min/100g tissue

Question 2

describe cardiac myocyte structure

Answer 2

1. y-shaped cells
2. striated
3. contain a single nuclei
4. limited ability to replicate
5. linked together by intercalated disks
6. lack of distinct fiber types
7. do not fatigue
8. all fibers contract with each beat
9. provide graded muscle contractions

Question 3

describe cardiac myocyte force production

Answer 3

1. all cardiac muscle cells contract regardless of HR and contractility
2. cardiac muscle cells regulate their force production by regulating availability of calcium to their sarcomeric proteins

Question 4

Describe the steps in a cardiac action potential

Answer 4

1. resting membrane at -90mV, all channels but K+ are closed
2. rapid Na influx through open fast Na channels results in rapid increase to >0
3. transient K+ channels open and K+ efflux returns TMP to 0mV
4. Influx of Ca through L-type Ca channels is electrically balanced by K+ efflux, through delayed rectifier K+ channels
5. Ca channels close by delayed rectifier K+ channels remain open and return TMP to -90mV

Question 5

compare the time of a skeletal muscle AP to a cardiac muscle AP

Answer 5

1. skeletal → 4-8 ms
2. cardiac → 100-300 ms

Question 6

what are refractory periods?

Answer 6

periods of time in which cardiac muscles allow complete emptying of the ventricles prior to next contraction

Question 7

list the refractory periods

Answer 7

1. ARP (absolute refractory period)
2. ERP (effective refractory period)
3. RRP (relative refractory period)

Question 8

how is SV calculated?

Answer 8

LVEDV - LVESV

Question 9

how is EF calculated?

Answer 9

(LVEDV - LVESV)/LVEDV * 100

Question 10

define the relative norm values for various cardiac values like EDV, ESV, SV, and EF

Answer 10

1. EDV → 110-120 mls
2. ESV → 40-50 mls
3. SV → ~70 mls
4. EF → 60% is WNL

Question 11

T/F: changing EDV or ESV does not impact SV

Answer 11

FALSE

changing either will result in a change to SV

Question 12

T/F: Atrial failure is often unnoticed

Answer 12

TRUE

but with A-Fib on the other hand, the atria contract to frequently which reduces the amount of blood flowing to the ventricles

Question 13

describe the process of filling the ventricles

Answer 13

1. substantial filling of the ventricles occurs during the first 1/3 of ventricular diastole
2. middle 1/3 of ventricle diastole there is a modest amount of blood entering the ventricles
3. during the last 1/3 the “tank is topped off” via the atrial contraction

Question 14

T/F: there is a substantial change in volume during a isovolumetric ventricular contraction?

Answer 14

FALSE

short (0.02 - 0.03 seconds)

all valves are shut and pressure is building but no volume loss/change

Question 15

when does ventricular contraction shift from isovolumetric to an ejection contraction?

Answer 15

once the LV SP >80 mmHg (in a normatensive pt)

rapid ejection → 70% occurs in first 1/3 of systole

Question 16

what is isovolumetric diastolic or relaxation of the LV?

Answer 16

this is when ventricular pressure < aortic pressure and thus the aortic valve closes and remains closed resulting in no net blood flow

• early part of ventricular relaxation
• LV pressure is decreasing
• both aortic and AV valves are closed
• AV valves open when atrial pressure is greater than that of the ventricles

Question 17

what does an elevated RV pressure suggest?

Answer 17

1. pulmonary HTN
2. RV failure
3. CHF
4. increased blood volume

Question 18

list the pulmonary pressures

Answer 18

1. systolic → 20-30 mmHg
2. diastolic → 8-12 mmHg
3. mean pulmonary pressure → 25 mmHg

Question 19

what allows the pulmonary ciruculation to function as a low pressure system?

Answer 19

1. RV pumps through a shorter length of pipe
2. pulmonary vasculature is more compliant
   
   • lower resistance to bloow flow
   • vessels walls are thinner and contain less muscle
3. expansion of lungs result in vascular dilation
4. much lower impact of gravity
5. benefit-reduced risk of pulmonary edema

Question 20

List clinical markers of cardiac function and how to determine them

Answer 20

1. Cardiac output
   
   • CO = HR * SV
   • ~4-8 L/min
2. EF (LVEF)
   
   • norm ~55-70%
3. Cardiac Index
   
   • ratio of CO to body surface area
   • CO/BSA
   • units = L/min/m2
   • norm → 2.5-4

Question 21

what EF is indicative of heart failure?

Answer 21

<40%

Question 22

List determinants of cardiac performance as defined by CO

Answer 22

1. HR
2. Contractility
3. Preload
4. Afterload

Question 23

what is contractility and how does it impact cardiac performance?

Answer 23

strength of contraction/force production

• if increased → increase in CO
• if decreased → decrease in CO

Question 24

how is preload a determinant of cardiac performance?

Answer 24

it reflects the amount of venous return which impacts how much myocardial distension occurs prior to contraction

Frank-Starling mechanism

Question 25

how is afterload a determinant of cardiac performance?

Answer 25

this is the force that the ventricles must overcome in order to eject blood and it is largely dependent on arterial BP and vascular tone if increased its makes the heart work harder (like riding uphill you slow down)

Question 26

list determinants of energy cost of cardiac performance

Answer 26

1. HR 2. contractility 3. wall tension * systolic force or work per surface unit 4. Double Product * SBP \* HR

Question 27

how does sepsis impact cardiac performance?

Answer 27

1. cytokines released during sepsis reduces cardiac contracility * impaired Ca handling 2. sepsis is usually associated with normal or elevated CO

Question 28

how do positive iontrophic drugs impact cardiac contractility?

Answer 28

they act to increase it typically used in heart failure pts to preserve CO examples: 1. Digoxin 2. Dopamine 3. Dobutamine 4. adrenaline 5. noradrenaline

Question 29

how is the SNS assessed in its ability to regulate HR?

Answer 29

1. resting HR (HR at rest) 2. HR recovery 3. HR response to standing 4. HR response to valsalva 5. BP response to standing

Question 30

list factors that control venous return

Answer 30

1. total blood volume 2. venous blood pressure (driving force for blood return) 3. muscle pump 4. respiration 5. gravity/posture 6. intrapericardial pressure 7. right atrial pressure/increased thoracic pressure

Question 31

T/F: the frank-starling mechanism is ideally suited for matching CO to venous return

Answer 31

TRUE a sudden excessive output by one ventricle very soon increases venous return to the other ventricle the stimulus to increase output in the 2nd ventricle is an increase in diastolic length with an accompanying increase in force of contraction

Question 32

what is Pulse Pressure?

Answer 32

SBP - DBP * normal = 40 * considered abnormally low if it is \<25% of SP (~30) * considered abnormally high if \>100 * PP = SV/arterial compliance

Question 33

what can decrease PP?

Answer 33

1. directly proportional to volume changes (SV) 2. inversely proportional to arterial compliance 3. in trauma, a low or narrow PP suggests sig blood loss 4. aortic valve stenosis 5. cardiac tamponade

Question 34

is it better to focus on PP or SBP?

Answer 34

while PP may be a better predictor of heart health than SP or DP independently it should not be taken out of context and doesn't provide unique info in most cases current recommendations are to focus on SBP in determining need for therapy

Question 35

what is mean arterial pressure (MAP)?

Answer 35

describes the average arterial pressure across a cardiac cycle * considered to be the perfusion pressure seen by organs in the body * MAP = CO \* SVR * SVR = systemic vascular resistance * MAP = DP + 1/3(SP - DP)

Question 36

MAP norms

Answer 36

* norm → 65-110 * MAP \> 60 → is enough to adequately perfuse organs of the average person * MAP \<60 → signals decreased organ perfusion * long term depressed MAP results in ischemia and injury

Question 37

decreased MAP can lead to \_\_\_\_, \_\_\_\_\_\_\_, and \_\_\_\_\_\_

Answer 37

poor perfusion of brain, LOC, and death

Question 38

other than the radius of the vessel what else impacts systemic vascular resisance?

Answer 38

1. Local control * NO release → results in vasodilation and diminished MAP * endothelin released in presence of decreased MAP leads to smooth muscle contraction 2. ANS * altered sympathetic and parasympathetic input to vascular smooth muscle 3. renal regulation of blood volume

Question 39

what is the Law of LaPlace?

Answer 39

T(LV wall stress) = Pressure \* radius T= PR the tension (T) in the wall of a hallow cylinder is directly proportional to the cylinder's radius (r) and the pressure (p) across the wall caused by the flow inside

Question 40

for a given blood pressure ________ the radius of the cylinder leads to a _____________ in tension

Answer 40

increasing linear increase

Question 41

what is atrial natriuretic peptide?

Answer 41

* 28-amino acid peptide * synthesized, stored and relased by atrial myoctyes in response to: * atrial distension (hypervolemia) * angiotension II stimulation * sympathetic stimulation

Question 42

what are the action of the ANP?

Answer 42

1. involved in long term Na and water balance, blood volume and arterial pressure * increases diuresis and sodium loss → reduces blod volume * decreases renin release which further leads to diuresis and Na loss 2. dilates veins reducing central venous pressure, preload and CO 3.