Regulation of Heart Rate and Myocardial Performance

Question 1

list the 3 major principles guiding circulatory function

Answer 1

1. maintain blood flow that meets each tissue’s needs
2. maintain a cardiac output that matches input (pump our all blood heart receives)
3. maintain a constant, normal, MAP

Question 2

cardiac output is the product of what?

Answer 2

cardaic output is the sum of flow to all tissues of the body, so
CO (mL/min) = heart rate (beats per minute) X stroke volume (mL/beat)

Question 3

what is stroke volume determined by?

Answer 3

end diastolic volume minus end systolic volume

Question 4

list the 3 major determinants of cardiac output

Answer 4

stroke volume X HR = CO, and the 3 determinants of SV are
1. preload
2. afterload
3. contractility
so these are also the determinants for cardiac output

Question 5

define cardiac preload

Answer 5

1. preload is the stretch/tension o nrelaxed myocytes pre-contraction/systole
2. preload is determined by end-diastolic volume and end-diastolic pressure

(the pressure that primes the pump)

Question 6

define cardiac afterload

Answer 6

1. the tension of myocytes DURING systole
2. the resistance that must be overcome to push blood out of the ventricle; felt by muscle AFTER it starts to contract
3. determined by resistance of ventricular outflow tract and aorta, and also by systemic vascular resistance
4. is approximated by mean arterial pressure (MAP)

Question 7

define cardiac contractility

Answer 7

1. also called inotropic state
2. the intrinsic ability of the heart to generate the force required to eject blood, independent of preload and afterload
3. increased contractility means increased velocity and peak force of contraction, so heart generates force faster and to a greater final degree

Question 8

how does the heart recruit its inotropic reserve?

Answer 8

1. intrinsic properties: length-dependent activation and response to changes in preload and afterload
2. extrinsic properties: effect of the ANS and other hormones/substances

Question 9

describe the Frank-Starling mechanism of the heart as an intrinsic way to recruit inotropic reserve

Answer 9

the greater the preload the greater the force of the next contraction; and increased contractility increases stroke volume which increases cardiac output

bottom line: when the amount of blood returning to the heart increases, this is a built-in mechanism to allow it to adapt by matching output volume to input volume

Question 10

describe how Frank Starling also helps the heart adapt to increased afterload

Answer 10

the first heart beat following an acute increase in afterload will have a greater than normal ESV, which will be added to the amount of blood entering the ventricle on the next beat, resulting in an increased EDV and activating Frank Starling as now preload is increased and there will be a resultant greater force of contraction, which is why changes in afterload don’t substantially reduce stroke volume or cardiac output unless change is severe or heart is diseased

Question 11

what determines preload? (4)

Answer 11

1. total blood volume
2. capacity of the venous system
3. heart rate (slower HR = longer filling time = more blood in chamber)
4. effect of gravity/changes in body position

Question 12

describe the effects of sympathetic stimulation on heart rate and myocardial performance

Answer 12

sympathetic stimulation (and parasympathetic inhibition) increases contractility! the heart’s intrinsic mechanisms can only increase cardiac output by 2.5x normal, but with sympathetic stimulation, can increase to 5x normal