Week 1: cardiac mechanics, cardiac factors in the regulation of cardiac output

Question 1

Define cardiac output (CO).

Answer 1

the volume of blood pumped by the left ventricle in one minute.
CO=Stroke volume X heart rate

Question 2

What is stroke volume?

Answer 2

SV=end diastolic volume (EDV)- end systolic volume (ESV)

Question 3

End diastolic volume is determined by what three factors?

Answer 3

1. Effective filling pressure
   - primary determinant is pressure at outlet of systemic vascular system, sustained by venous return (pressure at end of great veins)
2. compliance of the ventricle
   - ease at which ventricular wall can be stretched
3. heart rate (diastolic time)
   - HR greater than 180 beats/min are inadequate for ventricle to fill to its existing capacity

Question 4

End systolic volume depends on what two principal factors?

Answer 4

1. Contractility
   - increased contractibility decreases ESV
   - used to indicate forcefulness of contraction of heart muscles
   - influenced by length-tension and force -velocity
2. Aortic pressure (outflow resistance)

Question 5

Describe the length-tension curves in cardiac muscle.

Answer 5

Length on x axis, tension on Y axis

• active tension=total tension-passive tension
• bell curve
• force/tension in isometric contraction increases as the initial length of unstimulated muscle increases up to a max beyond which the contractile tension declines

Question 6

Describe how the length-tension curves apply to the heart lung preparations by Frank Starling, which addressed the response of the heart to 1)increased venous pressure supplying the right atrium, 2)increased resistance to outflow (increased aortic pressure)

Answer 6

1) Increased venous pressure (constant arterial BP)
-increased filling of ventricle (greatly increased EDV). ESV increases, but not as much.
-SV increased
-increases initial fiber length, leads to increased force of contraction and increased CO (cardiac output)
2) Increased arterial pressure
-incomplete emptying of ventricle increases blood in ventricle, results in greater EDV, and greater force of contraction
-SV unchanged (EDV and ESV both increase), CO unchanged. Same output with greater pressure.
Meaning: EDV is increased in both. Useful for balancing right and left sides of heart. This is an important reserve mechanism, it is believed that actual adjustment of cardiac output to meet changing depends depends on changing cross bridging instead of length of muscle fibers.

Question 7

Describe the force-velocity relationship in the heart and its importance.

Answer 7

-Ionotropic agents can change force of contraction by changing intra cellular chemistry of sarcomeres
Force-velocity test
-Preload=EDV (initial length)
-afterload=Aortic pressure
Result of test:
-preload 1 = control
1. w/ preload 2 (greater than preload 1), has greater velocity of shortening, but Vmax is same as. 2. With preload2+norepi, velocity if shortening is even greater, and a increase in Vmax.
-Vmax reflects dynamics of actin/myosin
3.with constant preload and increasing after load, as after load increases, tension before shortening increases, degree of shortening decreases, velocity of shortening decreases
Meaning: this appears to be main mechanism by which heart responds to changing demands for cardiac output

Question 8

What factors affect aortic pressure?

Answer 8

1. Total peripheral resistance (TPR)
   - Most important. sum of resistances that arise as blood flows through various organ circulations
2. Stroke volume
3. aortic compliance
   - compliance determines how much the pressure will rise above diastolic due to injection of blood during systole

Question 9

Describe two ventricular function curves

Answer 9

1. Cardiac output vs right atrial pressure
   - plots with 3 curves: maximal activation (e.g. exercise), resting, and failing
   - a 4mmHg increase in filling pressure can cause 5x increase in CO at maximal activation
2. Pressure-volume graph
   - ventricular pressure is Y axis, ventricular volume is x axis
   - two curves: potential max and diastolic (passive)
   - similar to length-tension curve
   - potential max affected by inotropes

Question 10

How do you measure energy required by the heart?

Answer 10

-determination of oxygen consumption. Blood flow should match oxygen demand, linear relationship.
Pressure-Time-Index (PTI) is a simpler measure
PTI=HR x BP

Question 11

What are the external influences regulating cardiac output (of the cardiac factors)?

Answer 11

• heart rate
• contractility: length-tension and force-velocity
• Total peripheral resistance (affecting aortic pressure)

Question 12

Whats the effect of HR on cardiac output?

Answer 12

-Increased HR increases cardiac output (with not much change in stroke volume until about 180 beats/min)
-CO reaches a peak at 180 and then decreases
-Stroke volume has a slight increase than drops after 180 beats/min
Remember: CO=SV x hr