Answers: Cardiac Correlate Question Set

Problem 1 (Page 73 Physiology Handout)

1. To what on the skeletal muscle length-stress relation are ventricular volume and ventricular pressure analogous?

Ventricular volume is analogous to length. Ventricular pressure is analogous to stress. Both pressure and stress are in units of dynes/cm².

2. Although time is not indicated on Figure 1, it can be used to trace through the events that occur during one filling and ejection cycle of the heart. If end diastolic pressure is 8 mm Hg. what will be the volume in the ventricle (end diastolic volume) just prior to contraction? Which curve did you use to determine volume at this pressure?

At an end diastolic pressure of 8 mm Hg, the ventricular end diastolic volume (EDV) will be 160 ml. The passive tension curve is used.

3. The ventricle now contacts. Assume that pressure in the aorta is kept at 100 mm Hg during the entire cardiac cycle. What will ventricular volume be as ventricular pressure increases from 8 mm Hg to 100 mm Hg?

Ventricular volume will remain at 160 ml as intraventricular pressure increases from 8 to 100 mm Hg. This is the period of isovolumetric contraction.

4. What happens to ventricular volume once ventricular pressure reaches 100 mm Hg? What minimal ventricular volume (end systolic volume) is reached? What fraction of the end diastolic volume has been ejected?

Ventricular volume begins to decrease once the ventricular muscle develops enough stress to overcome the afterload of 100 mm Hg thus opening the aortic valve. The volume of the ventricle then decreases because blood is being ejected into the aorta. Ejection will continue until ventricular volume decreases to 80 ml (end systolic volume). Volume does not decrease below 80 ml because this is the smallest volume (shortest muscle cell length) at which ventricular muscle can develop enough stress to match an afterload of 100 mm Hg. The decrease from 160 to 80 ml means that the stroke volume is 80 ml, and the ejection fraction is 50% (SV/EDV)

5. The ventricle now relaxes. Assume that pressure in the aorta is kept at 100 mm Hg and that venous pressure is 8 mm Hg. What happens to ventricular volume and ventricular pressure during relaxation? What is the volume reached? What is the minimum pressure reached?

Ventricular volume remains at 80 ml (end systolic volume) as ventricular pressure falls from 100 mmm Hg to 1 mm Hg (period of isovolumetric relaxation). The minimum volume reached is 80 ml and the minimum pressure is 1 mm Hg. As ventricular pressure falls below 8 mm Hg and toward 1 mm Hg, he atrio-ventricular valve opens and blood flows into the ventricle increasing its volume and increasing intraventricular pressure along the passive tension curve. The final diastolic volume reached is 160 ml at an end diastolic pressure of 8 mm Hg.

6. Using the information you have just deduced, trace on Figure 1 the pressure and volume changes associated with a single cardiac cycle Identity the following phases or events on the tracing.

Refer to this resource to identify the appropriate phases.

The R wave of the QRS complexes occurs at the EDV of 160 ml

The T wave is roughly associated with closure of the aortic valve (Volume = 80 ml Hg, Pressure = 100 mm Hg)

The P wave is roughly associated with the point having a volume coordiante of approximately 150 ml and a pressure coordinate of 7 mm Hg.

What is Stoke volume? 80 ml

If the heart is paced a 70 beats/min, what is the cardiac output? 80 ml x 70 = 5.6 L/min

Problem 2 (Page 75 Physiology Handout)

1. Would the patient's heart be able to pump blood if the end diastolic pressure was 8 mm Hg and the mean aortic pressure were 100 mm Hg as in the normal situation? Defend your answer.

No. At an EDP of 8 mm Hg, the EDV would be 160 ml as in the normal case (Note that we have made the assumption, which may or my not be true that the passive properties of U.T.'s heart have not changed.) With the reduced contractility of U.T.'s heart, it could only develop 70 mm Hg pressure. This would not be enough to match the afterload of 100 mm Hg and open the aortic valve.)

2. If the patient's left ventricular end diastolic pressure ( = pulmonary capillary wedge pressure) were 23 mm Hg and her mean aortic pressure were 69 mm Hg, calculate the following:

end diastolic volume	EDV= 200 ml (use passive tension curve)
stroke volume	SVC = 20 ml (i.e. 200 ml - 160 ml)
end systolic volume	ESV = 160 ml
Ventricular Ejection Fraction	EF = SV/EDV = 40/200 = 20 %

When the enlarged ventricle contracts, ventricular pressure rises from 23 to 69 mm Hg during the period of isovolumetric contraction. At that point, the aortic valve opens and blood is ejected until the ventricular volume decreases to 160 ml. Again, this is the smallest volume at which a stress large enough to overcome an afterload of 69 mm Hg can be developed.

How do these relate to the markedly enlarged heart and pulmonary edema seen on the chest X-ray?

U. T.'s heart is operating at larger end-diastolic and end-systolic volumes, and this shows up on the X-ray as cardiac enlargement. Elevated ventricular end diastolic pressure means that pulmonary venous pressure is also elevated, and these elevated pressure are reflected back into the pulmonary capillaries and filtration occurs, resulting in pulmonary edema.

3. Discuss the sequence of events that most likely occurred in cardiac muscle mechanics, ventricular volumes, venous pressures and arterial pressures as the patient progressed from normal to the time of this admission to the hospital.

The patient has heart failure in that contractility decreased. As contractility was decreasing, the cardiac function curve was shifting downward, and thus cardiac output was decreasing and venous pressure was increasing. Because cardiac output was decreasing, mean arterial pressure was decreasing. This elicits a baroreceptor reflex to cause increased sympathetic tone leading to an increase in heart rate, and increase in total peripheral resistance, a decrease in unstressed volume (raising mean circulatory filling pressure via venoconstriction). This increase in MCFP and TPR and the decreased contractility would be such that the new operating point (on a Guytoon) would be at a lower than normal cardiac output and a higher than normal RAP.

Important

If you have not done so already, you should work through the

Pressure-Volume Laboratory