0702 - Cardiac Work and Venous Return - RM Flashcards
What occurs in Phase I (bottom line) of the PV diagram?
Period of filling. This phase in the volume-pressure diagram begins at a ventricular volume of about 50 ml and a diastolic pressure of 2 to 3 mm Hg. The amount of blood that remains in the ventricle after the previous heartbeat, 50 ml, is called the end-systolic volume. As venous blood flows into the ventricle from the left atrium, the ventricular volume normally increases to about 120 ml, called the end-diastolic volume, an increase of 70 ml. Therefore, the volume-pressure diagram during phase I extends along the line labeled “I,” from point A to point B, with the volume increasing to 120 ml and the diastolic pressure rising to about 5 to 7 mm Hg.
What occurs in Phase II (RHS vertical) of the PV diagram?
Period of isovolumic contraction. During isovolumic contraction, the volume of the ventricle does not change because all valves are closed. However, the pressure inside the ventricle increases to equal the pressure in the aorta, at a pressure value of about 80 mm Hg, as depicted by point C.
What occurs in Phase III (top) of the PV diagram?
Period of ejection. During ejection, the systolic pressure rises even higher because of still more contraction of the ventricle. At the same time, the volume of the ventricle decreases because the aortic valve has now opened and blood flows out of the ventricle into the aorta. Therefore, the curve labeled “III,” or “period of ejection,” traces the changes in volume and systolic pressure during this period of ejection.
What occurs in Phase IV (LHS vertical) of the PV diagram?
Period of isovolumic relaxation. At the end of the period of ejection (point D), the aortic valve closes, and the ventricular pressure falls back to the diastolic pressure level. The line labeled “IV” traces this decrease in intraventricular pressure without any change in volume. Thus, the ventricle returns to its starting point, with about 50 ml of blood left in the ventricle and at an atrial pressure of 2 to 3 mm Hg.
What are the energy requirements of cardiac work?
Amount of work per cycle is the area spanned within a PV diagram. Energy/Work=PressurexVolume=work. All work is done during systole, particularly during the isovolumetric contraction and fast ejection phases.
What is the difference between internal and external cardiac work?
Two types of cardiac work:
External (physical) - pressure-volume work and kinetic work (actually pumping).
Internal (largely heat) - isovolumetric work (most of it), electrical activity/pumps and channels, sounds/murmurs from turbulent flow, and base metabolism.
Internal work is 5-20 times energy use of external work.
What is the hidden cost of internal work?
Internal work can’t be determined directly - only measured as heat. The vast majority of internal work comes from isovolumetric contraction to hit the right pressures. It is determined by Laplace’s Law (bigger the ventricle, bigger the tension), and the duration of systole during the cycle (high HR=more heat, bring it down for efficiency).
Thus, if CO is increased via HR, systole becomes more dominant, and internal work increases (lots of isovolumetric contraction) - requires much more energy. If CO is increased via SV, external work is increased, requiring less energy for the same gain. Useful for treating CAD.
Why and how is venous return homeostatically matched with CO?
VR is matched with CO over time to ensure the effective operation of the cardiac pump - with no additional inputs, if there was a ‘leak’ less VR would lead to less CO, and the cycle would continue.
They are only matched when RA pressure=0, and there is a balance must be found as what increases VR limits CO. When RA pressure rises, it pushes more blood into pulmonary circulation, increasing CO, but obviously decreasing VR, and vice versa.
How is VR determined by MSF and RA pressures?
MSF pressure is generated by vessel wall tension when heart is stopped (around 7 torr). If greater than RA pressure, this will aid flow into RA. When the pressure is too low, there is venous pooling until it rises again. Can be modulated by SY stimulation and infusion (+), and NO donors and bleeding (-).
How is VR determined by venous resistance and compliance?
More peripheral resistance drops MSF pressure, thus lowering VR. More compliance volume (therefore more blood pooling in veins), increases MSF pressure and thus venous return.
How is VR determined by pleural pressure?
Heart and VC are exposed to pleural pressure - if it is more negative, draws blood towards the heart, with an initial drop in CO, later becomes an increase in CO.
