Pulsatile Perfusion

1. Pulsatile Perfusion

2. Pulsatile vs Continuous Flow Why use pulsatile flow? Inherently more physiologic What is flow like in our vessels?? Is there a difference between: a pulsatile mean arterial pressure of 60 mmHg a non-pulsatile mean arterial pressure of 60 mmHg??

3. Pulsatile Flow in Vessels

4. Resistance to Pulsatile Perfusion • Perception of greater complexity w/marginal benefit • Fear of hemolysis/shear stress/intimal damage • Comfort level w/non-pulse systems • Why change what works??

5. Theories of Pulsatile Flow • Energy Equivalent Pressure • Capillary Critical Closing Pressure • Neuroendocrine reflex mechanisms effected by baroreceptor discharge

6. Energy Equivalent Pressure • Theoretical advantage that the production of pulsatile flow depends not on a pressure gradient, but on an energy gradient • EEP represents the energy content of the pulsatile arterial wave. • EEP = ƒpfdt/ƒfdt • P = pressure (mmHg, f = flow ml/sec, • dt =change in time at a specific point

7. Energy Equivalent Pressure • Using this formula it was determined that the energy needed to deliver pulsatile flow was up to 3.4 x that required to produce non-pulsatile flow for the same levels of mean pressure and flow. • What does this extra energy mean? • Thought to be available to the tissues by capillary patency, increased lymph flow, oscillatory movements at the cell level

8. Capillary Critical Closing Pressure • Studies have suggested a reduction in capillary blood flow and a significant reduction in cerebral capillary diameter with non-pulse as opposed to pulsed flow. • (1) Suggests that capillary patency is preserved longer by the peaks of systolic pressure. • (2) That non-pulsatile flow produces more microcirculatory shunting and reduced capillary perfusion

9. Endocrine reflex mechanism effected by baroreceptor discharge • Transition from pulsatile to non-pulsatile flow results in: • Marked increase in discharge frequency from baroreceptor in carotid sinus • This may initiate reflex neuroendocrine responses which remain operative throughout the non-pulsatile phase.

10. Hemodynamic Effects of Pulsatile Flow • Non-pulse flow is associated with progressive elevation in SVR • Renin angiotensin activation • Continues into the post CPB period • Pulsatile perfusion is associated with significantly lower levels of vascular resistance • Benefits: • Improved tissue perfusion • Lower afterload for ventricle at the end of the perfusion period

11. Metabolic Effects of Pulsatile Flow • Cellular level • Pulsed flow is associated with higher rates of oxygen consumption and reduced metobolic acidosis • ??enhanced energy may maintain microcirclatory patency and optimize tissue perfusion. • Organ Level • Kidney: better urine output w/pulse • Brain: reduced cerebral acidosis/markers of brain injury • Hepatic, gut, pancrease: funtions better preserved possibly because of reduction of mucosal ischemia which can induce endotoxemia

12. Producing Pulsatile Flow • Roller Pump • Pump-head accelerates in systolic phase and deaccelerates during diastole • Sub-optimal waveform • Centifugal pumps • Afterload dependance makes them unreliable for this purpose.

13. The Natural Pulse

14. What the Natural Pulse Does • Steep front (blood acceleration) induces a pressure wave • Pressure wave creates a wave phenomenon which: • expands walls of blood vessels, allowing more blood flow to move with lower pressure • reinforces muscles of blood vessels • removes obstacles inside the blood vessels • pushes blood through tiny capillaries

15. Obstacles to Transmitting Pulsatile Flow • Distensibility of tubing • Resistance and damping of the: • Membrane • Arterial line filter • Cannula • The small size of the aortic cannula creates the equivalent of AS resulting in circuit stresses and possibly hemolysis