Pulsatile Perfusion

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??

Pulsatile Flow in Vessels

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??

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

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

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

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

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.

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

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

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.

The Natural Pulse

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

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