PULSATILE PERFUSION (FLOW) DURING CPB

Pulsatile flow

Introduction

• Importance of pulsatile flow lies in the fact that systemic arterial vasoconstriction occurs with nonpulsatile CPB. This vasoconstriction leads to reduced visceral perfusion.
•  At separation from CPB when left ventricle is already functionally compromised from the insult of the operative procedure increased afterload predisposes to a low cardiac output syndrome and visceral organ damage.
•  Increased SVR occurs due to increased circulating catacholamines,vasopressin and activation of renin angiotensin system.

The fundamental difference between pulsatile and non pulsatile flow is that additional hydraulic energy is required and applied to move blood when pulsatile flow is used.This extra kinetic energy improves RBC transit, increase capillary perfusion and lymphatic drainage.

The hydraulic power of pulsatile flow is the sum over time of the product of instantaneous pressure and instantaneous flow.

Indices of pulsatility :

Pulsatility Index – It quantifies the relative sharpness of a given waveform with respect to its mean flow.

Pulse Power Index – It quantifies the power of a pulsatile waveform compared with non pulsatile equal flow.

Grossi et al developed these two indices. With their specific combinations with pulse rate, the lactate production was less than non pulsatile flows.

Physiologic Benefits :

1. Direct transmission of energy to the blood and then to tissues
2. It depends on geometry of waveforms : Rate of rise of pressure in aorta & amplitude of pulse

Theories proposed for pulsatile perfusion :-

3 theories-

1. Energy equivalent pressure
2. Capillary critical closing pressure
3. Neuroendocrine reflex mechanism

1)Energy equivalent pressure:-

States that benefits of a pulse wave are related to the contents in the pulsation

         EEP = P.f .dT / f. dT

                    P= Pressure mmHg

                    f= flow ml/ sec

                   dT=change in time

Increase in energy by pulse waveform is transmitted to the tissues and this causes-

• Maintain capillary patency
• Increase lymph flow
• Stimulate metabolism

2)Capillary Critical Closing Pressure :-

The peak pressure of pulsatile systolic pressure maintains capillary calibre for longer time.

Thus, the critical closing pressure at precapillary arteriole which obliterates tissue perfusion is higher, so beneficial.

3)Neuroendocrine Reflex Mechanism :-

Baroreceptor respond to both static and pulsatile aspects of waveform.

The baroreceptor mechanism of nonpulsatile causes increase in carotid sinus baroreceptor discharge frequency causing reflex vasoconstriction.

Metabolic effects of Pulsatile CPB -

Pulsatile CPB and cell metabolism: Non Pulsatile CPB is associated with development of metabolic acidosis and decreased tissue oxygen consumption whereas pulsatile flow has higher rate of oxygen consumption and decrease in metabolic acidosis.This is because of the enhanced energy associated with pulsatile blood flow that maintains microcirculation and improves delivery of nutrients.

Pulsatile CPB and kidneys : Improved kidney function is the result of better gas exchange at the capillary level together with the maintenance of normal lymph flow.

Pulsatile CPB and Brain :

• Pulsatile flow prevents the cerebral acidosis often observed during the early phase of nonpulsatile CPB. This may be due to better preservation of regional cerebral blood flow.
• Pulsatile flow reduces cerebral vascular resistance by as much as 25%. The improvement in regional blood flow distribution causes reduced cerebral lactate production .
• Anaerobic metabolism is suppressed with pulsatile blood flow, particularly during the critical cooling and rewarming phases.

Pulsatile CPB and Pancreas, Liver :

• Preserves pancreatic function better than nonpulsatile CPB.
•  "Normal function" of the pancreatic -cells with pulsatile blood flow and reduced function in the nonpulsatile flow is observed.
•  Reduced incidence of elevated amylase levels in patients undergoing CPB with pulsatile flow.
• Hepatic function is also preserved as reflected in postoperative SGOT level. Hepatic blood flow shows a vasoconstrictive response to nonpulsatile CPB with decrease in hepatic oxygen consumption.

Pulsatile blood flow and the gut :

• Postoperative gastrointestinal morbidity occurs in undergoing CPB.The cause is mesenteric hypoperfusion leading to ischemia  CPB associated with endotoxemia which in turn causes increases in gut permeability.
•  Pulsatile blood flow results in improved blood flow to the gut, reducing mucosal ischemia and increasing oxygen delivery.

Interaction between pulsatile flow and membrane oxygenators & heat exchanger -

The gaseous microembolic activity associated with the combination of pulsatile blood flow and a membrane oxygenator is significant, a screen filter downstream from the membrane oxygenator is included to reduce it.

Pulsatile blood flow can enhance gas exchange within the membrane by generating secondary flows at the membrane–blood interface, and by breaking down boundary layers in case of an increase in oxygen demand.

 Pulsatile flow enhances the performance of the heat exchanger, the boundary layer effect within the heat exchanger is broken down, leading to improved heat transmission. This is important because the reduced systemic vascular resistance encountered during pulsatile CPB may increase demand on the heat exchager during the rewarming phase.

Systems for delivering pulsatile flow –

1)Roller blood pump

The roller pump mechanism is simple and reliable works on the principle of two or more diametrically opposed rollers "milking" a constrained piece of tubing.

2)Ventricular blood pumps

The most physiologic method for generating pulsatile blood flow in that they operate in a similar manner to the ventricle of the heart. In simple terms, ventricular systems consist of compressible sac and two one-way valves permitting blood to flow into and out of the ventricle in only one direction.

3)Compression plate pumps

Like ventricular pumps, compression plate pumps can produce only pulsatile flow. A length of tubing of known diameter is placed on a rigid back plate and compressed by a moving plate that descends for a preselected stroke length, thereby ejecting a volume of perfusate from the tube .The direction of blood flow is ensured by valves positioned at the inlet and outlet of the ventricle or sac.

4)Centrifugal blood pumps

These are pumps in which the working elements rotate a drive shaft, they can be axial, nutational, or rotary in nature. Centrifugal pump-generated pulsatile flow has not been well accepted because generating physiologic pulsatility is not possible with currently available mechanisms.

5)Pulsatile Assist Device

Is an intermittent occlusive device that employs an intraaortic balloon pump apparatus to produce pulsatile blood flow in the arterial line of the CPB circuit.

The pulse is generated by occluding the arterial line of the circuit under flowing conditions,thereby creating a large pressure and volume delay within the arterial side of the circuit.on deflation of the balloon in the arterial line, the pressure and volume are released in the aorta to the patient as a pulse.

Concerns with this device are

• It increases the complexity of the circuit
• Fear of balloon rupture in the arterial line
• Hemolysis
• Gaseous microemboli may form.

Benefits of pulsatile perfusion

1. Increase blood flow in microcirculation
2. Decrease release of baroreceptor reflex hormones, limiting vasoconstriction
3. Increases renal, cerebral, pancreatic flow
4. Changes in blood flow distribution
5. Facilitate aerobic metabolism due to increase microcirculation
6. Attenuation of the systemic inflammatory response during CPB.
7. Attenuation of hormonal responses : Catacolamines, Renin Angiotensin Aldosterone, Antidiuretic hormone, Cortisol, Thromboxane, Prostacyclin.
8. Increase in Lymphatic function
9. Increase capillary perfusion
10. Increase RBC transit
11. Reduced markers of endothelial damage
12. Improved gastric mucosal oxygenation
13. Decreased need for inotropic support, shortened hospital stay, and superior organ preservation

References –

1.Terry Gourlay,Kenneth M Taylor.Cardiopulmonary bypass: Principles and Practice

2.Cardiopulmonary bypass: Principles and Practice – Glenn P.Gravlee

3.Kaplan’s Cardiac Anesthesia – The Echo Era 6^th Edition

4.Pulsatile versus nonpulsatile flow during cardiopulmonary bypass: microcirculatory and systemic effects. O'Neil MP Ann Thorac Surg. 2012 Dec;94(6):2046-53.

5.Effects of pulsatile perfusion during cardiopulmonary bypass on biochemical markers and kidney function in patients undergoing cardiac surgeries.Alireza Mohammadzadeh, Am J Cardiovasc Dis. 2013; 3(3): 158–162.

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- by Dr Amarja