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 :
Theories proposed for pulsatile perfusion :-
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-
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 CPB and Pancreas, Liver :
Pulsatile blood flow and the gut :
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
Benefits of pulsatile perfusion
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2.Cardiopulmonary bypass: Principles and Practice – Glenn P.Gravlee
3.Kaplan’s Cardiac Anesthesia – The Echo Era 6th Edition
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.