Department of Anaesthesia, The Hospital for Sick Children and University of Toronto, 555 University Avenue, Toronto, Ontario, Canada, M5G 1X8*Corresponding author: Department of Pediatric Anesthesia and Intensive care Medicine, La Timone Childrens hospital, 365 Bd Jean Moulin, F-13385 Marseille cedex 5, France
Accepted for publication: September 24, 2001
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Abstract |
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Methods. Using a transcranial Doppler apparatus, we measured the peak velocity of whole blood cells pumped by a cardiopulmonary bypass (CPB) circuit, through a 0.15-cm internal diameter segment of rigid tubing. BF and BFV relationships were obtained at temperatures of 19, 28, and 37°C and at Hct of 0.05, 0.22, 0.39, and 0.54, by altering CPB flow over a range from 10 to 100 cc/min. Linear regression analysis was performed.
Results. The relationship between velocity and flow for the pooled Hct data was y=(0.43)x+0.86, r2=0.998 and 95% CI (0.9991) whereas the association for the temperature data was y=(0.42)x+0.02, r2=0.9998 and 95% CI (0.9990.9997). Changes of blood viscosity had no effect on velocity at a given flow rate. The combined effect of Hct and temperature on velocity for the relationship with flow is expressed by: y=1.3+2.4x.
Conclusion. In fixed diameter vessels with laminar flow, the linear relationship between flow and velocity is not affected by changes in temperature and Hct in clinical ranges. These results are explained by the FahraeusLindquist effect. They support the use of transcranial Doppler sonography to estimate cerebral blood flow in infants who may have large variations of Hct and/or temperature during bypass.
Br J Anaesth 2002; 88: 2779
Keywords: anaesthesia, paediatric; equipment: transcranial Doppler apparatus; blood, haemodilution; complications, hypothermia
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Introduction |
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Methods and results |
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Comments |
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In experimental and clinical settings, many physiologic factors affect cerebral blood flow velocity. Among them, Hct and core temperature have been widely studied. In adults, normovolaemic haemodilution with a decrease of Hct from 38 to 30% was associated with an average 16% increase in cerebral blood flow velocity.3 In children, an inverse relationship between cerebral blood flow velocity and Hct has been demonstrated during deep hypothermic cardiopulmonary bypass.4 During deep hypothermia, cerebral blood flow was reduced by 40%.5 The cerebral blood flow velocity was also correlated with cerebral blood flow. In eight children undergoing cardiac surgery with deep hypothermia, the authors demonstrated that cerebral blood flow velocity was reduced to 33% of the control value, whereas oxygen consumption was decreased to 20% of control.1
The effects of Hct and temperature on cerebral blood flow velocity can be related either to their effect on blood viscosity, on cerebral blood flow adaptation to cerebral metabolic rate for oxygen, or both. Changes in Hct can change blood viscosity. Moderate haemodilution (Hct 22%) decreases blood viscosity by 3050% at a low blood temperature2 whereas an increase in Hct increased blood viscosity and decreased CBF.6 Blood viscosity increases when blood temperature decreases; however, the increase in viscosity is observed mostly at temperature below 15°C.2 The cerebral blood flow adapts with metabolic demand. It is reduced during hypothermia when oxygen consumption is decreased.1 During polycythaemia, there is an increase in arterial oxygen content and a decrease in cerebral blood flow velocity.6 In animals, a nitroprusside infusion administered to normalize the arterial oxygen content in polycythaemia allowed cerebral blood flow velocity to return to baseline values.6
In cerebral resistance blood vessels, the pressure-flow relationships are given by Poiseuilles law (equation 1):
Q = Pr4/8l
(1)
where Q is blood flow, P is the pressure gradient across the vessel, r is the vessel radius, is blood viscosity and l is the vessel length. Given that cerebral blood flow velocity (CBFV) is expressed by equation 2
CBFV=Q/r2(2)
We can replace Q in equation 2 to obtain equation 3
CBFV=Pr2/8l(3)
In this equation, if P, r2 and the vessel length are constant, cerebral blood flow velocity is inversely proportional to viscosity. In animals a decrease in Hct was associated with only a small 4.4% change in basilar artery diameter,7 whereas the pial arterioles constricted as Hct decreased.8 These results are consistent with the finding that the hyperaemia accompanying haemodilution is the result of a decrease in blood viscosity and/or an adaptive phenomenon to variations in cerebral metabolism.7 9
As tube length, vessel radius, and pressure are maintained constant, one would expect cerebral blood flow velocity, at a given flow, to vary proportionally with blood viscosity. In the present study, the blood flow velocity varied directly with blood flow, independently of Hct and temperature, hence independently of viscosity. Several general properties about biomechanics of blood explain these observations. The FahraeusLindquist effect confirms that the apparent viscosity of whole blood varies with the Hct. Normally, plasma viscosity is 1.21.3 times that of water, whereas the blood viscosity is 2.4 times that of plasma. Furthermore, the effect of viscosity of whole blood depends critically on the size of the vessel through which it flows. When blood is flowing through vessels narrower than 2 mm in internal diameter, the apparent viscosity tends toward that of plasma.10 This explains the present observation and confirms that, in a 1.5-mm internal diameter tube intended to simulate the diameter of an infants mean cerebral artery, viscosity does not affect blood flow velocity recordings.
In conclusion, we found that in a small vessel of fixed diameter, wide variations of temperature, Hct and viscosity did not affect the relationships between blood flow velocity and blood flow. It supports the clinical usefulness of TCD to assess cerebral blood flow in infants who have large changes of Hct or temperature which can occur during cardiopulmonary bypass.
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Acknowledgement |
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References |
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