“BRAIN SPARING” EFFECT

Animal and human studies have shown that there is preferential shunting of blood to the fetal brain, heart and adrenals in the stressed fetus (e.g. IUGR) (1,2). This increased blood flow the to fetal brain can be demonstrated by analyzing the waveform in the middle cerebral (MCA) or internal carotid arteries (1). Splanchnic vasoconstriction and cerebral vasodilatation results in an increased forward component to the waveform in diastole. This results in a decrease in the Pulsatility or resistance indices.

• The brain sparing effect may be transient (3).
• The MCA PI is below the normal range when the PO2 is reduced. The maximum reduction in the PI is reached when the fetal PO2 is 2-4 SD below the normal value for that gestation (4).
• Severe brain edema results in a tendency for the PI to rise. The disappearance of the brain sparing effect appears to precede fetal death (5). This concept is, however, difficult to demonstrate.
• MCA/UA (middle cerebral artery to umbilical artery) ratio of less than one signifies shunting of blood to the fetal cranium as a compensatory mechanism.
• The increased diastolic component of the MCA waveform is due to cerebral vasodilatation.

REFERENCES

1. Mari G, Deter RL. Middle cerebral artery flow velocity waveforms in normal and small for gestational age fetuses. Am J Obstet Gynecol 1992;166:1262-1270.
2. Wladimiroff JW, Tonge HM, Stewart PA. Doppler ultrasound assessment of cerebral blood flow in the human fetus. Br J Obster Gynaecol 1986;93:471-475.
3. Richardson BS, Rurak D, Patrick JE et.al. Cerebral oxidase metabolism during sustained hypoxemia in fetal sheep. J Dev Physiol 1989;11:37-43.
4. Vyas S, Nicolaides KH, Bower S et.al. Middle cerebral artery flow velocity waveforms in fetal hypoxemia. Br J Obstet Gynaecol 1990;97:797-803.
5. Mari G, Wasserstrum N. Flow velocity waveforms of the fetal circulation preceding fetal demise in a case of lupus anticoagulant. Am J Obstet Gynecol 1991;164:776-778.