LETTER TO EDITOR

Transcranial Doppler Studies of Middle Cerebral Artery Blood Flow Following Different Test Conditions

Naveen K.V., Nagendra H. R., Shirley Telles, Garner C.

Vivekananda Kendra Yoga Research Foundation, Bangalore, India.

Transcranial Doppler ultrasound (TCD) is a noninvasive method of studying cerebral haemodynamics allowing the measurement of blood velocity in the major intracranial vessels using ultrasound signals transmitted through bone1. Cerebral circulation is controlled mainly by auto regulation and fluctuations in blood CO2 and O2 with scarcely any chances when arterial pressure is constant between 50 and 140mmHg2. We recorded the blood velocity in both right and left middle cerebral arteries (MCA) in a 26 year old male volunteer with TCD (DWL Ultrasonic Doppler System, Sipplingen, Germany). There were six repetitions on each side, and recordings were made before and immediately after breath holding, Valsalva maneuver, mental arithmetic, mental route finding, a yoga rapid shallow breathing at 120 times per minute and hyperventilation. A 2 MHz doppler probe, connected to a TCD device was mounted at a 30 degree angle over the squamous temporal bone using an elastic head band. The high pass filter was 250 Hz with emitted ultrasonic power of 56.0 mW.

There was a significant increase in systolic and diastolic blood flow velocity (sbfv, dbfv) following valsalva maneuver for both right and left MCA (P<.005, t test for paired data in all comparisons against the preceding baseline). In contrast following hyperventilation and rapid yoga breathing there was a significant decrease in dbfv for the right MCA (P<.002, both cases). Following, Hyperventilation the sbfv of the left MCA reduced (P<.001). There were no changes following breath holding, isometric contraction, mental arithmetic and mental route finding. An increase in blood flow velocity may be correlated with increased cerebral blood flow or reduced vessel diameter.3 Immediately after the valsalva maneuver the systemic blood pressure has been reported to be approximately 170 mmHg4. As the BP rises above 140 mmHg, cerebral blood flow is known to increase as was demonstrated here by increased bfv. Breath holding may not have increased systemic BP to the same extent as valsalva, hence not influencing cerebral blood flow. Hyperventilation is known to reduce CO2 in the blood and hence cerebral blood flow would reduce,1 this was reflected in reduced bfv. The same result followed rapid yoga breathing, suggesting that this practice, like hyperventilation, reduces cerebral blood flow, possibly also by carbondioxide washout.

Hence TCD appears to allow simple, rapid and repeatable measures of changes in cerebral blood flow related to changes in systemic pressure and blood CO2. In the present study changes in cerebral blood flow related to cognitive activities (such as mental route finding) were not recorded.

References

1. Peters P, Datta K : Middle cerebral artery blood flow velocity studied during quiet breathing, reflects hypercapnic breathing in man. In: Modelling and control of ventilation, Semple SJG, Adams L and Whipp BJ (eds). Plenum Press New York 1995; 293-295.
   

2. McHenry C : Cerebral blood flow and metabolism. In : Cerebral Vascular Diseases, Harrison MJ and Dyken ML (Eds). Butterworths & Co. London 1983; 67-85.
   

3. Chan KH, Dearden WM, Miller JD : The significance of post- traumatic increase in cerebral blood flow velocity: A Transcranial doppler ultrasound study. Neurosurgery 1992; 30 697-700.
   

4. Ganong WF: Review of medical physiology, California: Lange Medical Publication, 1981.