Background and Purpose Transcranial Doppler (TCD) monitoring of mean blood flow velocity (mV) during head-upright tilt can allow testing of cerebral autoregulation. Nonetheless, head-upright tilt can induce changes in the ventilation-perfusion relationship and/or respiratory activity that might influence TCD data.
Methods Forty-eight healthy volunteers underwent monitoring of mV and end-tidal CO2 in the horizontal position and during head-upright tilt.
Results Both mV and end-tidal CO2 significantly decreased in orthostasis (P<.01). Linear regression analysis showed a significant linkage between end-tidal CO2 and mV changes (r=.83, P<.01).
Conclusions Changes in ventilation-perfusion ratio and in the respiratory pattern induced by head-upright tilt can significantly influence TCD data by determining a Pco2 decrease.
Transcranial Doppler (TCD) has been widely used to investigate cerebral hemodynamics and cerebral autoregulation in response to changes in systemic hemodynamics induced by several different methods, including postural stimuli.1 2 3 4 Orthostasis can induce changes in the respiratory pattern and especially the ventilation-perfusion ratio, thus determining changes in Pco2. Because the effect of Pco2 change on TCD recordings of mean blood flow velocity (mV) has been well documented,5 the present study aimed to test whether slight changes of the respiratory-perfusion pattern induced by head-upright tilt (HUT) can affect the interpretation of TCD data in orthostasis.
Subjects and Methods
Forty-eight healthy men (mean age, 38.8 years; range, 22 to 50 years) gave their informed consent to the study, which was approved by the local ethics committee. All the subjects were free from hypertension, diabetes, cerebrovascular disease, and significant stenosis of intracranial and extracranial cerebral arteries, as shown by physical examination, clinical history, and preliminary ultrasonographic investigations. All the subjects were tested in the afternoon; each subject was lying on a proper tilt table, and the room temperature was kept constant at 20°C to 22°C. After the subjects had rested for 20 minutes, the devices for noninvasive monitoring of the chosen biological parameters were positioned. Recordings of cerebral blood flow velocity were performed with a 2-MHz probe (Multidop L, DWL) on the middle cerebral artery of the dominant hemisphere; the position of the probe was kept constant by means of a mechanical probe holder equipped with an elastic band that was fastened around the skull. The insonation depth was set between 51 and 57 mm, depending on the optimization and stability of the signal, and the horizontal sweep speed was 10 seconds. Continuous end-tidal CO2 was measured with a capnographic monitor (Capnogard, Novametrix) while the subjects were spontaneously breathing room air. Blood pressure and heart rate were noninvasively monitored (Finapres Ohmeda). Respiratory activity was monitored by means of a piezoelectric transducer (OS-9000SRS, Goldstar) fastened around the bottom of the chest with an elastic band, as previously described.6 During TCD at the horizontal position, data were collected over approximately 20 consecutive cardiac cycles, at least 5 minutes after the devices for noninvasive monitoring had been applied, when the monitored parameters reached a steady state. The table was then turned up at 60°, and recordings were again performed after at least 5 minutes. For each subject, mV and end-tidal CO2 were calculated as mean values in each position. Differences between the horizontal and standing positions for mV, end-tidal CO2, and rate of breathing were statistically evaluated by paired t test. Linear regression analysis was performed between percent changes of mV during HUT and absolute changes (in millimeters of mercury) of end-tidal CO2.
Results of TCD and capnometry are shown as mean±SD in the Table⇓. During HUT, the subjects displayed a significant drop of end-tidal CO2. mV values also showed a significant drop during HUT. Mean arterial blood pressure did not change significantly with HUT (92.6±13.0 mm Hg at baseline, 94.3±10.8 mm Hg during HUT); heart rate slightly increased (from 66.2±11.6 to 74.8±8.4 beats per minute; P<.05). The rate of breathing did not change significantly in orthostasis (from 16.0±4.94 to 15.33±3.94 breaths per minute), although the piezoelectric transducer recorded wider expansions of the chest wall in the upright position (Fig 1⇓). Linear regression analysis (Fig 2⇓) showed a significant (r=.83; P<.01) linkage between end-tidal CO2 and mV changes, expressed by a constant of 2.97 (percent mV change/mm Hg).
Head-upright tilt induced slight but significant drops in both end-tidal CO2 and mV in the healthy subjects examined in the present study. The mV drop was mostly determined by end-tidal CO2 changes, as demonstrated by the significant result of linear regression analysis, with a 2.97% change of mV for each mm Hg change of end-tidal CO2, which is similar to what was previously found in other different studies.7 Results for mV during HUT were not affected by the autoregulatory response because data were collected at least 5 minutes after tilt, when it is likely that the dynamic autoregulatory response already was completed.8 Although the validity of TCD measurements during HUT has been questioned by some authors2 because of concerns about a possible change in the diameter of the middle cerebral artery during HUT, further studies3 demonstrated that such changes of the vessel cross-sectional diameter are negligible during HUT. Because the rate of breathing did not change significantly during tilt, the observed changes of end-tidal CO2 values are likely to depend on changes of the ventilation-perfusion relationship during HUT. In fact, in the normal upright lung, the blood flow per unit volume decreases from bottom to top, reaching very low values at the apex, where perfusion is possible only during systole; in addition, ventilation, despite a less marked change, increases from top to bottom in the upright lung because of the wider expansions of the bottom of the chest wall that are allowed by the upright position (Fig 1⇑). The changes of the ventilation-perfusion relationships during HUT result in a sort of shunt effect that does not affect blood oxygen saturation but allows, at the bottom of the lungs, a better clearance of CO2, which diffuses through the membrane about 20 times as rapidly as oxygen, thus inducing a slight hypocapnia. The results from the present study suggest that TCD studies on cerebral autoregulation in response to postural stimuli should include careful examination of end-tidal CO2 values, at least if the data analysis is performed as a comparison between horizontal and standing positions. Furthermore, the results from the present study stress the need for end-tidal CO2 monitoring to attempt to explain the paradoxical constriction described during presyncope in some subjects,9 since syncopal subjects often show marked changes in the respiratory pattern during presyncope.
Reprint requests to Alfonso Lagi, MD, Via G Mameli 44, 50131 Firenze, Italy.
- Received January 14, 1997.
- Revision received March 6, 1997.
- Accepted March 21, 1997.
- Copyright © 1997 by American Heart Association
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