Transcephalic electrical impedance provides a means for quantifying pulsatile cerebral blood volume changes following head-up tilt

Cerebral hemodynamics in preterm infants during positional intervention measured with diffuse correlation spectroscopy and transcranial Doppler ultrasound

Four very low birth weight, very premature infants were monitored during a 12 degrees postural elevation using diffuse correlation spectroscopy (DCS) to measure microvascular cerebral blood flow (CBF) and transcranial Doppler ultrasound (TCD) to measure macrovascular blood flow velocity in the middle cerebral artery. DCS data correlated significantly with peak systolic, end diastolic, and mean velocities measured by TCD (p(A) =0.036, 0.036, 0.047). Moreover, population averaged TCD and DCS data yielded no significant hemodynamic response to this postural change (p>0.05). We thus demonstrate feasibility of DCS in this population, we show correlation between absolute measures of blood flow from DCS and blood flow velocity from TCD, and we do not detect significant changes in CBF associated with a small postural change (12 degrees ) in these patients.

A pilot study of changes in cerebral blood flow velocity, resistance, and vital signs following a painful stimulus in the premature infant

PURPOSE

The purpose of this pilot study was to determine the cerebral blood flow velocity and resistance changes and vital signs following a painful stimulus in the premature infant.

SUBJECTS

A convenience sample of 12 infants was randomly assigned to one of 2 treatment groups. In the final analysis, there were 10 infants younger than 24 hours of age and between 25 and 32 weeks' gestational age.

DESIGN

A randomized 2-period, 2-group, crossover design was used.

METHODS

Cerebral blood flow velocity and resistance were measured via a Doppler head ultrasound transducer placed over the anterior fontanel. Vital signs were measured with a cardiorespiratory monitor. The infant then received the heel stick procedure or the sham procedure (heel preparation with no heel puncture). Each infant served as his or her own control. After each procedure, there was ultrasound and vital sign measurement at 15, 60, 120, 180, 240, and 300 seconds. Thereafter, the alternate treatment was used and 6 more measurements were taken.

MAIN OUTCOME MEASURES

Cerebral values: peak systolic velocity (PSV) and resistive index (RI); vital signs: heart rate, respiratory rate, oxygen saturation (SpO2), and blood pressure.

RESULTS

Treatment groups were similar at baseline except for gestational age. There were no carryover or period effects in the crossover design for the primary outcomes except for SpO2. There was a significant group effect (heel stick compared with sham) (P = .009) for peak systolic velocity; however, there were no significant differences between groups at each time point. Two subjects had a distinctive pattern based on simultaneous changes in flow and resistance: when flow velocity increased, resistance decreased. This may be reflective of risk for intraventricular hemorrhage (IVH). Mean arterial blood pressure (MAP) was not significant. However, heart rate was significantly different between stick and sham at 15 seconds (P = .022); respiratory rate was significant at 180 seconds (P = .029); and SpO2 was significant at 3 different time points. There were no significant correlations between PSV and mean arterial blood pressure and PSV and SpO2 when comparing stick to sham.

CONCLUSIONS

This is a study based on a small sample size. However, the Doppler-measured peak systolic velocity increases significantly after a painful stimulus. The clinical implication of this finding needs to be established.

Pulsatile electrical impedance response from cerebrally dead adult patients is not a reliable tool for detecting cerebral perfusion changes

The original objective of this work was to verify the possibility of using electrical pulsatile cerebral impedance measurements as a diagnostic aid for assessing the brain-death condition in adults; a subordinate target was to validate a simple method for detecting perfusional changes in the brain. To this end, impedance signals were recorded, for a comparative study, from both live subjects and brain-dead patients, using a simple four-electrode arrangement. Rather unexpectedly, pulsatile transcephalic impedance waveforms exhibiting a temporal dependance similar to those of live subjects were detected in artificially ventilated, cerebrally dead, adult subjects; distributions of the time delays between impedance peaks and ECG peaks were also recorded for the two groups (dead and live subjects). These data provided no evidence, at the 1% significance level, against the hypothesis that the two sample groups are drawn from identical populations. The detection of impedance variations from brain-dead patients can be explained by the residual persistence of blood flow through the scalp, by mechanical variations synchronous with the heart beat and by the presence of the oscillating flow and the systolic spikes that precede the final blood flow arrest. The fact that impedance variations can be traced back to a multiplicity of causes, unrelated to the normal unidirectional flow, renders the transcephalic impedance method inappropriate for detecting cerebral perfusion changes in adults. This conclusion is also strengthened by some theoretical results recently derived from a multilayer model of the head.