W. Weyland

Cerebrovascular tone rather than intracranial pressure determines the effective downstream pressure of the cerebral circulation in the absence of intracranial hypertension.

Cerebral perfusion pressure is commonly calculated from the difference between mean arterial pressure and intracranial pressure because intracranial pressure is known to represent the effective downstream pressure of the cerebral circulation. Studies of other organs, however, have shown that effective downstream pressure is determined by a critical closing pressure located at the arteriolar level. This study was designed to investigate the effects of PCO2-induced variations in cerebrovascular tone on the effective downstream pressure of the cerebral circulation. Sixteen patients recovering from head injury were studied. Intracranial pressure was assessed by epidural pressure transducers. Blood flow velocity in the middle cerebral artery was monitored by transcranial Doppler sonography. Effective downstream pressure was derived from the zero flow pressure as extrapolated by regression analysis of instantaneous arterial pressure/middle cerebral artery flow velocity relationships. PaCO2 was varied between 30 and 47 mm Hg in randomized sequence. Intracranial pressure decreased from 18.5 ± 5.2 mm Hg during hypercapnia to 9.9 ± 3.1 mm Hg during hypocapnia. In contrast, effective downstream pressure increased from 13.7 ± 9.6 mm Hg to 23.4 ± 8.6 mm Hg and exceeded intracranial pressure at hypocapnic PaCO2 levels. Our results demonstrate that, in the absence of intracranial hypertension, intracranial pressure does not necessarily represent the effective downstream pressure of the cerebral circulation. Instead, the tone of cerebral resistance vessels seems to determine effective downstream pressure. This suggests a modified model of the cerebral circulation based on the existence of two Starling resistors in a series connection.

Flow velocity measurements as an index of cerebral blood flow. Validity of transcranial Doppler sonographic monitoring during cardiac surgery.

Background:Transcranial Doppler sonography is increasingly used to monitor changes in cerebral perfusion intraoperatively. However, little information is available about the validity of velocity measurements as an index of cerebral blood flow (CBF). The purpose of this study was to compare invasive and Doppler-derived measurements of cerebral hemodynamic variables during coronary artery bypass graft surgery. Methods:In 15 male patients, measurements of CBF and middle cerebral artery flow velocity (VMCA) were performed before and after induction of fentanyl-midazolam anesthesia, during hypothermic cardiopulmonary bypass (CPB), and at the end of the surgical procedure. Transcranial Doppler sonography recordings of systolic, diastolic, and mean VMCA, and derived parameters such as pulsatility (PI) and resistance (RI) indexes were recorded from the proximal segment of the right middle cerebral artery. CBF was measured by the Kety-Schmidt inert gas saturation method with argon as a tracer. To facilitate comparisons of CBF and VMCA measurements, changes between consecutive measurements were expressed as percentage values. Calculations of cerebral perfusion pressure and cerebral vascular resistance (CVR) were based on jugular bulb pressure. The cerebral metabolic rate for oxygen was calculated from CBF and the arterial — cerebral venous oxygen content difference. Results:Changes In mean VMCA paralleled changes in mean CBF except for hemodynamic changes associated with hypothermic CPB. At this stage of surgery, mean VMCA increased while actual CBF decreased. Separate analysis of the periods before and after CPB revealed a poor association between percentage changes In CBF and VMCA (r=0.26, P=0.36; r=0.51, P=0.06, respectively). Mean values of CVR, PI, and RI showed consistent changes after induction of anesthesia. After termination of CPB, mean CVR significantly decreased, whereas mean PI and RI remained virtually unchanged. Neither before nor after CPB was a clinically useful correlation found between percentage changes in PI, RI, and CVR (PI r=0.28, P=0.34; r=-0.47, P=0.09, respectively; RI r=0.16, P=0.59; r=-0.53, P=0.06, respectively). Conclusions:Hypothermic CPB seems to alter the relation between global CBF and flow velocity in basal cerebral arteries. Inconsistency in directional changes in CBF and VMCA at this stage of surgery might be attributable to changes in middle cerebral artery diameter, red blood cell velocity spectra, and regional flow distribution. Although changes in mean VMCA before and after CPB appear to parallel changes In mean CBF, individual responses of VMCA cannot reliably predict percentage changes in CBF. Furthermore, Doppler sonographic PI and RI cannot provide an approximation of changes in CVR during cardiac surgery.

Comparison of forced-air warming systems with lower body blankets using a copper manikin of the human body

Background: Forced‐air warming with upper body blankets has gained high acceptance as a measure for the prevention of intraoperative hypothermia. However, data on heat transfer with upper body blankets are not yet available. This study was conducted to determine the heat transfer efficacy of eight complete upper body warming systems and to gain more insight into the principles of forced‐air warming.

Conductive Heat Exchange with a Gel-Coated Circulating Water Mattress

The use of forced-air warming is associated with costs for the disposable blankets. As an alternative method, we studied heat transfer with a reusable gel-coated circulating water mattress placed under the back in eight healthy volunteers. Heat flux was measured with six calibrated heat flux transducers. Additionally, mattress temperature, skin temperature, and core temperature were measured. Water temperature was set to 25°C, 30°C, 35°C, and 41°C. Heat transfer was calculated by multiplying heat flux by contact area. Mattress temperature, skin temperature, and heat flux were used to determine the heat exchange coefficient for conduction. Heat flux and water temperature were related by the following equation: heat flux = 10.3 × water temperature − 374 (r2 = 0.98). The heat exchange coefficient for conduction was 121 W · m−2 · °C−1. The maximal heat transfer with the gel-coated circulating water mattress was 18.4 ± 3.3 W. Because of the small effect on the heat balance of the body, a gel-coated circulating water mattress placed only on the back cannot replace a forced-air warming system.

A new paediatric metabolic monitor

AbstractObjectiveA paediatric option for the measurement of

Severe accidental hypothermia : rewarming strategy using a veno-venous bypass system and a convective air warmer

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Infrarot-Temperaturmessung im Gehörgang mit dem DIATEK 9000 Instatemp und dem DIATEK 9000 Thermoguide Einflußgrößen und Vergleich mit anderen Methoden der Temperaturmessung des Körperkerns

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