Pulse oximeter perfusion index as an early indicator of sympathectomy after epidural anesthesia.

BACKGROUND: The pulse oximeter perfusion index (PI) has been used to indicate sympathectomy-induced vasodilatation. We hypothesized that pulse oximeter PI provides an earlier and clearer indication of sympathectomy following epidural anesthesia than skin temperature and arterial pressure. METHODS: Forty patients received lumbar epidural catheters. Patients were randomized to receive either 10 ml 0.5% bupivacaine or 10 ml 0.25% bupivacaine. PI in the toe, mean arterial pressure (MAP) and toe temperature were all assessed at baseline and at 5, 10 and 20 min following epidural anesthesia. The effect of epidural anesthesia over time was assessed by repeated measures analysis of variance. Additionally, we defined clinically evident sympathectomy criteria (a 100% increase in the PI, a 15% decrease in MAP and a 1 degrees C increase in toe temperature). The numbers of patients demonstrating these changes for each test were compared using the McNemar test for each time point. RESULTS: Twenty-nine subjects had photoplethysmography signals that met a priori signal quality criteria for analysis. By 20 min, PI increased by 326%, compared with a 10% decrease and a 3% increase in MAP and toe temperature, respectively. For PI 15/29, 26/29 and 29/29 of the subjects met the sympathectomy criteria at 5, 10 and 20 min, respectively, compared with 4/29, 6/29 and 18/29 for MAP changes and 3/29, 8/29 and 14/29 for toe temperature changes. CONCLUSIONS: PI was an earlier, clearer and more sensitive indicator of the development of epidural-induced sympathectomy than either skin temperature or MAP.

Synthetic Capillaries to Control Microscopic Blood Flow.

Capillaries pervade human physiology. The mean intercapillary distance is only about 100 µm in human tissue, which indicates the extent of nutrient diffusion. In engineered tissue the lack of capillaries, along with the associated perfusion, is problematic because it leads to hypoxic stress and necrosis. However, a capillary is not easy to engineer due to its complex cytoarchitecture. Here, it is shown that it is possible to create in vitro, in about 30 min, a tubular microenvironment with an elastic modulus and porosity consistent with human tissue that functionally mimicks a bona fide capillary using "live cell lithography"(LCL) to control the type and position of cells on a composite hydrogel scaffold. Furthermore, it is established that these constructs support the forces associated with blood flow, and produce nutrient gradients similar to those measured in vivo. With LCL, capillaries can be constructed with single cell precision-no other method for tissue engineering offers such precision. Since the time required for assembly scales with the number of cells, this method is likely to be adapted first to create minimal functional units of human tissue that constitute organs, consisting of a heterogeneous population of 100-1000 cells, organized hierarchically to express a predictable function.

Photoplethysmographic measurement of changes in total and pulsatile tissue blood volume, following sympathetic blockade.

Epidural anaesthesia, used for pain relief, is based on blocking the sensory and the sympathetic nerves in the lower part of the body. Since the sympathetic nervous system regulates blood vessel diameter, the sympathetic block is also associated with several haemodynamic changes. In the current study photoplethysmography (PPG) was measured on toes and fingers of patients undergoing epidural anaesthesia. Three parameters, which are related to the change in total and pulsatile tissue blood volume, were derived from the PPG baseline and amplitude. All parameters showed statistically significant increase in the toes after the sympathetic block, indicating higher arterial and venous blood volume and higher pulsatile increase in the arterial blood volume (higher arterial compliance) in the toe. These haemodynamic changes originate from the lower tonus of the arterial and venous wall muscles after the sympathetic block. In the fingers the PPG parameters based on the change in PPG amplitude decreased after the sympathetic block, indicating lower compliance. The measurement of the haemodynamic changes by PPG enables the assessment of the depth of anaesthesia, and can help control the adverse effects of the blockade on the vascular system.

Increase in pulse transit time to the foot after epidural anaesthesia treatment.

Epidurally induced anaesthetic treatment is a routine treatment for pain relief during surgical procedure, based on blocking the sensory and sympathetic fibres that mediate pain. The epidural sympathetic block results in relaxation of the muscle walls in the lower limbs, which can be assessed by the resultant haemodynamic changes. In the current study, the difference tt,f in the transit time of the blood pressure pulses between the toe and the finger is measured by photoplethysmography (PPG). Fifteen patients are administered 10 ml 0.25% of bupivacaine, ten patients are administered 10 ml 0.5%, and 17 patients are administered 40 ml 0.0625%. tt,f decreases as a function of the patient's age and blood pressure, both before and after the sympathetic block, owing to the decrease in arterial compliance with age and blood pressure. The time delay tt,f increases after the epidural treatment by 10.1+/-7.0 and by 16.8+/-10.8 ms for the 0.25% and the 0.5% concentrations, respectively. The time delay increase for the lowest concentration is not statistically significant. The toe-finger time delay change is found to reflect the haemodynamic changes induced by the sympathetic block with higher reliability than the routine methods of skin temperature or arterial blood pressure.