Investigation of splanchnic perfusion utilising an intraluminal opto-electronic sensor placed in the duodenum
The splanchnic region (abdominal gastrointestinal organs) is sometimes known as the “canary of the body” for its susceptibility to develop hypoxia at an earlier stage, analogue to the old times practice in coal mining. When the neuroendocrine response is activated, it exhibits regulation of blood flow and extraction of oxygen, facilitating redistribution of blood to vital organs. This can ultimately lead to systemic inflammatory response and multiple organ failure. The vital need to monitor the perfusion of the splanchnic region in critically ill patients has not yet been met by existing techniques. The goal of this research was to evaluate the feasibility of using the technique of photoplethysmography intraluminally in the duodenum in order to measure the haemodynamic changes occurring in the splanchnic circulation in a minimally invasive fashion. A bespoke processing system and data acquisition virtual instrument were designed and developed to allow continuous and simultaneous monitoring of two probes: an existing miniaturised PPG probe intended for intraluminal use and optically- identical finger PPG probe. Nine anaesthetised patients undergoing elective open laparotomy surgery were recruited and consented for the clinical trial at The Royal London Hospital. Due to the great proximity to the surgical site, monitoring of duodenal pulse oximetry signals could not be done in a continuous way. Also, the presence of moderate respiratory modulation in otherwise good quality, high amplitude signals seemed to result in an underestimation of arterial blood saturation of 2%. A frequency domain algorithm was thus applied to the data with results in agreement with both the finger PPG probe and commercial pulse oximeter. Blood oxygen saturation estimation at respiratory frequency yielded values within the physiological range expected for venous blood. For three of the patients, PPG signals were also acquired from the stomach, with results showing a similar pattern to the ones obtained from the duodenum. During the clinical trials, two patients experienced hypotension. PPG signals obtained before, during and after showed a great decrease in estimated blood oxygen saturations, which remained low even when monitored haemodynamical variables were back to normal values. Finger PPG probe estimations and commercial pulse oximetry values did not demonstrate this change. This suggests the possibility of photoplethysmography identifying changes in tissue oxygenation and blood volume in the splanchnic circulation resulting from external and/or internal regulatory mechanisms. This clinical trial thus show the great promise of pulse oximetry as complementary monitoring for patients at risk of developing splanchnic ischaemia.
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