Tissue Near InfraRed Spectroscopy (NIRS) in Critically Ill Patients

Overview

Hypotheses: 1. In the acute phase of early illness, tissue oxygen saturation and vascular occlusion test (VOT) are important adjuncts in differentiating severe critical illness requiring ICU admission from patients benefiting from less aggressive therapies (non-ICU admission). 2. An early feature of severe sepsis is abnormal microcirculatory vasoreactivity. 3. Microvascular reactivity can be measured by means of vascular occlusion test (VOT) using non-invasive near infrared spectroscopy (NIRS) to measure tissue Oxygen saturation (StO2). 4. Microvascular reactivity is significantly deranged in patients with early severe sepsis and is quantifiably different from non-septic critically ill patients. 5. Other measures of microcirculatory perfusion ie. sublingual orthogonal polarization spectral (OPS) video microscopy, are abnormal and will directionally correlate with VOT and StO2 in severe sepsis 6. In severely septic patients response to therapy can be assessed by VOT and StO2 monitoring and will correlate with prognosis. 7. A management protocol incorporating VOT and StO2 monitoring in addition to conventional hemodynamic and biochemical parameters as a guide to therapy will result in improved outcomes in severely septic patients. This project will investigate the use of a non-invasive near infrared light (NIR) device in conjunction with brief arm compression to measure the microcirculation in critically ill patients. It is know that in patients with overwhelming infections, their blood vessels do not respond normally and the NIR device can help measure how abnormal their blood vessels are. This type of testing is non-invasive and can be performed repeatedly without harm to the patient and may provide an earlier way to determine whether they have overwhelming sepsis and also may help to optimize the treatments they receive and better tailor their treatments to the degree of blood vessel abnormalities that are found. If our hypotheses are correct, this simple non-invasive test could provide a very rapid means of assessing patients that could be done more safely and quickly than some of the current methods. This would have an important effect to enhance patient safety and improve outcomes in such critically ill patients.

Project Overview: This project is focused on the early diagnosis and management of septic shock using tissue near infrared spectroscopy (tNIRS) for continuous and non-invasive assessment of microcirculatory vasoreactivity as a diagnostic and prognostic indicator in critically ill patients. A novel aspect of this study is that point of entry of patients will be via the Critical Care Outreach Team (CCOT) which will ensure patients are assessed in the early phases of their disease prior to and co-incident with admission to Critical Care Unit (CCU). The investigators will employ tNIRS to assess peripheral tissue oxygen saturation (StO2) in all patients assessed by CCOT (n = 1031 patients in 2009) and in addition, the investigators will employ the NIRS vascular occlusion test (VOT) using a brief episode of forearm ischemia to quantify microvascular (dys)function. All patients admitted to CCU via CCOT (n = 230 patients in 2009) will have StO2 monitored and VOT assessment sequentially throughout the course of their critical illness. Biomarkers to assess severity of illness, inflammatory processes and microcirculatory reactivity will be obtained, and in a subset of patients (n = 30), correlations between these parameters and other indices of microvascular perfusion using sublingual orthogonal polarization spectral (OPS) imaging videomicroscopy will be sought. As it is estimated that 20-30% of CCOT admissions are for sepsis (n = 40-60), the inclusion of other critically ill non-septic patients will enable discrimination of various parameters of VOT to be contrasted between groups to determine relative diagnostic and prognostic significance. Statement of Objectives: 1. To assess the diagnostic utility of tNIRS StO2 and VOT in early assessment of critically ill patients. 2. To determine whether significant and pathognomonic differences exist in StO2 and indices of VOT between septic patients versus other non-septic critically ill patients and healthy age matched volunteers. 3. To determine whether correlations exist between biomarkers of inflammation and vasoreactivity and measures of StO2 and VOT in severely septic patients. 4. To determine whether incorporation of StO2 and VOT into a treatment protocol can positively impact clinical outcomes in critically ill patients.