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Purpose: Near infrared spectroscopy (NIRS) monitors absolute changes in concentration of chromophores from an unquantified baseline. A change in oxygenation is required to obtain information. Clinically, knowing initial chromophore concentration is desirable. We sought absolute initial concentrations by combining NIRS measurements with simultaneous Eppendorf histograph measurement (absolute values of tissue partial pressure of oxygen (pO2)). Methods: There were 22 trials on 9 occasions in 2 adult volunteers. Following local anaesthetic, the histograph probe was inserted into the forearm muscle and NIRS optodes applied. A sphygmomanometer cuff was inflated over the upper arm to produce ischaemia and hypoxia. Change in pO2 was determined as a percentage of the initial pO2. Change in HbO2 was measured, and the initial HbO2 calculated by assuming the percentage change in HbO2 equaled the percentage change in pO2. Results: Mean percentage change in pO2 was 20.7 ± 9.6. Mean change in HbO2 was 22.4 ± 3.4 µmol/l(tissue). Calculated initial HbO2 was 146.4 ± 108.7 µmol/l(tissue), range 51 to 508 µmol/l(tissue). Correlation (r2) between pO2 and HbO2 was 0.97 ± 0.03. Conclusion: Although changes in pO2 and HbO2 were highly correlated, calculation of initial absolute concentration of HbO2 was not achieved by this method. A reliable method of obtaining absolute concentrations of HbO2in vivo remains to be found.

During cardiac surgery, bypass pumps rely on pressure monitors to evaluate flow. We studied whether it would be possible to optimize pump flow by monitoring changes in cerebral cytochrome a,a3 using NIRS to maintain cyt redox status at its pre-bypass level. Method: 18 healthy 7–45 kg swine were placed on bypass for repeated cycles of cooling and re-warming from 36 to 15 to 36°C in 3°C steps. Between each cycle, the swine's bypass pump blood flow rate was adjusted to restore cytochrome redox status to its pre-bypass value. Results: In all swine trials, the number of pump flow alterations imposed by NIRS monitoring ranged from 0 to 42, the average being 14 per trial. The best trial had 22 pump flow adjustments during which the range of cytochrome redox status change was 0.50±0.06 µmol?l–1. The average trial had a range of cytochrome redox status change of 1.50±0.22 µmol?l–1. Conclusion: NIRS-driven alterations in pump flow rate to maintain pre-bypass cytochrome redox status can be achieved successfully in the animal model.

Near infrared spectroscopy (NIRS) monitors changes in oxygenated haemoglobin (HbO2), and redox status of cytochromeaa3 (cyt) continuously and non-invasively in living tissue. We present examples where clinically relevant changes in HbO2 and/or cyt were detected in real time, allowing intervention to avert potentially harmful hypoxic-ischaemic damage to the brain and/or spinal cord. Brain monitoring: In children undergoing surgery on cardiopulmonary bypass, observations include that: atrial fibrillation (cardiac arrhythmia) lowered cerebral HbO2 concentration; concealed haemorrhage decreased cerebral HbO2 concentration; inadequate level of anaesthetic resulted in spikes of changes in volume with interventions such as suturing; circulatory arrest reduced brain HbO2 and cyt redox status; and bypass pump problems compromised cerebral blood flow. Spinal cord monitoring: In the experimental animal, we observed that NIRS detected ischaemic change immediately following aortic compression, spinal column distraction (instrumentation to separate the vertebrae), and hypoxia. In an infant requiring release of a congenitally tethered spinal cord, we observed that traction on the spinal cord of the infant resulted in decreased total haemoglobin concentration. Summary: NIRS brain monitoring probably represents the “standard of care” during cardiac surgery because adverse events can be detected and quantified. Similarly, spinal cord monitoring could reduce ischaemic spinal cord damage in spinal cord surgery and aortic aneurysm repair.