Noninvasive Optical Quantification of Cerebral Venous Oxygen Saturation in Humans

Rationale and Objectives

Cerebral oxygen extraction, defined as the difference between arterial and venous oxygen saturations (SaO 2 and SvO 2 ), is a critical parameter for managing intensive care patients at risk for neurological collapse. Although quantification of SaO 2 is easily performed with pulse oximetry or moderately invasive arterial blood draws in peripheral vessels, cerebral SvO 2 is frequently not monitored because of the invasiveness and risk associated with obtaining jugular bulb or super vena cava (SVC) blood samples.

Materials and Methods

In this study, near-infrared spectroscopy (NIRS) was used to noninvasively measure cerebral SvO 2 in anesthetized and mechanically ventilated pediatric patients ( n = 10). To quantify SvO 2 , the NIRS signal component that fluctuates at the respiration frequency is isolated. This respiratory component is dominated by the venous portion of the interrogated vasculature. The NIRS measurements of SvO 2 were validated against the clinical gold standard: invasively measured oxygen saturations from SVC blood samples. This technique was also applied in healthy volunteers ( n = 5) without mechanical ventilation to illustrate its potential for use in healthy populations with natural airways.

Results

Ten pediatric patients with pulmonary hypertension were studied. In these patients, SvO 2 in the SVC exhibited good agreement with NIRS-measured SvO 2 ( R 2 = 0.80, P = .001, slope = 1.16 ± 0.48). Furthermore, in the healthy adult volunteers, mean (standard deviation) NIRS-measured SvO 2 was 79.4 (6.8)%. This value is in good agreement with the expected average central venous saturation reported in literature.

Conclusion

Respiration frequency-selected NIRS can noninvasively quantify cerebral SvO 2 . This bedside technique can be used to help assess brain health in neurologically unstable patients.

Oxygen extraction is an important parameter for assessment of brain tissue health in many critically ill patient populations at risk for altered cerebral hemodynamics. Knowledge of cerebral oxygen extraction helps the clinician balance oxygen delivery with cerebral demand thus preventing cell death secondary to metabolic failure. This quantity can be approximated from the difference between the saturation of arterial blood received by the brain and saturation of the venous blood leaving the brain. However, the gold standard for measurement of cerebral venous saturation remains a highly invasive procedure: central venous catheterization followed by direct measurement of oxygen saturation in the jugular bulb by oximetry . Although jugular catheters are commonly placed in adult critical care patients, these catheters pose substantial challenges in younger/smaller patients. Smaller, shorter necks make placement of these catheters difficult, and in-dwelling catheters can lead to complications such as line-associated thrombosis and an increased risk of line-associated infection . Thus, an easy-to-acquire, noninvasive measurement of central venous oxygen saturation (SvO 2 ) would potentially enable continuous assessment of cerebral oxygen extraction. Further, this development would provide an opportunity for the clinician to make informed decisions in balancing systemic hemodynamics to achieve optimal cerebral oxygen delivery.

Near-infrared spectroscopy (NIRS) is a widely accepted noninvasive modality for quantifying blood volume and oxygen saturation . Although it is mostly used to measure tissue oxygen saturation, a handful of publications have shown that NIRS can measure venous oxygen saturation. Typically, such schemes require a functional perturbation of cerebral blood volume to isolate the venous component of the optical signal, such as tilting of the head or venous occlusion . However, these measurements are feasible only during perturbation and thus do not offer the possibility of continuous monitoring of SvO 2 . Furthermore, the potential to use such perturbations in clinical settings is limited. Postural manipulation in preterm infants, for example, has been shown to cause changes in intracranial pressure, increasing the risk of intracranial hemorrhage .

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Materials and methods

Population

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Study Protocol

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NIRS: Theory, Instrumentation, and Analysis

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Statistical Analysis

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Results

Validation Study

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Table 1

Median (IQR) of Patient Demographics

Median (IQR) Age (y) 5.3 (1.3, 9.7) Weight (kg) 15.3 (8.7, 24.1) Height (cm) 99.9 (69.9, 124.8) Sex (male:female) 5:5

IQR, interquartile range.

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Table 2

Median (IQR) of Patient Measurements during Cardiac Catheterization

Median (IQR) Respiration rate (breaths/min) 20 (17, 20) Heart rate (beats/min) 103 (87, 134)SVOSVC2 SVO

2

SVC (%) 64 (62, 67)SvONIRS2 SvO

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NIRS (%) 59.6 (56.4, 65.9)SNRHb S

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2 , signal-to-noise ratio of the oxy-hemoglobin signal; SvONIRS2 SvO

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NIRS , venous saturation measured noninvasively with near-infrared spectroscopy; SVOSVC2 SVO

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SVC , cerebral venous oxygen saturation from a superior vena cava blood sample.

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Healthy Volunteers

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Discussion

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Conclusions

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Acknowledgments

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