Extracranial contamination in the INVOS 5100C versus the FORE-SIGHT ELITE cerebral oximeter: a prospective observational crossover study in volunteers

Abstract

Purpose Previous studies have found that most cerebral oximeters are subject to inaccuracies secondary to extracranial contamination of the cerebral oximetric signals. We hypothesized that the more advanced secondgeneration FORE-SIGHT ELITE cerebral oximeter would be significantly less affected by extracranial tissue hypoxemia than the more widely used first-generation INVOS TM 5100C monitor. Methods Twenty healthy volunteers aged 18-45 yr had the INVOS and FORE-SIGHT probes placed on their forehead in a random sequence while in the supine position. A pneumatic head cuff was then placed around each volunteer's head just below both the oximeter and a concomitantly placed scalp forehead pulse oximeter probe. The subjects' scalp cerebral oxygen saturation (SctO 2 ) values were measured and compared using the two different devices in sequence, both before and after scalp tissue ischemia was induced by the pneumatic cuff. Results Extracranial ischemia resulted in a significant reduction in SctO 2 values from baseline in both devices. The INVOS 5100C recorded a median [interquartile range] decrease in SctO 2 from baseline at five minutes of 15.1% [12.6 -17.6], while that recorded by the FORESIGHT ELITE device was 8.6% [4.0 -12.3] at five minutes (median difference, 7.9%; 99% confidence interval, 1.9 to 16.5; P = 0.002). Conclusion Updated technological algorithms employed in the FORE-SIGHT ELITE cerebral oximeter may be responsible for less extracranial contamination than was observed in the previous-generation INVOS 5100C device. The impact that this extracranial contamination may have on the clinical use of these devices remains to be determined. RésuméObjectif Selon les études réalisées par le passé, la plupart des oxymètres cérébraux sont sujets à des inexactitudes suite à une contamination extracrânienne des signaux d'oxymétrie cérébrale. Nous avons émis l'hypothèse que l'oxymètre cérébral FORE-SIGHT ELITE, l'oxymètre cérébral de deuxième génération le plus innovant à ce jour, serait significativement moins affecté par l'hypoxémie des tissus extracrâniens que le moniteur de première génération INVOS TM 5100C, plus répandu.

Cerebral oxygenation and vascular resistance changes during cardiopulmonary bypass – where is the proof?

“…Indeed, a vasoconstrictive effect on rSO 2 is far more likely a manifestation of the inherent extracranial contamination that variably occurs with cerebral oximeters, particularly the one that they used in their study (INVOS 5100 cerebral oximeter; Somanetics Company, Troy, MI, USA) [6]. It is far more likely that this is an extracranial vasoconstrictive effect, as noted by Sørensen et al [8] who found that a a 1 agonist (norepinephrine) caused a reduction in directly measured cutaneous blood flow and a reduction in the overall rSO 2 using the same oximeter as Sperna Weiland et al…”

“…It seems that NIRS measures both intra-and extracranial perfusion and this may have influenced our results [8]. However, others have demonstrated an increase in transcranial Doppler-derived mean cerebral artery blood flow velocity with a converse decrease in rSO 2 after phenylephrine infusion [3].…”

“…Perhaps the way forward might be to repeat this experiment in a larger number of patients, and with other cerebral oximeters that cause less extracranial contamination [8], as opposed to using the oximeter that has the greatest degree of extracranial contamination and thus the highest likelihood of extracranial-related rSO 2 measurement errors. We agree with Grocott that a 1-2% decrease in rSO 2 is small and will probably not directly lead to worsened clinical outcome.…”

Brain natriuretic peptide (BNP) may play a major role in risk stratification based on cerebral oxygen saturation by near-infrared spectroscopy in patients undergoing major cardiovascular surgery

“…However, significance of absolute ScO 2 values has not been established, since they are influenced by multiple factors such as a composition of focal arterial/venous blood components, oxygen saturation in extra-cerebral tissues, blood hemoglobin concentration (Hgb), and the skull thickness [2, 13–17]. In addition, ScO 2 values derived from different NIRS devices can differ even within the same subjects [14, 15, 17, 18]. Therefore, ScO 2 currently is used as a trend monitor rather than as an absolute index of cerebral oxygenation [2].…”

Phenylephrine and paradoxically increased muscle tissue oxygenation: is the mechanism related to local venoconstriction or augmented venous return?

“…As the NIRS SmO 2 signal is typically derived assuming a fixed ratio of arterial:venous hemoglobin, decreasing the venous component (due to venoconstriction) would increase the more oxygenated arterial component with a resulting overall increase in SmO 2 . Although this could very well be the mechanism at work, the NIRS device that was used in their study (Invos 5100, Medtronic, Dublin, Ireland) uses an algorithm in which the arterial:venous ratio of blood is fixed and is based on that derived from studying the brain [2,3], not skeletal muscle, adding uncertainty to this proposed mechanism. Additional uncertainty is that the increase in SmO 2 could also have been simply a function of an increase in venous return, which the α-adrenergic agonist phenylephrine is known to affect by augmenting the stressed component of the venous compartment [4], and consequently the cardiac output which could increase oxygenation of the muscle.…”