INVOS Frequently Asked Questions

What is the INVOS?

The INVOS^® Cerebral Oximeter is a trend monitor of brain hemodynamics. It is a device which uses near-infrared spectroscopy to measure changes in the balance between oxygen supply and demand, predominately in the brain.

What does INVOS mean?

In-Vivo Optical Spectroscopy.

How does it work?

The INVOS Cerebral Oximeter passes harmless, low-intensity near-infrared light into the patient's forehead where it penetrates the skull and passes through the cerebral cortex. By measuring the returned light at two distances from the light source (3 and 4 cm), the spectral absorption of blood in the brain can be determined.

How many wavelengths are used?

The INVOS Cerebral Oximeter generates two wavelengths of light (730 and 805 nm) using two LEDs (light-emitting diodes) which are alternately illuminated.

How deep in the brain does the measurement reach?

Because of the characteristics of extracranial tissue and skull, light which is detected at least 2.5 cm from the light source has penetrated to the dura mater and cerebrospinal fluid (CSF).^{1, 2} At 3 and 4 cm, the detected light absorption originates primarily in the gray matter blood with some white matter blood. Of course, this is variable across individuals.

How does the light get through the skull?

In the near-infrared band, light scatters easily through mammalian tissues. Even visible light diffuses through bone which is somewhat translucent due to lower blood volumes.

Where do I place the SomaSensor^®? Can I place it over hair or on other areas of the head?

The SomaSensor is designed to be placed in the front-temporal region only (forehead, either right or left of midline). Placement over scalp, even when shaved, will result in very low return signals due to absorption of light by hair follicles.

Can I reuse the sensors if I'm careful?

When the sensor has been removed from the patient, it has a layer of dead skin cells on the adhesive. This interferes with the adhesive's optical properties and adhesion, resulting in signals which may reflect a normal baseline but will not respond to changes in rSO₂ accurately.

Why is the Cerebral Oximeter a bilateral device?

Many times a patient may exhibit unequal perfusion patterns bilaterally, especially patients with atherosclerotic vascular disease or elderly patients. Bilateral monitoring can help distinguish between unilateral oxygen imbalances, i.e. focal ischemia, blood steal, versus those caused by systemic pathologies.

What is the rSO₂ index?

The rSO₂ index is a measure of the oxygen saturation of the mixed arterial and venous blood in the brain cortex. Since the volume of blood is predominately venous, it reflects the balance between oxygen supply and demand in the brain. It is referred to as an index because it is not easily validated in vivo, is regional in nature and may not reflect events occurring at a distance from the sensor.

What is the update rate for the displayed rSO₂?

A new rSO₂ value is displayed on the screen approximately every four seconds. Each four-second sample represents the most recent data (there is no averaging with previous samples).

How accurate is the INVOS?

Comparison with blood samples was evaluated in a recent volunteer hypoxia study where 42 healthy subjects were measured at five levels of oxygenation, which were then repeated at a higher cerebral blood flow.³

How accurate is the pediatric sensor?

A study of 22 congenital heart malformation patients in the pediatric Cardiac Cath Lab compared oximetry data with blood sample co-oximeter analysis of jugular venous saturation and arterial saturation combined in a 3:1 ratio. Over a range of 50% to 95%, trend accuracy was ±1.6% and absolute accuracy was ±4.7%, both one standard deviation.

Where does the 3:1 venous to arterial compartment partitioning come from?

Like other tissue, the cerebral vasculature has been previously described as having a greater volume of venous blood than arterial blood. Recent research suggests that between 60% to 100% of cerebral blood is venous by volume.⁴ Although this volume cannot be validated in vivo, INVOS values correlate well when a 75% venous volume⁵ is assumed during volume changes which occur as a result of changes in PaCO₂.³

How does an arterial-venous compartment shift affect the accuracy of the measurement?

First, the 75/25 ratio is only a rough average of what we believe the mean ratio in most healthy individuals to be. Second, when compartment ratios change without a change in blood volume, the accuracy of the oximeter does not change at all. What changes is the comparison between the rSO₂ index and a known measurable parameter such as jugular venous oxygen saturation (SjvO₂). This change does not affect clinical utility however since the rSO₂ index still represents oxygen availability in the brain. Third, INVOS responses to known compartmental shifts are very small. Consider the response during retrograde cerebral perfusion (RCP).

Describe the INVOS Cerebral Oximeter's response to retrograde cerebral perfusion (RCP).

During RCP, highly oxygenated blood is pumped into the cerebral venous system and flows retrograde into the arterial system, providing oxygen to the brain as it passes through. When RCP is initiated, there is an instantaneous shift from predominantly venous to predominantly arterial blood in the brain as the two compartments are swapped. This change, however, does not cause the expected spike in rSO₂ index but is represented by a more gradual change in saturation which can increase as a result of the incoming oxygen or decrease slowly, as shown in the graph, during low flow.

How do changes in blood volume affect the accuracy?

With regard to changing blood volumes, the volunteer hypoxia validation study³ examined accuracy at normo- and hypercapnia. A mean decrease of 5 mmHg in etCO₂, which would cause about a 5% decrease in blood volume in the cortex,⁶ causes a small increase in the slope of the fSO₂-rSO₂ regression, increasing the sensitivity of the INVOS Cerebral Oximeter to brain oxygenation changes. This is consistent with the change in light absorption, which decreases when hemoglobin decreases, resulting in an increase in the depth of penetration of light. This change is small: theoretically, a 20% change in blood volume will cause a 10% saturation change in the patient to be in error by 0.8. The rSO₂ reading would reflect either +9.2 for increased hemoglobin volumes or would overestimate decreases reading -10.8, erring on the positive side.³

Does temperature affect the accuracy of the readings?

No. The SomaSensor is designed to measure temperature changes and compensate for intensity changes as a function of temperature. However, the rSO₂ reading is not corrected for right-left shifts in the oxyhemoglobin dissociation curve and the data must be interpreted according to the patient's temperature and blood pH, also like a co-oximeter or pulse oximeter.

How does extracranial blood affect the measurement?

Extracranial blood has a very small effect on rSO₂, except in cases where the extracranial blood volume is artificially increased, such as with a head tourniquet or hematoma. Several studies of carotid endarterectomy (CEA) where the external carotid artery was clamped separately to assess the effects of scalp ischemia on rSO₂ have shown its effect to be quite small, assuming the external carotid is not perfusing brain tissue.^7,8 Another study showed a significant correlation between rSO₂ and SjvO₂ during CEA but no correlation between rSO₂ and the oxygen saturation of blood in the facial vein during clamping, despite the fact that changes were more pronounced in the facial vein than in the jugular.⁹ Another CEA study showed that the use of dual detectors by the INVOS Cerebral Oximeter resulted in a 4 times reduction in the effect of extracranial blood than a similar device with only one detector.¹⁰

How much of a change is significant? What is the threshold of concern?

In every controlled study of carotid endarterectomy to date, changes in the rSO₂ index of 12-20 points (absolute) or 20%-30% (relative) correlated with changes in the patient's neurological status.^11-14 In these studies, as well as in a large study of cardiac surgery patients, values of rSO₂ index less than 50 were associated with higher probabilities of a poor outcome.

Why do some patients with low values show no pathology?

The threshold of concern for rSO₂ values represents an average value where most subjects will eventually experience problems, similar to thresholds of blood gases or blood pressure. However, there are individuals who can tolerate lower values or longer durations, seemingly without pathology. Actually, patients can experience a frontal lobe injury which is not immediately apparent (frontal lobe injury affects executive function and is generally non-eloquent). Typically, patients with pre-existing pathologies are less tolerant of low rSO₂ values and end up doing worse following a desaturation.

Can the INVOS Cerebral Oximeter measure cerebral blood flow (CBF)?

No. The INVOS readings may not even correlate with changes in CBF unless there is an ischemic episode. In a healthy individual, CBF can change continually in response to changes in blood pressure, carbon dioxide and metabolic rate, while rSO₂ index will likely remain relatively constant. The INVOS Cerebral Oximeter is designed to measure changes in brain blood oxygen saturation and can provide an early warning of cerebral hypoperfusion.¹²

Can the INVOS Cerebral Oximeter measure cytochrome oxidase?

No. Cytochrome is an oxygen-dependent chromophore found in the mitochondria of brain cells in quantities typically 10 times less than hemoglobin. Because it represents oxygenation at the cellular level, it provides information on tissue oxygen uptake. In addition to low volumes, the absorption spectra of cytochrome varies over a range which is 10 times smaller than hemoglobin, making the optical signal two orders of magnitude (100 times) less than the hemoglobin signal. Cytochrome monitoring is virtually impossible to validate using other monitors and it is not a familiar parameter with most clinicians. For these reasons, the INVOS Cerebral Oximeter was not designed to measure cytochrome oxidase.

Does the INVOS Cerebral Oximeter measure tissue oxygen uptake?

No, it measures blood oxygen saturation of hemoglobin in a region of the brain. If the oxygen affinity of hemoglobin is left-shifted (higher affinity) and off-loading of oxygen to tissue is impaired, the INVOS values may not reflect this and may show high or normal values during these periods as does a pulse oximeter. This condition may occur during cardiac surgery where the patient is hypothermic and frequently acidotic. However, numerous studies in cardiac surgery have shown that hypoperfusion is much more common and that simple interventions aimed at increasing perfusion and/or oxygenation can improve patient outcomes and reduce costs.^14-17

What happens when I place the sensor on a brain-dead patient?

Following herniation of the brain, normal to slightly elevated rSO₂ values can be expected. This is also the case when the sensor is placed over a previously infarcted area of brain tissue in a stroke victim.¹⁸ This is assumed to be caused by normal oxygen diffusion through tissues in the absence of blood flow or drainage of scalp blood into the cranial vault through the diploic veins. If oxygen consumption is non-existent, a normal to moderately high rSO₂ value can be expected. On the other hand, the penumbra area around an infarct can be expected to be low in the absence of therapeutic interventions.¹⁸ In cases where the sensor is placed on cadavers and near normal readings are obtained, the expected cerebral venous saturation after death can range from 0% to 95% and is dependent on the conditions of storage, not the cause of death.¹⁹

Why does the sensor sometimes give a reading when it is not on the patient?

Like a television remote control, the sensor can bounce near-infrared light off the wall or ceiling of a room. When room light is normal, the INVOS Cerebral Oximeter is designed to detect the room light and reject these signals from the sensor. In a darkened room, however, the ambient light may not be bright enough to trigger rejection and this may produce readings typically around 70. This can also occur when the sensor is adhered to certain types of tabletop such as laminated plastic, which mimics a critical optical property of human tissue. However, when the sensor is properly adhered to the patient, it will reject signals which do not represent brain tissue as described by Sehic et al. in their case report of a patient with a frontal skull defect.²⁰

What are the sensitivity and specificity of the INVOS Cerebral Oximeter?

Since cerebral oximetry is a relatively new monitoring modality, data is limited as to its sensitivity and specificity in detecting true cerebral hypoxia. While cerebral oximetry has the potential to identify global events such as hypoperfusion, hypoxia or anemia, its regional nature limits its ability to detect focal events distant from the sensor. In a 99-patient study of the first-generation INVOS 3100, the sensitivity and specificity in detecting changes in neurologic status during awake carotid endarterectomy were 80.0% and 82.2% respectively.¹² Negative predictive value (NPV), the ability of the monitor to indicate that everything is OK, was very high, 97.4%. In another awake 50-patient study,¹¹ a -25% relative change from baseline was 100% sensitive and specific while a 54-patient study²¹ comparing the INVOS 3100A to EEG changes found a sensitivity of 94.0% and specificity of 82.0%.

How can I retrieve data using the INVOS Cerebral Oximeter?

The INVOS 4100 and 5100 have four methods to retrieve and archive rSO₂ data: an analog output for use with a data collection or recording system, an RS-232 output suitable for communications with any Windows-based PC, an optional 3.5" floppy disk drive or an optional printer.

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