Near-infrared spectroscopy (NIRS) is a technique that measures regional cerebral oxygen saturation (rScO2). By using one detector near, and one or more distant, from the light source, it is theoretically possible to subtract the signal from superficial layers and obtain a signal from the cerebral tissue with limited extra-cerebral contamination.1
To be able to rely on such measurements carried out with the patient in the prone position as, for example, during spine surgery, it is important to know whether the rScO2 readings are affected by the external pressure on the forehead sensor in this position. In a previous study, it was suggested that in patients placed in the prone position, a relieved pressure on the extra-cranial tissues below the sensor (head lift from the head support) might distort the rScO2 value on the monitor.2 The aim of the present study was to confirm and quantify this in a protocolled study. The primary endpoint was the increase in rScO2.
The prospective study was approved by the Danish Regional Scientific Ethics Committee of the Capital region (approval number: H-4-2013-FSP 78) as a quality assurance project on 5 September 2013 (chief Adviser Lone Gundelach) and was registered at Clinical Trials.gov (ID NCT01971996). The study was approved by The Danish Data Protection Agency. Written and informed consent was obtained from all patients. Patients scheduled for lower back surgery were enrolled. Inclusion criteria were age above 18 years and free painless extension of the neck. Exclusion criteria were known disease of the cervical spine.
A self-adhesive NIRS pad (Adult SomaSensor; Covidien, Troy, Michigan, USA) was attached on the right side of the forehead, 1 to 2 cm above the eyebrow and at least 2 cm lateral to the midline so that the diode and optodes were clear of the frontal sinus. The pads were connected to an INVOS oximeter model 5100C (Covidien, Troy, Michigan, USA). A cerebral state monitor (CSM, Danmeter, Denmark) was used to register the depth of anaesthesia during the measuring period. Anaesthesia was induced and maintained with propofol and remifentanil. After tracheal intubation, the lungs were ventilated using a volume controlled mode with a 1 : 1 oxygen air mixture. The patient was then placed in the prone position. The head was supported on the forehead, the cheek and the jaw by a foam head support (Proneview Helmet, Dupaco, California, USA). Steady-state anaesthesia was defined as obtained when there had been no change in positioning of the operating table or in the inspiratory oxygen fraction (FiO2), no use of vasoactive drugs or inotropes and no change in drug infusion rate for a period of at least 30 min. During steady-state anaesthesia, measurements were carried out in three series of 5 min, one with the head resting in the head support, one with the head lifted a few mm to alleviate pressure and one with the head again resting in the head support. During each series, the NIRS values were registered every 15 s; cerebral state index (CSM) was registered every minute, and blood pressure, heart rate (HR), SpO2 (all measured on an Intelli Vue, MP70, Philips, Denmark) and EtCO2 (Primus, Dräger, Germany) were registered immediately before and after each 5 min series.
Using a type I error of 0.05 and a type II error of 0.8, the sample size was calculated using data from the above-mentioned study and 10% was chosen to be a clinically relevant change.1 Fifty-seven patients completed the study. The results of the NIRS measurements are shown in Fig. 1. A transient increase in rScO2 was observed when the head was lifted from the support. The maximum difference in median value was 5 rScO2 units (7%). There was no difference between the rScO2 values obtained in the two series of measurements during head rest (P = 0.4). SpO2, mean aterial pressure, HR EtCO2 and cerebral state index were all constant during the three measurement series.
We found no difference between the rScO2 values obtained in the two series of measurements during head rest (P = 0.4), but a transient increase in rScO2 was seen during head lift. The change in measurement is largest in the beginning of the head lift and decreases within a few minutes. Such an initial response is supportive of the idea that there is some ischaemia in the scalp followed by hyperaemia as the pressure on the sensor is relived, and that this is responsible for the signal differences. The gradual shift back to baseline values that we observed during the head lift might support this hypothesis.
On the basis of the relatively small changes in the rScO2 readings induced by the head lift, we conclude that the prone position and the concomitant pressure on the tissue at the NIRS pad do not change rScO2 to a clinically relevant magnitude.
Acknowledgements relating to this article
Assistance with the study: we would like to thank all the dedicated anaesthesia nurses for their assistance with the study.
Financial support and sponsorship: the Toyota foundation is thanked for the donation of the NIRS monitor.
Conflicts of interest: none
1. Murkin JM, Arango M. Near-infrared spectroscopy as an index of brain and tissue oxygenation. Br J Anaesth
2009; 103 (Suppl 1):i3–i13.
2. Andersen JD, Baake G, Wiis JT, Olsen KS. Effect of head rotation during surgery in the prone position on regional cerebral oxygen saturation: a prospective controlled study. Eur J Anaesthesiol