Effects of Non-invasive Ventilation (NIV) on Cerebral Oxygenation.

Near infrared spectroscopy (NIRS), is a non-invasive method for the measurement of blood flow in tissues, first used for cerebral tissue oxygenation in 1977. NIRS is a spectroscopic technique, which uses electromagnetic waves (700-950nm), an emitter and a detector. In the last 20 years there was an enormous development in the instrumentation and application of NIRS. This technique now allows to measure the oxygenation of the brain tissue. The INVOS Brain oximeter is a reliable trend monitor for changes in regional cerebral oxygenation (rSO2) and correlates with the hemoglobin saturation in venous, capillary and arterial blood, using an algorithm based upon the Beer-Lambert law. The INVOS Cerebral oximeter system uses light, with wavelengths between 730-810 nm, that penetrates layers of the human body, among them the skin, the scull and the brain. It is either scattered within the tissue or absorbed by present chromophores. In the rather transparent near infrared region, there are many absorbing light chromophores, but only three are important as far as the oxygenation is concerned, namely haemoglobin (HbO2), deoxyhaemoglobin (Hb) and cytochrome oxidase (CtOx). Oxygenated and deoxygenated haemoglobin absorb light at different wavelengths, allowing a differentiation of these two forms of haemoglobin. The sensors, ("SomaSensors"), are applied to the patients forehead with an integrated medical-grade adhesive. The method is applied by using two source-detector distances in the sensor: a "near" one (shallow), 3 cm from the source and a "far" one (deep), 4 cm from the source. Both samples penetrate the tissue beneath the light source equally well, with the difference that the 4cm source-detector measures signals deeper in the brain or other parts of the body. The subtraction of the near sample from the far one should leave a signal originating predominantly in the brain cortex.

The proper management of brain oxygenation is an essential component of all anesthesiologic and intensive-care procedures. Nevertheless, the brain remains one of the least monitored organs in the perioperative phase and intensive care therapy. Up until now, the anesthesiological application of NIRS as a method for measuring the cerebral oxygenation has only been investigated in a health population, patients undergoing cardiac surgery or cerebrovascular surgery, elderly patients undergoing major abdominal surgery and neonatal infants. If NIRS detects a decrease of rSO2 in these fields, there are specific guidelines of interventions to regain a sufficient oxygen supply in the brain.

Therefore, it is already strongly involved in the patients care in the perioperative setting. A routine use of NIRS on the ICUs has not yet been established. For this reason most studies on rSO2 were performed during surgery. These studies indicate that the measured cerebral oxygenation is affected by the relative proportion of blood in the arterial or venous part of the capillary bed, the haemoglobin concentration, and the systemic saturation.

The precise consequences of alterations in the systemic saturation (SaO2) on the cerebral oxygenation (rSO2) remain unknown. It is a current assumption that rSO2 is directly correlated to Sa02, so that an increase of SaO2 also leads to an increase of rSO2.

The amount of oxygen in the arterial blood depends on the inspired oxygen and the pulmonary gas exchange. These two starting points are primarily affected by the individual's respiratory ventilation. Patients with chronic respiratory failure, or just a temporary (acute) breathing deficiency (e.g. after general anaesthesia), are routinely treated with the continuous positive airway pressure (CPAP) therapy, a non-invasive form of ventilation. It is also commonly used in the treatment of sleep apnea and in neonates (especially premature infants). In these patients CPAP ventilation may prevent the need of tracheal (re-) intubation, or enable earlier extubation. The therapy was developed by Dr. George Gregory and colleagues in the neonatal ICU at the University of California, San Francisco in 1971, and then modified by Professor Colin Sullivan at Royal Prince Alfred Hospital in Sydney, Australia, in 1981. Initially the CPAP therapy was mainly used by patients for the treatment of obstructive sleep apnea at home. Nowadays it is commonly used in ICUs as a form of non-invasive mechanical ventilation. There it is usually reserved for subgroups of patients where the oxygen treatment via a facemask is insufficient. Patients on CPAP therapy are closely monitored in the ICU setting. The treatment supports the patient's spontaneous breathing by building up a positive end expiratory pressure (PEEP). Typically CPAP is a discontinuous therapy on the ICU that means Patients take it off during meals, or just use it if they require temporarily respiratory relief. The pressure applied by most patients ranges between 5 and 12 cmH2O. The patient can individually determine his or her own respiratory frequency as well as the depth of respiration.