Non-invasive methods to monitor ventilation are valid surrogates for arterial blood gas analysis.
Growing use of non-invasive ventilation, growing numbers of procedures carried out under conscious sedation and growing awareness of the adverse effects of sleep apnoea are only three of various trends in modern medicine that call for continuous monitoring of ventilation, preferably continuous non-invasive monitoring of ventilation.
A variety of methods to monitor ventilation are available. For a better understanding of the advantages and limitations, it might prove helpful to recall the link between ventilation and the basic physical phenomenon of passive diffusion, i.e. the movement of molecules from a region of high partial pressure to a region of low partial pressure. This phenomenon is utilized even by the most primitive forms of life in order to ensure the necessary gas exchange between the medium surrounding the organism and the internal metabolism. For the multi-cellular human organism, however, diffusion through the body surface alone is insufficient, especially in order to supply adequate oxygen to and to eliminate carbon dioxide from cells and tissues. Therefore the respiratory system facilitates the passive diffusion of these gases by providing a huge additional respiratory surface, the walls of the alveoli. Ventilation, which can be spontaneous (breathing) or artificial (e.g. mechanical ventilation), describes the movement of air between the environment and the alveoli walls, where the gas exchange by passive diffusion can occur between blood and air at high rates. The purpose of ventilation is to ensure the efficiency of the passive diffusion through the walls of the alveoli by maintaining higher concentrations of oxygen and lower concentrations of carbon dioxide in the alveolar gas than those prevailing in the blood flowing in the alveolar capillaries. In other words, ventilation enables the organism to use the phenomenon of passive diffusion to maintain physiologically normal partial pressures of oxygen and carbon dioxide. The efficiency of ventilation can be monitored well by assessing the resulting arterial levels of oxygen and carbon dioxide.
Analysis of arterial blood samples has been the standard method to measure ventilation parameters like arterial oxygen partial pressure (PaO2), arterial oxygen saturation (SaO2) and partial pressure of carbon dioxide (PaCO2). But sampling arterial blood is invasive, carries the risk of infections, involves big amounts of disposables and provides only snapshot information. In the case of intermittent arterial puncture, it is also associated with pain and discomfort for the patient.
Pulse oximetry (SpO2), end-tidal capnography (PetCO2) as well as transcutaneous oxygen partial pressure (PtcO2) and transcutaneous carbon dioxide tension (PtcCO2) measurement are used as non-invasive approaches to either continuously estimate arterial levels of these parameters or determine trend changes.
Pulse oximetry optically assesses the colour of the blood, which changes with the oxygen saturation (SpO2) of the haemoglobin. The method is used to detect hypoxia, while over-oxygenation remains undetected due to the S-shape of the oxygen-haemoglobin-dissociation-curve. Arterial oxygen partial pressures of above 60 mmHg correlate with arterial oxygen saturation readings above 90%1. Further increases of PaO2 cause the saturation readings to near or equal 100%, but will not cause them to exceed 100% even in the case of extreme oxygen partial pressures. This particular shape of the oxygen-haemoglobin-dissociation-curve is also the reason for one of the major limitations of the method in terms of ventilation monitoring: while supplemental oxygen is administered to the patient, the partial pressure of oxygen might be elevated above physiologically normal levels. Then, in an event of hypo-ventilation, the oxygen saturation readings provided by optical pulse oximetry will remain above 90% until the partial pressure of oxygen has dropped to less than approximately 60 mmHg. Clinical research shows that a decline in SpO2 appears to be a reliable indicator of ventilatory abnormalities during room-air breathing, while the detection of such abnormalities may be delayed or even remain undetected in the presence of supplemental oxygen2. Under the administration of supplemental oxygen, the detection of adverse physiological trends in ventilation might be facilitated by additionally monitoring arterial carbon dioxide partial pressure, as the adverse physiological trend may be reflected by changes in the PaCO2 despite the still adequate arterial oxygen saturation displayed by an oximeter3.
End-tidal capnometry measures PetCO2 in the exhaled gas during expiration. The flow of the exhaled gas diminishes towards the end of the expiration, and is inexistent during inspiration, leading to the typical waveform recorded by capnometers. Closest to alveolar PCO2, and thus to blood gas levels, is the ultimate reading obtained at the end of one expirational tide, on the verge of the next inspiration. This end-tidal value, although representing a mixture of tracheal, bronchial and alveolar gas, can be used to estimate arterial PCO2. The accuracy of this estimation seems to depend on various criteria, of which dilution by room air or supplemental oxygen and, in case of nasal cannula, mouth breathing are understood to have major significance4. The error levels in terms of absolute PCO2-values estimated can infringe with medically tolerable inaccuracy5. Technical development and improved algorithms constantly reduce the inaccuracies, and a sufficient accuracy of the trend in PCO2 changes indicated by this type of measurement6 seems to explain the widespread use of end tidal capnometry.
Transcutaneous measurement of blood gas levels is possible because the above mentioned gas diffusion through the body surface, while not sufficient for our multicellular organism, nevertheless takes place. Under adequate perfusion and skin conditions, the gas diffusion through the skin is closely correlated with vascular gas pressures. Transcutaneous gas pressure sensors heat the measurement site to normally 41°C to 45°C to increase gas diffusion speed. When the site is warmed, the sensors collect the gas diffusing through the skin. Within minutes a pressure equilibrium between the skin and the sensor is established. From this moment on, a continuous and very accurate estimate of e.g. the arterial PO2 and or PCO2 is provided for by elaborate algorithms.
Recent microelectronic developments have allowed the components of transcutaneous PCO2 and pulse oximetry sensors to be integrated within one digital sensor. Concomitant pulse oximetry and PtcCO2-measurement is possible by applying one single sensor to the earlobe. Sensing gases in immediate proximity to the preferred central circulation of the cranium, the digital technology assures fast and reliable measurements. Validation studies have shown excellent correlations of transcutaneous measurements in comparison with arterial blood gas analysis for both pulse oximetry and PtcCO27. The single sensors are heated to warm up the measurement site and thus to increase the local blood flow. A good perfusion does not only add to PtcCO2-measurement accuracy but also improves the quality of the SpO2 signal8. Modern PtcCO2-sensors, which only heat up to 42°C, can be kept safely on one measurement site for periods of up to eight hours9. Digital sensors amplify, digitise and pre-analyse the measurement signals directly at the measurement site. Combined earlobe sensors were found to detect changes in SpO2 5 to 37 seconds faster than an analogue finger sensor, and changes in PtcCO2 9 to 48 seconds faster than a cutaneous sensor fixed to the upper arm10. In centralised patients, a warmed sensor applied to the earlobe might be less affected by low signal conditions than peripheral sensors. PtcCO2-values can be influenced by factors such as hypoperfusion at the site of measurement, shock, oedema, skin thickness and vasoconstricting drugs11.
The clinical applications of combined pulse oximetry and transcutaneous PCO2-measurement potentially include all settings when ventilation needs to be monitored, especially when supplemental oxygen is administered. A combined non-invasive sensor for pulse oximetry and PtcCO2, measuring all three parameters continuously and in real-time, might enhance patient safety in a variety of clinical situations. During medical and surgical procedures, sedatives are very commonly used and are often administered in the absence of an anaesthetist. Following the administration of sedative medication, respiratory depression can occur, potentially requiring treatment with antidotes to the sedative and eventually causing need for assisted ventilation12. PtcCO2 rises reflecting hypoventilation have been reported e.g. in patients undergoing various endoscopic procedures (thoracoscopy, bronchoscopy, colonoscopy) under sedation and administration of supplemental oxygen13. Measuring the trends of oxygenation and carbon dioxide elimination might be useful to monitor the success of extubation and the adequacy of non invasive ventilation. Diagnosis of suspected hyperventilation can be evaluated by studying the trend of PtcCo214. Recently, combined pulse oximetry and PtcCo2 have been used along with polysomnography studies to titrate non-invasive positive pressure ventilation during the night in patients with chronic respiratory failure15.
Non-invasive methods to monitor ventilation are valid surrogates for arterial blood gas analysis when SaO2 and PaCO2 are to be assessed, and additionally provide continuous information. The different approaches have distinct limitations and should therefore be chosen carefully. Combined pulse oximetry and transcutaneous monitoring of estimated arterial PCO2 can provide the clinician with information on respiratory status that can assist with decisions to provide ventilation support. In situations where supplemental oxygen is administered to the patient, a combined measurement might facilitate the detection of ventilation abnormalities. Continuous monitoring with a combined pulse oximetry and PtcCO2 sensor has the potential to enhance patient safety in clinical settings where, without limitation, the ventilation of patients is impaired by overmedication or sedation, by obstructive or neurological causes, in patients with acute or chronic pulmonary diseases or where patients receive assisted ventilation or oxygen supplementation.
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