Transcutaneous Carbon-dioxide Monitoring in Electrophysiological Procedures in Deep Sedation

Complex catheter ablations for treatment of supraventricular and ventricular tachycardias are performed under moderate to deep sedation.

These interventions include electrophysiological examinations such as cryo-pulmonary vein isolation as well as complex radiofrequency ablation using a 3D mapping system. In order to avoid complications and to achieve a successful ablation result, a quiet position of the patient should be ensured during the examination, which usually lasts several hours. Specific reasons for the need for sedation include:

1. Femoral access The access is, in most cases, the right femoral vein. By transseptal puncture of the atrial septum, the catheters are inserted into the left atrium, the most common target structure of the above mentioned procedures. In order to insert the catheter safely into the left atrium, the path from the right groin to the left atrium is secured by a guide rail, a so-called sheath. Due to this relatively rigid guide rail, unconsciously movements of the patient should be avoided during the examination.
2. 3D mapping system The 3D mapping system is used to create an individual, virtual, electro-anatomical map ("map of the structure and electrical activity of the heart") of the left atrium of the patient. The prerequisite for a millimeter accurate determination of the catheter position by means of magnetic fields is the patient's quiet position. Already deep breaths can endanger the catheter stability and thus the ablation result. By movements of the patient there is also the risk that the virtual 3D map no longer matches the real anatomy, this results in an increased risk of perforation in the left heart with the result of a pericardial effusion or tamponade.
3. Patient positioning The patient has to lie flat and quiet during the procedure. Keeping this position presents a challenge even for younger patients and short examination times and is therefore impractical for the patient for several hours for reasons of comfort.

The sedation usually consists of a combination of midazolam and propofol. At the beginning of the study, a midazolam bolus is administered and a low-dose propofol perfusor is started, which is increased during the course until an optimal sedation depth is reached. During ablation, opiates are also added for analgesia, depending on the procedure, consisting of fentanyl single doses or continuous administration of remifentanil with perfusor.

The sedation depth is primarily controlled clinically. The above-mentioned substances all have a respiratory depressive effect and can cause respiratory complications, in the sense of hypercapnia or hypoxia. Therefore, standard monitoring involves the continuous measurement of oxygen saturation by means of pulse oximetry (spO2) as well as the half-hourly analysis of a venous blood gas analysis to evaluate the pH and to monitor the carbon dioxide partial pressure (pCO2). For interventions in the left atrium there is also the possibility of an arterial blood gas analysis from the left atrium or, if present, arterial blood gas analysis from an arterial sheath. If abnormalities occur in the blood gas analysis, the depth of sedation is adjusted accordingly or the dosage of the various components of the sedation is adapted. In addition, there is a continuous heart rate measurement and regular non-invasive blood pressure measurements. The nurse, assisting the sedation, also provides a dedicated sedation protocol, with explanations of any change in sedation management during the procedure.

In the electrophysiology laboratory of the ulm university hospital is the option of continuous, transcutaneous CO2 monitoring using TCM 400 (Radiometer). For this purpose, an adhesive electrode (Severinghaus electrode) is attached to the forehead of the patient. Hereby, the measurement of the O2 and CO2 partial pressure in the underlying tissue can take place and with good blood flow, this value approaches the invasively gained gas values. This method ensures a continuous, non-invasive CO2 measurement. The collected parameters are stored by the TCM 400 device in an Excel spreadsheet and can be exported and analyzed after the procedure. The method was already used during complex catheter ablation as part of a small observational study, but does not yet count as standard monitoring.

So far, it is unclear to what extent a continuous, transcutaneous CO2 monitoring can prevent sedation-associated complications. The aim of this research project is the prospective, randomized analysis of the benefits of continuous, transcutaneous CO2 monitoring + standard monitoring in comparison to the previous standard monitoring. In particular, it should be investigated to what extent sedation-associated complications, such as oxygen saturation decreases, hypercapnia and respiratory acidosis can be prevented by continuous, transcutaneous CO2 monitoring.