A large number of interventional radiologic procedures are performed under monitored anaesthesia care (MAC). This entails the administration of low doses of intravenous (i.v.) anaesthetic drugs, oxygen via a nasal cannula or face mask and standard monitoring in spontaneously breathing patients . Despite its benefits in improving patient comfort, procedural sedation/analgesia may potentially worsen airway patency, leading to apnoea, hypoventilation and hypoxia. Capnography monitoring can detect early signs of respiratory depression during MAC . The CapnomaskTM (GHW group, Meylan, France) is a newly developed oxygen face mask with an end-tidal CO2 (etCO2) sampling line intended for use in spontaneously breathing patients under light sedation. We examined its feasibility for capnography monitoring and patients' tolerance in the supine and prone positions during MAC.
Forty-five patients (ASA II-III, 24 males/21 females, age: 65.5 ± 12.9 yr, height: 164 ± 8.1 cm and weight: 66.4 ± 9.7 kg) scheduled for radio-guided percutaneous vertebroplasty or nucleotomy were prospectively included. The CapnomaskTM was placed on the patient's face with oxygen delivery (6 L min−1) and the CO2 sampling line was connected to a capnometer. All the patients received i.v. midazolam (0.03 mg kg−1) and alfentanil (15 mcg kg−1 h−1). Non-invasive blood pressure (BP), heart rate (HR), etCO2, pulse oxymetry (SPO2) and respiratory rate (RR) were noted at steady state in the supine and 5 min after placement in the prone position. Sedation (i.e. the patient's response to verbal command) was rated as mild (spontaneous), moderate (after tactile stimulation) or deep (not easily aroused). The patients' tolerance of the CapnomaskTM was evaluated, in the recovery room, using a 100 mm visual analogue scale (0 = uncomfortable to 100 = extremely comfortable).
Additionally, the CapnomaskTM was also used in 20 patients (ASA II-III, 11 males/9 females, age: 55.6 ± 11.9 yr, height: 164.5 ± 5.8 cm and weight: 62.7 ± 8.5 kg), with arterial catheters, recovering from general anaesthesia. Their blood gas analysis permitted us to calculate the difference between arterial and etCO2 partial pressures ((a-et) PCO2) in the supine position.
The main cardiorespiratory changes during the procedures are presented in Table 1. The data were similar in the two positions except for the PetCO2, RR and patient tolerance where a significant difference was observed. The (a-et) PCO2 was within the usual reported range of 3-9 mmHg. Sedation was mild in 38 out of 45 (84%) patients and moderate in seven out of 45 (16%) patients. Transient apnoea was detected, within 5 s, in two patients during the procedure with rapid recovery after verbal stimulation. Two patients had nausea and one patient vomited after the procedure. Each of these incidents was effectively treated with i.v. ondansetron (Zophren®; GlaxoWellcome, Marly-le-Roi, France) 4 mg. The rest of the study was uneventful.
Our data suggest that the CapnomaskTM is feasible for oxygen delivery and CO2 sampling during MAC. Moreover, it permitted the early detection of apnoea during the procedure. However, PetCO2 and patient satisfaction were significantly lower in the prone than in the supine position.
The differences in PetCO2 between the two positions could plausibly be explained by the improper size of the CapnomaskTM or sampling line obstruction or leakage . Indeed we used a unique adult size for all the patients despite their morphological differences. This was the only size available at the inception of our study. However, the same mask was used in the two positions for each patient.
The CapnomaskTM is particularly useful for MAC procedures because it offers the possibility of capnographic monitoring in non-intubated spontaneously breathing patients. Other devices, including nasal cannulae and face masks, have previously been used for the same purpose [4-6]. Each type of device has its advantages and limitations. The nasal sampling tubes are more comfortable, permitting very low-flow oxygen delivery rates . However, they are easily dislodged from the proper position or occluded against the nasal mucosa. Moreover, when the patients convert to mouth breathing, the nasal devices simply do not work. The CapnomaskTM is a face mask that samples expired CO2 from both the nose and the mouth. In their document on ‘Practice Guidelines for sedation and analgesia by Non-Anesthesiologists', the Task Force American Society of Anesthesiology stated that the primary causes of morbidity associated with sedation/analgesia are drug-induced respiratory depression and airway obstruction .
A major limitation of this study is the fact that arterial CO2 measurements were only done in the supine position. Since our main intention was to detect respiratory depression, it was not necessary for us to know the precise value of the arterial PCO2 in this setting.
In conclusion, the CapnomaskTM is feasible for CO2 sampling during MAC. However, its reliability and tolerance are significantly reduced in the prone position. The main advantage of the device is the early detection of apnoea, not the exact etCO2 measurement.
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