THRIVE Ventilation for Operative Hysteroscopy Under General Anesthesia

Background Transnasal Humidified Rapid-Insufflation Ventilatory Exchange (THRIVE), term coined by Patel and Nouraei, refers to a technique that allows to extend the apnoeic window in patients undergoing general anesthesia by delivering 100% of heated and humidified oxygen at 70L/min using the Optiflow THRIVE-TM system (Fisher and Paykel Healthcare Ltd, Auckland, New Zealand), in order to maintain viable gas exchange during an extended period of cessation of muscular activity.

Apnoeic oxygenation is a physiological phenomenon in which oxygen flow into the lungs is driven by a negative pressure gradient generated by the difference between the alveolar rates of oxygen absorption and carbon dioxide excretion.

Potentially rapid and dangerous rise in carbon dioxide concentration is prevented by the underlying mechanisms occurring during high-flow nasal oxygenation under apnoeic conditions.

As proposed by Slutsky and more recently by Hermez, cardiogenic oscillations (changes in airway gas flow and pressure synchronous with the cardiac cycle) and their interactions with supraglottic turbulence generated by high-flow nasal oxygenation might be the underlying mechanisms of THRIVE and of the enhanced clearance of carbon dioxide.

Many clinically relevant benefits have been associated with nasal high flow. It provides a constant FiO2 by delivering the gas at flow rates that exceed the patient's peak inspiratory flow rate. It is effective in providing a flow-dependent effect of continuous positive airway pressure of about 1 cmH20 for every 10 L/min increase in gas flow within a range of 30-50 L/min, up to 11.9 cm H20 at a maximum flow of 100 L/min. Moreover, it reduces respiratory rate, it clears in a dose-dependent manner dead space of the upper airways and possibly even below the soft palate, leading to a reduction in rebreathing of expired air, with the potential to increase the alveolar volume and to improve alveolar ventilation and gas exchange. It is likely that, especially in patients with mild to moderate respiratory failure, enhanced comfort from heated humidification and flushing of dead space, lessen the work of breathing, thus improving thoraco-abdominal synchrony. By contrast, in spontaneously breathing patients, as a consequence of the large amount of air inflated into the stomach, higher number of reflux events could be detected increasing the risks of aspiration.

This technique has been successfully applied in several clinical settings to extend the apnoeic window after the induction of general anesthesia and during laryngoscopy in predicted difficult airway management. As long as airway patency was present, it succeeded in extending the apnoeic window up to 65 minutes. PaO2 and peripheral oxygen saturation remained stable and the rate of rise of CO2 during apnoea was reduced from earlier findings of 0.5 kPa/min with conventional low-flow systems to 0.15 kPa/min.

THRIVE has also been used as unique airway management technique for procedural sedation or when general anesthesia was performed for brief surgical procedures: it was able to keep patients with mild systemic disease well oxygenated, maintaining PaO2 stable with decreased rates of rise of CO2 during apnoea, as compared to conventional low-flow systems for oxygen therapy, such as the Venturi mask.

Operative hysteroscopy is a brief surgical procedure (< 30 minutes), usually performed in a day-surgery regimen. Typical anesthesiologic management in this setting includes general anesthesia with intravenous agents (propofol plus fentanyl) and positive pressure ventilation through laryngeal mask (I-gel). The application of THRIVE in this setting has potential advantages either for the patient, including minimal airway manipulation and greater comfort, either for the anesthesia provider, facilitating procedural technique and airway management.

Aim of the study The aim of this study is to investigate the effects of THRIVE apnoeic ventilation during operative hysteroscopy under general anesthesia.

Monitoring Standard perioperative monitoring will include non-invasive blood pressure (NIBP) with an upper arm cuff, 3-lead ECG, peripheral oxygen saturation (SpO2) and bispectral index (BIS). NIBP will be measured every 5 minutes before induction of anesthesia and throughout the procedure. Transcutaneous carbon dioxide (tcCO2) will be continuously monitored by Radiometer TCM5 monitor.

Study protocol Upon arrival on the operating theatre, each patient will be placed in supine position in lithotomic position; a peripheral venous cannula will be inserted on the hand or forearm and 500 ml Ringer-Lactate solution infusion will be started. Omeprazole 40 mg and dexametasone 4 mg will be given preoperatively. 3 minutes of pre-oxygenation will be performed with 100% oxygen 30 L/min delivered via dedicated Optiflow THRIVE-TM nasal cannulae. General anesthesia will then be induced with target-controlled infusion (TCI) of propofol (7 mcg/kg) through Orchestra Infusion system (Fresenius Kabi) plus fentanyl (1,5 mcg/kg). The range of maintenance of propofol concentration will be 4-6 mcg/ml, titrated to maintain a level of anesthesia monitored by BIS between 40 and 50. With the onset of general anesthesia oxygen flow will be increased to 70 L/min and maintained throughout the procedure. Head repositioning, jaw thrust, oropharyngeal cannula placement could be used to improve airway patency during the procedure. In case of episodes of oxygen desaturation, defined as SpO2<94%, or rise of tcCO2 over 45 mmHg, positive pressure ventilation by facial mask or laryngeal mask will be applied by the practitioner based on clinical judgement.

At the end of the procedure, propofol will be stopped. Paracetamol 1g, ondansetron 4 mg and ketorolac 30 mg will be administered as routine clinical practice.

The awake patient will be moved to the PACU for three hours of post-operative monitoring as required by internal practice for day-surgery procedures. Standard monitoring will include NIBP measured every 15 minutes, 3-lead ECG, and peripheral oxygen saturation (SpO2).

One hour after the end of the procedure and before discharge from the PACU the patient will be asked to rate its perceived degree of dyspnoea with the Borg dyspnoea score and its degree of comfort by Visual Numerical Scale (VAS).

In case of episodes of oxygen desaturation, defined as SpO2<94%, supplemental oxygen will be initially provided to the patient by low-flow Venturi mask with FiO2 40%.

Discharge from the PACU will require an Aldrete score of 9-10.

Sample size calculation For this pilot study no sample size calculation was performed.