Spontaneous Breathing

Over the past two decades it has become a well-established fact that general anesthesia may impair the respiratory system. Neuromuscular blockade and subsequent controlled ventilation are a suspected cause of respiratory impairment. Besides parameters for controlled ventilation, intraoperative spontaneous breathing and spontaneous ventilation may be an option for further improvement.

The discussion on protective ventilation in the OR has focused on how parameters of mechanical ventilation can be optimized to protect a patient’s lungs. Isn’t there more to this topic than parameters of mandatory mechanical ventilation? Wouldn’t spontaneous breathing be more beneficial, either as early as possible towards the end of general anesthesia, or as soon as possible after securing the airway?

There is literature suggesting the positive effects of intraoperative spontaneous breathing¹. While lung aeration during mandatory ventilation is impaired and may lead to compromised ventilation/perfusion mismatch, spontaneous breathing appears to favor the dependent parts of the lungs and thus may lead to a more physiological ventilation distribution. 

As the literature suggests, spontaneous breathing of the anesthetized patient alone might be insufficient to achieve the targeted oxygenation; it might be better supported, for example by pressure support ventilation (PSV). Potential advantages of PSV are better patient-ventilator synchrony and the associated decrease in the work of breathing and improved breathing comfort². In addition, one trial has confirmed that intraoperative PSV in patients with a laryngeal mask airway (LMA), as compared to continuous mandatory ventilation, reduces anesthesia emergence time and Propofol consumption³.

Emergence from anesthesia and the entire postoperative phase are critical periods where pulmonary complications are most frequent. Anesthesia care providers usually strive to keep the depth of anesthesia as flat as the surgical procedure allows. Towards the end of surgery, the anesthesia care provider discontinues anesthetic delivery to allow for spontaneous breathing and a subsequent extubation. 

The wash out of volatile anesthetic drugs may be performed gradually or quickly by means of high fresh-gas flows and/or hyperventilation. The latter happens at a cost of a decreased PaCO₂ and the subsequent risk of reduced cerebral blood flow. In addition, this quick approach may not leave sufficient time for appropriate drug redistribution from less perfused tissues with higher solubility. A rise of anesthetic drug levels in the central compartments and a subsequent respiratory depression in the recovery room could follow. Furthermore, lowering PaCO₂ reduces respiratory drive and may delay the return of spontaneous breathing^4,5.

A smoother approach to emergence could be the reduction of the depth of anesthesia well before the end of surgery, promoting the early onset of spontaneous breathing.

Particularly after long cases, the reduction in the depth of anesthesia may be initiated earlier rather than later. Consequently, spontaneous breathing may also return earlier but is likely to be insufficient to maintain proper arterial oxygenation. To prevent hypoxaemia during this phase, adequate support for the spontaneous breathing effort may be beneficial⁶. It has been stated that spontaneous breathing during general anesthesia is associated with hypercapnic acidosis and an increased work of breathing in both healthy patients and those with known comorbidities. In order to smooth this process, assisted ventilation modes have been introduced in anesthesia machines. 

Pressure support ventilation (PSV) is now commonly available and is intended to support spontaneous breathing while reducing patient-ventilator dysynchrony⁶. Beyond this, PSV has also been shown to provide more effective gas exchange compared to unassisted CPAP ventilation during anesthesia with a laryngeal mask airway (LMA). PSV supports the achievement of optimal tidal volumes and, in comparison to unassisted spontaneous breathing, increases minute volume, lowers etCO₂ and improves oxygenation^6,7. In addition, PSV-supported intraoperative spontaneous breathing was found to reduce LMA removal time, emergence time and even Propofol consumption⁶.

In summary: The transition from mandatory ventilation to spontaneous breathing is a common and important aspect of general anesthesia.