The role of an ambient pressure oxygen source during one-lung ventilation for thoracoscopic surgery

Abstract

Video-assisted thoracoscopic surgery is facilitated by prompt collapse of the non-ventilated (‘operated’) lung, and interrupted and impeded if there is a need for oxygen (O2) delivery by continuous positive airways pressure in order to manage hypoxaemia. It has been proposed that connecting an ambient pressure O2 source to the airway of the non-ventilated lung at the time one-lung ventilation is initiated and before the chest is opened will, by avoiding entrainment of ambient nitrogen, serve to facilitate lung collapse. It has also been proposed that leaving the O2 source connected will enable, not only ongoing apnoeic oxygenation before the chest is opened, but also the thoracoscopic procedure to commence with the operated lung fully pre-oxygenated (with an inspired oxygen fraction of 1), and apnoeic oxygenation to continue throughout the operative procedure in those patients who exhibit a degree of small airways patency at ambient pressure. In reality, several factors can influence the speed of collapse of the operated lung, and very many factors can influence the incidence of hypoxaemia during one-lung ventilation. It therefore appears unlikely that the necessary evidence to support these proposals will be forthcoming from randomised clinical studies on large numbers of patients. Rather, the necessary evidence may only be provided by specifically designed within-patient clinical measurement studies. Nevertheless, it is argued that, in the meantime, there is already sufficient rationale for an ambient pressure O2 source to be connected to the airway of the non-ventilated lung, and for it to remain connected for the duration of one-lung ventilation.

Evidence-based medicine: time to upend the pyramid for some clinical situations?

“…Another 'point of view' article, published a decade earlier, analysed the likely beneficial role of apnoeic oxygenation of the non-ventilated lung at the initiation and sometimes during one-lung ventilation. 7 This hypothesis is further advanced in the more recent 'point of view' article, 3 but 'where is the evidence?' Is it sacrilege to say that the evidence might lie in a lowly 'level of evidence' such as a case report?…”

“…For instance, clinical best-practice for the anaesthetic management of one-lung ventilation will be unlikely to derive from randomised controlled trials because of the markedly differing respiratory and cardiovascular physiologies and pathophysiologies seen in thoracic surgical patients. 3,4 Perhaps in such situations, reviewers and editors should consider 'upending the pyramid' when assessing submitted manuscripts, demoting the heavy reliance on the randomised controlled trial, and considering lesser 'levels of evidence' such as within-patient clinical measurement studies and persuasive and convincing case reports.…”

“…These changes led in turn to the view that one particular aspect of one-lung ventilation should be adopted as routine, at least until its theoretical clinical and hence patient benefits are shown not to pertain. 3 This conclusion was published as a 'point of view' article, a classification of publication that possibly allows an editor to sympathise with at least some of an author's perceived biases.…”

“…11 Oxygen saturation remained in the high 90s for the duration of one-lung ventilation and neither the ventilated lung nor the non-ventilated lung was exposed to high concentrations of oxygen because nitrous oxide in the pulmonary blood equilibrates with any gas entering the alveoli via patent small airways of the non-ventilated lung (Fig. 1 from the study by Pfitzner 3 ).…”

Reasons for using nitrous oxide in one-lung ventilation, especially for thoracoscopic surgery

“…2 I previously found in a sheep model that the ‘operated’ lung collapsed faster when N 2 O was used rather than 100% oxygen. 3,4 Subsequently, I found that if a N 2 O:O 2 gas mixture was used and nitrogen was meticulously excluded from the non-ventilated lung (by connecting an oxygen source to its open airway to prevent entry of ambient air, 5 and by avoiding air in the inspiratory gas), the need for continuous positive airway pressure (CPAP) to the ‘operated’ lung to manage hypoxaemia was rare. 6,7 Because the application of CPAP first interrupts and then impedes thoracoscopic surgery, possibly at a clinically critical time, I recommended that during thoracoscopic surgery, air should be used only after the surgeon has indicated that operative access will not be impaired by re-expansion of the ‘operated’ lung.…”