This Invited Commentary accompanies the following original article:
Birch M, Kwon Y, Loushin MK, et al. Intrathoracic pressure regulation to treat intraoperative hypotension. A phase II pilot study. Eur J Anaesthesiol 2015; 32:376–380.
A consensus definition of intraoperative hypotension (IOH) remains to be achieved,1 though commonly used definitions include a reduction in SBP below 80 mmHg and an episode of SBP more than 20% below baseline or normal for the patient. Without an agreed definition, the true incidence is difficult to estimate, but IOH is common and occurs in between 5 and 99% of patients undergoing anaesthesia.1
Despite the lack of a consensus definition, it is clear that IOH can harm patients. Reduction of the mean arterial pressure below 55 mmHg, for as little as 1 to 5 min, can carry a significantly increased risk of acute postoperative kidney injury and myocardial ischaemia.2 In addition, IOH has been associated with peri-operative stroke.3 It has a negative influence on outcomes not only in the immediate postoperative period but also up to 1 year after surgery.4
What is unclear from the outcome studies of IOH is why harm occurs. Brady and Hogue4 suggest that adverse events could be due to the use of vasopressor agents in the treatment of IOH rather than the hypotension itself. In this issue of the Journal, Birch et al.5 offer a novel, nonpharmacological treatment for IOH. Here, an intrathoracic pressure regulation device is attached to the endotracheal tube to reduce intrathoracic pressure throughout the respiratory cycle. The authors hypothesise that the device works by reducing intrathoracic pressure and increasing venous return to the heart, leading to the correction of hypotension. This device reduces the increase in intrathoracic pressure associated with positive pressure ventilation and exerts negative transpulmonary pressure during the expiratory phase.
Although the intrathoracic pressure regulation device may appear to offer an innovative solution to IOH, there are a number of issues that require further investigation. Firstly, the device requires the use of positive pressure ventilation without positive end-expiratory pressure (PEEP). Although the correct level of PEEP during surgery is yet to be established,6 application of PEEP appears to be important. Lack of PEEP during general anaesthesia results in ventilator-induced lung injury and has been associated with an increase in mortality.7,8 The authors report that atelectasis and airway de-recruitment events were not observed during the trial. However, the number of patients (n = 17) was too small to detect the likely adverse event rate, and because this was only a pilot study, there was no control group. The authors state that there were no changes in pulse oximetry measurements during the trial, but they report only the mean fraction of inspired oxygen for the whole patient cohort, making detection of desaturation events difficult. Secondly, application of the device requires an increase in gas flow through the anaesthetic machine. This reduces efficiency in the use of volatile anaesthetics with low flow systems and therefore increases gas scavenging requirements and costs. Finally, reversal of IOH occurred at a mean of 8 min after applying the intrathoracic pressure regulation device. This period is in excess of that shown to be potentially harmful (<5 min) and slower than the correction of IOH that would be expected with vasopressor agents.
Intrathoracic pressure regulation offers an interesting and novel mechanical approach to the management of IOH. Birch et al.5 raise a number of important issues relating to the management of IOH and application of the intrathoracic pressure regulation device, which can only be answered by further well conducted randomised controlled trials.
Acknowledgements relating to this article
Assistance with the commentary: none.
Financial support and sponsorship: none.
Conflicts of interest: none.
Comment from the editor: this invited commentary was checked and accepted by the editors but was not sent for external peer review. MC is an associate editor of the Eur J Anaesthesiol.
1. Bijker JB, van Klei WA, Kappen TH, et al. Incidence of intraoperative hypotension as a function of the chosen definition: literature definitions applied to a retrospective cohort using automated data collection. Anesthesiology
2. Walsh M, Devereaux PJ, Garg AX, et al. Relationship between intraoperative mean arterial pressure and clinical outcomes after noncardiac surgery: toward an empirical definition of hypotension. Anesthesiology
3. Bijker JB, Gelb AW. Review article: the role of hypotension in perioperative stroke. Can J Anaesth
4. Brady K, Hogue CW. Intraoperative hypotension and patient outcome: does ‘one size fit all?’. Anesthesiology
5. Birch M, Kwon Y, Loushin MK, et al. Intrathoracic pressure regulation to treat intraoperative hypotension. A phase II pilot study. Eur J Anaesthesiol
6. PROVE Network Investigators for the Clinical Trial Network of the European Society of Anaesthesiology. High versus low positive end-expiratory pressure during general anaesthesia for open abdominal surgery (PROVHILO trial): a multicentre randomised controlled trial. Lancet
7. Acute T, Distress R, Network S. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. The Acute Respiratory Distress Syndrome Network. N Engl J Med
8. Levin M, McCormick PJ, Lin HM, et al. Low intraoperative tidal volume ventilation with minimal PEEP is associated with increased mortality. Br J Anaesth