Intra-operative hypotension – how low should you go?

The February issue of Anaesthesia attempts to answer the age-old question plaguing anaesthetists – what blood pressure should we be targeting and how much does it matter? We are delighted to present the first publication from the iHype study – a prospective cohort study designed and conducted by the Research and Audit Federation of Trainees (RAFT). Data from 4750 patients aged >65 y were collected over a 48 h period. They found intra-operative hypotension, and its many different definitions, to be common, with incongruence between the blood pressure anaesthetists say they will treat and their actual treatment threshold. This study is a great example of what trainee research networks can achieve and how they are a relatively untapped resource. The podcast discussion with the authors is a must for any budding trainee researcher on how to plan and perform such a large and effective study. The associated editorial highlights the discrepancy between blood pressure recordings and explores why we feel may feel the need to “fudge” the anaesthetic chart. With electronic recording becoming more prevalent in the future there may be nowhere to hide! 

Figure 1 Thresholds at which anaesthetists give vasopressor treatments and their intended treatment thresholds. Y-axis denotes the proportion of patient episodes/respondents given the defining threshold on the x-axis for (a) absolute mean arterial pressure; (b) absolute systolic blood pressure; (c) relative change in mean arterial pressure from pre-operative blood pressure; and (d) relative change in systolic blood pressure from pre-operative blood pressure.

The effect of intra-operative hypotension on acute kidney injury, postoperative mortality and length of stay following emergency hip fracture surgery is a neat single centre study investigating the incidence of intra-operative hypotension in a vulnerable group. Interestingly, they did not find an association with intra-operative hypotension and acute kidney injury. However, delay to surgery was associated with increased mortality. North Shore Hospital Anaesthetic department do use electronic intra-operative recording and the MAP was (truly) rarely below 65mmHg in their cohort. 

Why does blood pressure drop during anaesthesia and is there anything we can do to predict which patients will be most affected? The editorial by Frandsen et al. goes back to basic physiology and highlights the often forgotten about autonomic system – it is not all about intra-vascular volume. Anaesthesia causes both sympatholysis and vagolysis. If a patient already has pre-existing autonomic dysfunction, despite adequate fluid volume, anaesthesia could and does provoke hypotension and cardiac instability. By using pre-operative markers of vagal tone, such as heart rate variability, can we identify high risk patients and tailor our anaesthesia accordingly?

The pandemic has sharpened and exacerbated health and social inequalities globally. There is no doubt that patients of non-white ethnicity have poorer outcomes from SARS-CoV-2 infection. The underlying cause for this is likely to be multi-factorial but there has been anxiety that pulse oximetry is less accurate in patients with darker skin pigmentations. Wiles et al. performed a retrospective observational study to address this important concern. They compared SaO2 and SpO2 measured concurrently in patients on different ethnicities and reassuringly found no clinically significant difference. However, as mentioned in the accompanying editorial – is ethnicity the correct question to be asking, and should we actually be comparing skin pigmentation levels in order to accurately validate the use of pulse oximetry in different groups? The inclusion of different ethnic groups in both clinical research and validation studies of commercial monitoring is improving, but this topic highlights the need for both patients and medical staff to be confident that what we are both doing and using to monitor patients in different diseases is correct for all patients. 

Figure 2 Bland-Altman plot paired measurements of oxygen saturation by arterial blood gas analysis and peripheral oxygen saturation in 194 patients with COVID-19 pneumonitis who were admitted to critical care for non-invasive respiratory support. Patient ethnic origin is shown by the colour of each datum (White = yellow; Asian = purple; Black = light blue; and Other = green). The solid line represents the bias between the two measurements, the dashed line represents the limits of agreement (bias ± 1.96 SD) and the dotted line represents the 95%CI for the limits of agreement. (a) shows all paired measurements and (b) shows only those measurements when the patient was hypoxaemic (defined as SaO2 ≤94%).

The pandemic has also made us aware of the vital importance of oxygen and how hypoxic patients do not always look the same! However, aside from being able to sketch out the oxygen dissociation curve, our actual understanding of oxygen physiology is limited. Slingo and Pandit present a narrative review aiming to introduce hypoxia-inducible factor and oxygen sensing pathways to the wider clinical community. The ability of cells to detect and respond to varying oxygen concentrations relies on a complex cascade. Manipulation of this pathway may have many clinical implications, including targeted oxygen therapy. This fascinating review also highlights the requirement for more basic science research in anaesthesia.

Figure 3 Several hundred genes are direct transcriptional targets of HIF, and are involved in diverse cellular and physiological processes in order to optimise oxygen supply and demand. A few examples are given here that will be of direct interest to clinicians in anaesthesia and critical care. VEGF, vascular endothelial growth factor; TF, transferrin; EPO, erythropoietin; Hb, haemoglobin; Hct, haematocrit; EDN1, endothelin 1; NOS, nitric oxide synthase; GLUT 1, glucose transporter 1; LDH, lactate dehydrogenase; PDK1, pyruvate dehydrogenase kinase 1.

The expanding field of peri-operative medicine is changing the focus of anaesthesia research. Along with clinical shared decision-making, patient involvement in study design is improving and with that the need for outcomes that are important for patients. Days alive and at home (DAH) is one such outcome that neatly encompasses the many factors of the post-operative journey for a patient. Ferguson et al. surveyed patients and found the minimal clinically important difference to be 3 days, with an early discharge and being discharged to home rated as important. This is important work that will guide study design in future trials. Indeed Moore et al. used a similar patient-centred outcome measure (days alive and out of hospital) to audit the implementation of the WHO Surgical Safety Checklist.

None of the above research into patient outcomes however is important if we do not have a world in which to practice anaesthesia! In light of COP26 in Glasgow in November the World Federation of Societies of Anaesthesiologists has produced a consensus statement on the principles of environmentally-sustainable anaesthesia. The main points are that patient safety should not be compromised and healthcare systems should be mandated to reduce their contributions to global heating in order to limit global heating to 1.5℃ by 2050. There is a lack of scientific evidence to underpin these guidelines and further work and investment is needed to ensure that we protect our world and current and future generations.

Elsewhere we also have a feasibility study on the use of cryoprecipitate in post-partum haemorrhage. Finally, we are recruiting! We have adverts for a Trainee FellowEditor-in-Chief and three Editors posts at Anaesthesia Reports! Join us!

Cara Hughes and Andrew Klein

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