Is research reporting intraoperative hypotension apt enough?

Intraoperative hypotension (IOH) needs urgent attention as it significantly affects organ perfusion and patient outcomes.[1] Depending on its severity and duration, it might result in major organ dysfunction, ischaemia, and failure, even leading to case fatality.[2-5] Despite being so crucial, there is no uniform definition or universally accepted threshold to define IOH.[2] A systematic review by Bijker JB et al.[6] identified 140 definitions of IOH provided in over 100 studies from January 2000 to April 2006. Frequently used definitions include systolic blood pressure (SBP) <80 mmHg, a decrease in SBP of >20% below baseline, and a combination of definitions consisting of an absolute SBP of <100 mmHg and/or a 30% decrease below baseline.[6] In adults undergoing non-cardiac surgery under general anaesthesia, the most common definitions of IOH are SBP <90 mmHg or mean blood pressure (MBP) <60 mmHg.[5] The scenario is the same even for obstetric anaesthesia. The systemic literature review by Klöhr S et al.[7] found 15 different definitions of hypotension in caesarean sections. The two most frequent definitions were a decrease below 80% baseline and the combined definition of SBP below 100 mmHg or a decrease below 80% baseline.[7] Such variabilities lead to a significant impact on the reported outcomes. Depending on which definition for IOH is used, the incidence varies between 5% and 99%.[2] Even the target thresholds for management get impacted, and the literature indicates a range of SBP thresholds ranging from 55 to 110 mmHg, MBP from 40 to 85 mmHg, and diastolic blood pressure (DBP) from 35 to 60 mmHg.[2] The variation is not only limited to the type of blood pressure value (SBP, DBP, or MBP), what value (absolute or percentage fall), and how much falls (20% or 30%) but is also noted for the timing of the baseline value reporting. We screened the original articles published in the Indian Journal of Anaesthesia in 2023. The research papers focusing on hypotension and its management as one of the key objectives were taken; 13 articles were found [Table 1].[8-20] The variation was evident because five considered MBP alone, three SBP alone, and two considered both SBP and MBP. Three were unclear about it. Similarly, six articles considered the absolute value of MBP or SBP for defining IOH. The combined use of percentage fall from baseline and absolute cut-off value of MBP and/or SBP was also noted in three.Table 1: Studies published in the Indian Journal of Anaesthesia during 2023 reporting intraoperative hypotension as an objectiveWhile each component of blood pressure reading has advantages and limitations, MBP is less affected while monitoring non-invasively using resonance technique and even invasively even with damping effects.[21] In the analysis of organ-specific outflow pressure, the primary indicator of end-organ perfusion is MBP.[2] Perioperative Quality Initiative statement on blood pressure that may be considered safe for patients under anaesthesia is that 'myocardial and renal injury is predicted by both absolute thresholds (MBP <65 mmHg) and relative thresholds (>30% drop from baseline)'. Therefore, either can be used to define hypotension and initiate management.[22,23] Nevertheless, the duration of hypotension is also crucial. In non-cardiac surgery, Sun et al.[24] found that acute kidney injury is linked to intraoperative MBP <55–60 mmHg for ≥11–20 min. Salmasi et al.[25] found that postoperative heart and kidney injury risk in non-cardiac surgery increases with a reduction in intraoperative MBP <50–60 mmHg for 1–30 min or 20%–50% of the preoperative value for ≥5 min. In a recent retrospective multi-centre cohort study including 316,717 non-cardiac surgical patients, an MBP of <55 mmHg was associated with an increased rate of postoperative delirium.[26] Applying personalised SBP targets, as opposed to standard blood pressure (BP) management, dramatically reduced the incidence of postoperative organ dysfunction as found in multi-centre INPRESS trials.[27] The baseline reading timing is also crucial as it can impact the definition of hypotension, especially when the percentage fall from the baseline is considered while defining IOH. In clinical practice, BP measurements taken just before induction of general anaesthesia are often used as a surrogate for the patient's baseline. However, a prospective observational study comparing ambulatory and perioperative BP in 370 patients showed that pre-induction MBP values do not reflect mean daytime MBP values.[28] Thus, pre-induction MBP values should not serve as a surrogate for the individual normal daytime MBP.[3] Blood pressure and heart rate in the preoperative area and operation theatre are significantly affected by preoperative anxiety.[29] Anxiety is frequently noted in the preoperative period, even after pre-anaesthesia evaluation and counselling.[30] Such anxieties can be because of multiple factors such as fear of needles, waking up and pain during surgery, and inadequate information about surgery and anaesthesia, leading to preoperative anxiety.[31] Our analysis found that the timing for the baseline value for BP was also variable. The first reading of the operating table was the most common, that is, 6 (46.15%). The researcher also took the ward and preoperative area values; one study did not report it. To conclude, the practice and research reporting have remained diverse in defining IOH. Although it might be difficult to put one BP above another, MAP can be chosen to define hypotension in most cases. A personalised approach might be the best approach. However, it might not always be feasible. Nonetheless, a fall of 20% from the baseline might be the nearest resemble of a personalised approach rather than an absolute value, especially in chronic hypertensive patients. The first reading on the operating table or preoperative area might be affected by anxiety or pre-medication; thus, taking the average ward BP reading as a baseline might be better in admitted patients.