PoCUS Guided Fluids to Prevent Post-induction Hypotension

Background: Hypotension is a common side-effect of general anesthesia induction, and is related to adverse outcomes, including significantly increasing risk of one-year mortality (intraoperative systolic hypotension increased mortality risk by 3.6% per minute below 80 mm Hg). Even short durations of intraoperative hypotension have been associated with acute kidney injury (AKI) and myocardial injury.

The feasibility of POCUS in directing fluid management to prevent hypotension has been demonstrated after the induction of spinal anesthesia but not general anesthesia.

Design: This will be a randomized controlled trial. All participating patients will undergo a bed-side ultrasound scan and assessment of the IVC-CI. Patients with collapsible IVC, defined as a CI equal or greater than 43%, will be randomized 1:1 to either receive a fluid bolus or not. We will test the potential efficacy of an ultrasound directed fluid bolus in reducing the hypotension associated with the induction of general anesthesia. This will be a single blinded study with the attending anesthesiologist unaware to patient group allocation and POCUS data.

The primary objective is to determine the efficacy of preoperative POCUS-guided intravenous fluid bolus administration in reducing the incidence of hypotension after the induction of general anesthesia in adults presenting for elective non-cardiac, non-obstetric surgery.

While in the preoperative area, patients who fulfil the eligibility criteria will be approached for recruitment, informed consent, and voluntary participation. Written informed consent will be obtained from all patients. Upon informed consent, baseline characteristics will be collected which include demographic information, past medical history including medical therapy, and the type of scheduled surgery. Thereafter the assessment of preoperative IVC-CI will be performed, with patients with an IVC-CI ≥ 43% randomized into the intervention and control groups.

Randomization: Patients who fulfil the eligibility criteria, provide informed consent, and are found to have an IVC-CI ≥ 43% according to a POCUS assessment, will be randomized prior to surgery while in the preoperative area. The allocation sequence will be created by a statistician through a computer-generated random table, and concealed in sequentially numbered, opaque, sealed envelopes, with a 1:1 allocation ratio and random block sizes of 4, 6, and 8 participants. Attending clinicians involved in the care of the patient will be blinded to the allocation and intervention.

Data collected in the preoperative area will include patient demographics [gender, age, height, weight, past medical history including coronary artery disease, heart failure, diabetes, smoking, peripheral vascular disease, hypertension, current treatment with beta-blockers, ACE-I or ARBs, and diuretics, type of surgery (coded as lower limb or upper limb orthopedic surgery, lower abdominal surgery, vascular, gynecology (non-obstetric) and urology), and the preoperative fasting duration.

In addition to the IVC measurements obtained by the POCUS scan, we will also document the duration of the POCUS scan and the fluid bolus administration, as well as the baseline perfusion index (PI), which estimates the pulsatility of blood in the extremities, calculated using infrared spectrum as part of plethysmography waveform processing, and has been shown to predict hypotension following propofol induction.

Once in the operating room, the information from the induction of the general anesthesia will be collected through the routine electronic chart including blood pressure and heart rate (before the induction of anesthesia), drugs administered during and after induction, and blood pressure over 20 minutes from the start of induction. In case of an arterial line inserted before induction to measure blood pressure invasively and continuously, which is indicated according to the discretion of the treating anesthesiologist, it will be used to collect the blood pressure and heart rate information.

A previously reported IVC-CI threshold of 43% had a positive predicting value (PPV) of 86% in predicting post-induction hypotension. As our sample will only include patients with a collapsible IVC (IVC-CI ≥ 43%), for the sample size calculation we will assume a 85% incidence of post-induction hypotension in the FB- group. Since no prior studies have assessed the effect of IVC-CI guided fluid bolus within the general anesthesia population, we will choose a relative risk reduction (RRR) of 50% with POCUS directed fluid optimization as clinically significant. Thus, assuming post-induction hypotension incidence of 85% and 42.5% in the FB- and FB+ groups, respectively, a sample size of 19 patients per group (95% confidence level) is required to reject the null-hypothesis with a risk of 0.05 and a power of 0.8.

Statistical analysis will follow the trial completion. No other interim analysis will occur during this trial. Data collected during the study will be compiled using Excel spreadsheets (Microsoft, USA). The lowest MAP recorded during the twenty minutes after the start of induction, defined as the first administration of an anesthetic agent at an anesthetic dosage, will be used to calculate the percentage decrease from baseline in each patient. Baseline will be defined as the first blood pressure measurement in the preoperative area or from the pre-anesthesia clinic visit. Descriptive data will be presented as means ± standard deviation (SD) for continuous variables and as absolute numbers and percentages for categorical variables. This data will include patient demographics, IVC-CI, the incidence of hypotension and its total duration, the total amount of fluids and vasopressors administered, the mean duration of the POCUS scan, and the incidence of surgical delays. For our primary outcome, the reduction in the incidence of post-induction hypotension between FB+ and FB- groups, we will use a χ2 test. Student's t test will be used for parametric data null hypothesis testing, while two-tailed Manne Whitney U test will be used to evaluate significance in non-normally distributed parameters.

In all cases a p value < 0.05 will be considered as statistically significant. Statistical analysis will be performed using SPSS version 20 (IBM, Chicago, IL, USA).