Frederic Michard, MD, PhD
Postoperative hypotension is common, largely undetected, and strongly associated with postoperative morbidity and mortality.
A recent study (www.ncbi.nlm.nih.gov/pubmed/30875354) showed that around 25% of postsurgical ward patients experienced at least one episode of mean arterial pressure (MAP) <70 mmHg lasting at least 30 minutes, and 18% had an episode of MAP <65 mmHg lasting at least 15 minutes. Nearly half the patients who had a MAP <65 mmHg for at least 15 minutes were undetected by routine vital signs assessment.
A large observational study (www.ncbi.nlm.nih.gov/pubmed/29189290) showed that hypotension occurring between day 1 and day 4 after surgery was associated with a 183% increase in postoperative myocardial injury and death. For sake of comparison, 10 minutes of hypotension during surgery was associated with a significant but 23 times lower (8%) increase in postoperative myocardial injury and death.
This could be described as a “monitoring paradox”: we monitor blood pressure very closely in the operating room, intensive care units (ICUs) and post-anaesthesia care units (PACUs) but not on the wards…and this is where most surgical patients develop complications and may eventually die (www.ncbi.nlm.nih.gov/pubmed/22998715). Therefore, a closer, nearly continuous monitoring of blood pressure may help for the early detection and treatment of postoperative hypotension and ultimately to improve outcome (www.ncbi.nlm.nih.gov/pubmed/30916008).
Over the last few years, multiple sensors have been developed to monitor vital signs, non-invasively and continuously in ambulatory ward patients. They include adhesive patches, necklaces and wrist devices enabling the monitoring of heart rate, heart rate variability (detection of cardiac arrhythmia), respiratory rate, SpO2 and/or axillary temperature (www.ncbi.nlm.nih.gov/pubmed/30336872). Very few are actually able to measure blood pressure.
Blood pressure monitoring on the awards
On surgical wards, blood pressure is classically measured by nurses every 4 to 6h with the oscillometric brachial cuff method. This universal approach is intermittent by nature, often inaccurate in obese patients and when blood pressure is very low or very high (www.ncbi.nlm.nih.gov/pubmed/21952254). In addition, repeated measurements may be bothersome and are not compatible with good quality sleep. Therefore, several alternatives are emerging. Existing methods include wireless brachial cuffs, volume clamp techniques, pulse wave transit time and pulse decomposition algorithms. Future sensors, such as electronic tattoos, have the potential to transform physiological monitoring in hospital wards.
Wireless brachial cuffs
Wireless brachial cuffs are now available for measuring blood pressure in ambulatory ward patients. They are powered by a light battery and transmit intermittent blood pressure measurements wirelessly (e.g. via Bluetooth) to a bedside or central monitoring system. When other wearables (abdominal patch to capture respiratory rate and finger pulse oximeter to capture pulse rate and SpO2) are used simultaneously, it enables the early detection of cardio-respiratory and septic complications. Such a system has been tested in a large (>4400 patients) before-after comparison study that demonstrated a significant decrease in cardiac arrest and mortality after implementation on the wards (www.ncbi.nlm.nih.gov/pubmed/28288655).
Volume clamp methods
As of today, volume clamp methods are part of monitoring systems designed for the operating room, not for the wards. These techniques, combining an air-filled finger cuff and a photoplethysmographic sensor, continuously determine the cuff pressure needed to maintain the finger artery blood volume constant. Then, the derived finger blood pressure signal is mathematically converted (transfer function) into a brachial waveform. Calibration with the oscillometric brachial cuff method is optional (depending on the system).
These methods are relatively accurate, meeting the current AAMI standards (bias < 5 mmHg, SD < 8 mmHg), pending the peripheral circulation is not shut down and patients do not receive vasopressors (www.ncbi.nlm.nih.gov/pubmed/29516119). These bulky and tethered monitoring systems cannot follow ambulatory patients. However, they can stay on crash carts, and be used on demand when ward patients develop haemodynamic instability (www.ncbi.nlm.nih.gov/pubmed/31446156). Of note, in addition to blood pressure measurements, volume clamp methods have the advantage to estimate the two main determinants of blood pressure, cardiac output and vascular resistance. This is useful to decide how to treat hypotension.
Pulse wave transit time methods
Both cardiac output and vascular resistance influence the pulse wave transit time (PWTT). The PWTT is the time needed by the blood to travel from the heart to a peripheral site of measurement. In practice, the R wave of the ECG (skin electrodes) is classically used to approximate when the blood is leaving the heart, and the upstroke of a pulse oximetry waveform to know when it arrives in the periphery. The PWTT decreases when cardiac output and vascular tone increase, both situations often associated with a rise in blood pressure. In other words, changes in PWTT may be useful to predict blood pressure changes, but less to assess absolute blood pressure values. In this regard, tracking changes in PWTT has been shown to be useful to detect hypotension induced by anaesthesia induction (www.ncbi.nlm.nih.gov/pubmed/23223109). More recently, the PWTT method has also been proposed to track changes in blood pressure in ambulatory ward patients (www.ncbi.nlm.nih.gov/pubmed/28679490).
The pulse decomposition analysis
The pulse decomposition method is relatively new (launched in 2018). A piezoelectric sensor captures the blood pressure signal from a small finger cuff inflated at constant low pressure (40 mmHg). The finger pressure signal is then “decomposed” by a proprietary algorithm to extract blood pressure values. Calibration with the oscillometric brachial cuff method is mandatory. A single study (www.ncbi.nlm.nih.gov/pubmed/28327093) suggests it is accurate and precise to monitor blood pressure in the operating room. Because it is easy to use, light and wireless, this technique has the potential for continuous ward monitoring.
In the future, volume clamp methods could be miniaturized and become wireless. Thanks to flexible electronics and new biomaterials, adhesive sensors (aka electronic tattoos) able to literally “feel” our pulse may become available as well (www.ncbi.nlm.nih.gov/pubmed/28605474). These innovations have strong potential to change our monitoring practices in hospital wards and beyond.