Fasten your seatbelts: innovation in regional anaesthesia is a bumpy ride

Seatbelts save lives. Early studies of motor vehicle accidents demonstrated a significant reduction in major and fatal injuries attributed to the use of seatbelts that ranged from 35% to 80% depending on the study 1. In the UK, registered cars were required to have front seatbelts by 1965 although mandatory use of them did not become law until 1981 2. Initially, seatbelts only offered two-point restraint (i.e. lap or diagonal, but not both). While mortality data strongly support any seatbelt over no seatbelt, the two-point diagonal design is considered the least effective and most dangerous in terms of causing injury 2. Diagonal two-point seatbelts can potentially cause soft tissue injury to the neck, chest and breasts as well as rib and sternal fractures 2. Potential injuries associated with two-point lap seatbelts include intra-abdominal trauma such as intestinal perforation, haematomas from mesenteric tears and vessel injury, gallbladder rupture, pancreatic damage and closed loop obstruction 2. Skeletal injury in the form of pelvic and spinal fractures or ligament disruptions has also been associated with lap-only seatbelts 1. Visible bruising or friction burns on the abdomen at the site of the seatbelt was termed a ‘seatbelt sign’ and demanded further investigation to rule out serious injury 2. Despite the safety strides achieved by early versions of two-point seatbelts, there was a clear need to improve the design 1. This year marks the 60th anniversary of the modern three-point seatbelt 3. A former aviation engineer, Nils Bohlin, is credited for developing this innovative seatbelt design for Volvo, and customers in the Scandinavian market had access to vehicles with three-point seatbelts as early as 1959 3. Although they have also been associated with seatbelt-related injuries 4, 5, 1991–1997 data from the US Fatality Analysis Reporting System on head-on collisions show a 72% decreased odds of death when passengers are using a three-point seatbelt compared with no seatbelt 6. This means more lives saved over the previous model of two-point lap-only seatbelts which were associated with a 57% decreased odds of death 6. Today, three-point seatbelts are standard, and all automobile manufacturers have implemented them. Why are we talking about seatbelts? Three-point seatbelts represent a modern innovation that has improved outcomes that matter to people and has changed the automobile industry. In this issue of Anaesthesia, Ferré et al. present the results of a randomised clinical trial comparing an anterior (proximal) with a posterior (distal) approach to the suprascapular nerve block in terms of hemidiaphragmatic paralysis assessed by ultrasound 7. All participants in the study underwent arthroscopic shoulder surgery and also received an axillary nerve block consistent with the selective ‘shoulder block’ concept introduced by Dr Price 8. The only comorbidity exclusion was pre-existing respiratory failure, and participants were generally healthy and non-obese. Not surprisingly, the block approach performed further away from the brachial plexus was associated with a lower rate of sonographically-demonstrated hemidiaphragmatic paralysis 7. This study, while meticulously performed, raises two important questions: is this a patient-centred outcome; and will the results change clinical practice? The Patient-Centred Outcomes Research Institute (PCORI) defines patient-centred as being “meaningful and important to patients and caregivers” 9. Using the seatbelt example, investigating innovations to decrease the potential outcomes of death and major injury from a motor vehicle accident fit the PCORI definition of patient-centred since “the focus includes outcomes that matter to patients” 9. This is an oft-discussed topic in many modern anaesthesia studies 10, and similarly, we question whether the sonographic detection of hemidiaphragmatic paralysis, the primary outcome of the present study 7, meets this criterion. The assessment of hemidiaphragmatic function was performed by clinicians skilled in point-of-care ultrasound. High-end portable ultrasound equipment was used to visualise the diaphragm and estimate the degree of diaphragmatic motion using M-mode. Study participants were asked to breathe normally and perform a ‘sniff test’ (i.e. forced inhalation through the nose) on command. Consistent with the methodology used in previous studies 11, 12, hemidiaphragmatic paralysis was considered complete if the reduction in movement was > 75% and partial if reduction was between 25% and 75% 7. This detailed measurement is appropriate for a research study but is not easy to implement in routine clinical practice. In the study by Kim et al. which evaluated the outcome of hemidiaphragmatic paralysis in a comparison of superior trunk and interscalene blocks, investigators noted difficulty in visualising the left hemidiaphragm 12. They stated, “Visualisation of the left-sided hemidiaphragmatic motion is more challenging because of the smaller window of the spleen, especially in patients with high body mass indexes” 12. For this reason, they excluded patients from the study who had a BMI over 35 kg.m−2 and developed an alternate technique for visualising the left hemidiaphragm in the case of a difficult scan 12. Determining the implications of research studies that primarily report changes in sonographically-detected diaphragmatic excursion from the patient perspective is nearly impossible. The clinical skills to obtain this measurement are not universally available in the peri-operative setting, and rarely do patients exhibit any symptoms from hemidiaphragmatic paralysis. Dyspnoea is a patient-centred outcome measure that can be a marker of underlying involvement of the phrenic nerve, pneumothorax or other sinister complications, and can indicate further treatment and prolonged hospital stay. However, even with sonographic evidence of hemidiaphragmatic paralysis, as well as a change in forced expiratory volumes and vital capacity, dyspnoea remains uncommon in modern practice 13. In patients who do report dyspnoea after interscalene or supraclavicular brachial plexus blocks following shoulder surgery, only a third to three-quarters actually have objective evidence of hemidiaphragmatic paralysis 11, 14. There are several reasons that patients may experience dyspnoea after regional anaesthesia, general anaesthesia and shoulder surgery; thus, this symptom is neither sensitive nor specific. Overall, the relationship between ultrasound-diagnosed hemidiaphragmatic paralysis and clinical symptomatology is unclear. The quantification of diaphragmatic motion by ultrasound is a decidedly clinician-focused metric and a distant surrogate marker for the clinically-relevant outcome of respiratory distress. Although it is measurable and therefore permits the estimation of a relatively smaller sample size for a clinical trial, the outcome that matters to patients is more likely to be clinical respiratory distress requiring intervention, emergency care or hospital admission. This occurred in only one participant from the 84 person study 7, an incidence of 1.1%. To reduce this incidence by half (0.55%) in a randomised clinical trial study (ɑ 0.05 and β 0.2), 3460 per group are required with a total enrolment of 6920 participants. Whilst not impossible, the feasibility of such a study is limited, and the rarity of this complication means that this may not be worth exploring. Indeed, serious postoperative complications after arthro-scopic shoulder surgery are uncommon with an estimate of all-cause readmission in the US of approximately 1% 15. Taking the above facts into consideration, studies of patient-centred and clinically-oriented outcomes in regional anaesthesia can be very challenging. The next important question is whether the results of this study will change clinical practice. The goal of reducing hemidiaphragmatic paralysis had been a priority for regional anaesthetists since Urmey et al. reported the 100% incidence of this effect following interscalene brachial plexus block in 1991 16. Since then, many modifications of and alternatives to the interscalene brachial plexus block have been described 13, 17 with the shoulder block (a combination of distal suprascapular and axillary nerve blocks) introduced in 2007 being one of them 8. A randomised clinical trial by Dhir et al. comparing the shoulder block with the interscalene brachial plexus block for arthroscopic shoulder surgery, and designed to test for equivalence, demonstrated that interscalene brachial plexus block provides more effective analgesia in the immediate postoperative period 18. In addition, even in the hands of regional anaesthesia experts who performed the procedures, the shoulder block on average took approximately twice as long to perform than the interscalene brachial plexus block, which makes sense since two individual nerves are blocked 18. This suggests that a high level of expertise is required as well as sufficient time. In the US, few patients having outpatient surgery tend to experience peri-operative nerve blockade, approximately 3% among all surgical subtypes, and the majority of patients who specifically undergo outpatient shoulder surgery (59%) do not receive a block 19. System- and training-related factors play a role, but every regional anaesthetist knows that the only block that a patient will get is the one that best combines speed and efficacy. Distal blocks for shoulder surgery likely have an indication in specific cases (e.g. patient with severe chronic pulmonary disease causing respiratory failure), but they will continue to be tools for experts who already practice regional anaesthesia. It is our opinion that this technique will not be regularly adopted by novices as the go-to block technique for shoulder surgery and therefore will not expand patient access to regional anaesthesia 20. American entrepreneur Katrina Cole advised us to “focus on things that are small enough to change but big enough to matter”. This could be the rallying cry for outcomes research in regional anaesthesia. While innovations in block techniques may never result in the same public health impact as the three-point seatbelt, they can aspire to address patient-centred outcomes, the things that matter to real people and lead to changes in clinical practice that ideally improve quality and access to regional anaesthetic techniques. This work was supported with resources and the use of facilities at the Veterans Affairs Palo Alto Health Care System (Palo Alto, CA, USA). The contents do not represent the views of the Department of Veterans Affairs or the United States Government. KE is an Editor of Anaesthesia. No external funding or competing interests declared.