The effect of neuromuscular blockade on the efficiency of mask ventilation of the lungs*

Abstract

SummaryMathematical formulae to calculate body surface area from measurements of height, weight and other parameters date from the late 19th century. Drug doses, fluid therapy, caloric requirements and physiological parameters such as cardiac output, glomerular filtration rate and a variety of respiratory function parameters are all frequently expressed in terms of a body surface area. Body surface area is often used in preference to body mass (weight). However, the original rationale for using body surface area as an estimate for metabolic rate has never been tested and the algorithms used to approximate body surface area have little evidence to support their use in this role. Recent developments in technology using indirect calorimetry allow easy measurement of metabolic rate in the clinical setting. Such measurements should be used for standardisation when weight alone is considered inadequate.Keywords Body surface area. Metabolic rate. Calorimetry. Rubner's Law, published in 1883, crystallised a belief among physiologists in the mid to late 19th century that, regardless of species, the heat production (i.e. metabolic rate) of an individual was proportional to the body surface area (BSA) [1]. This law was very difficult to disprove because BSA could only be measured by very cumbersome means, such as skinning the subject. Measurement of metabolic rate via direct calorimetry was possible but not widely available. Being unable to measure the metabolic rate in a clinical setting, measuring BSA seemed to provide an acceptable alternative to metabolic rate. There was an impetus around the beginning of the 20th century to derive simpler ways of calculating BSA using mathematical manipulations of various body dimensions. Throughout the remainder of the century, investigators attempted to simplify the equations and expand the application of the formulae. Because of the importance and widespread use of BSA formulae in many areas of medicine, we undertook a literature review to determine the source of and evidence for these equations. MethodsComprehensive literature reviews of documents published around 100 years ago are difficult. The electronic database ÔMedlineÕ only includes publications from 1966 onwards, whilst its paper-based predecessor, Index Medicus, extends to only 1960. The Medline database was searched by combining a medical subject heading (MeSH) search for Ôbody surface areaÕ, which prior to 1970 was classified under ÔanthropometryÕ. The following search strategies were used: MeSH search for ÔmathematicsÕ, or keyword searches for Ôcalculat*Õ, ÔformulaÕ or ÔduBoisÕ (duBois being the author of the most widely used BSA formula). Index Medicus was searched using the ÔanthropometryÕ MeSH term. Documents prior to 1960 were found from the references in articles identified by Anaesthesia, 2003, 58, pages 50-83 Ó 2003 Blackwell Publishing LtdMedline and each relevant document prior to 1960 also had its references searched. The development of Ôheight)weightÕ formulaeThe first published equation for estimating B...

The 5th National Audit Project (NAP5) on accidental awareness during general anaesthesia: summary of main findings and risk factors

“…Rapid sequence induction accounted for most occasions when thiopental was administered, and this combination appeared to be an important risk factor for AAGA. Relevant questions include: whether co-administration of opioids or other adjuncts lowers the risk of AAGA while still achieving the goals of rapid sequence induction; whether there is time to assess the effect of the induction agent and provide more if needed; whether the administration of the rapidly acting neuromuscular blocking drug can be delayed slightly to check the conscious level, and/or even check the ease of bag-mask ventilation [35][36][37][38], itself a test of depth of anaesthesia; and whether thiopental should continue to have a place. The high proportion of AAGA cases associated with failed, prolonged or difficult airway management indicates an overlap between the findings of NAP4 [39] and NAP5.…”

Checking the ability to mask ventilate before administering long‐acting neuromuscular blocking drugs

“…We appreciate the interest Dr Xue et al have taken in our recent article [1]. Our study was designed to assess the effect of neuromuscular blockade on facemask ventilation and we found that the former facilitates the latter.…”

“…In our own earlier study [1], checkers offered a different and more accurate argument: that if a difficult airway was predicted, mask ventilation will maintain oxygenation from the earliest opportunity while anaesthesia is deepened [3]. This logic of 'checkers' also makes proper distinction between short-and long-acting NBDs.…”

“…I write in response to the study by Warters et al [1], who concluded that neuromuscular blockade facilitates mask ventilation. This implies that when mask ventilation is difficult, a neuromuscular blocking drug (NBD) should be given, and also that the practice of confirming ease of mask ventilation before giving NBDs does not increase safety; indeed the authors made both these points.…”

Simulating hypoxia and modelling the airway

“…4 informs the debate concerning the role of checking the ability to mask-ventilate the lungs before administering neuromuscular blockade. Although some experimental results suggesting an improved ability to mask ventilate after neuromuscular blockade have been interpreted to imply that this 'checking' is not necessary [14,15], this is probably over-ridden by the theoretical considerations. If we do not know whether we can maintain oxygenation by mask ventilation, but proceed nonetheless to administer a long-acting neuromuscular blocking drug, and if we subsequently find inability to intubate the trachea and mask ventilate, then we have created a situation in which obstructive apnoea and the physiology described in this article ensues.…”