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The jfish Peripheral Nerve Stimulator
What is a peripheral nerve stimulator?
A peripheral nerve stimulator, or PNS, is a device used in anaesthesia or intensive care to assess the degree of neuromuscular blockade - that is, the degree to which the normal communication between motor nerves and skeletal muscle1) is blocked by a neuromuscular blocking drug, or ‘muscle relaxant’.2)
A PNS works by stimulating a peripheral nerve with a series of very brief, accurately timed pulses of electrical current. A clinician assesses the resulting muscle movement (twitch), providing an accurate indication of the degree of muscle blockade.
A PNS is typically a handheld, stand-alone, battery-powered device. More advanced monitors are available that incorporate accelerometry to accurately measure the degree of muscle twitch following a stimulation pulse. However, because of the properties of the pulse-patterns used in PNS, determination of muscle blockade using only touch is both accurate and objective. This is the most common way that paralysis is assessed during and after anaesthesia.
Why is neuromuscular monitoring important?
Muscle paralysis (neuromuscular blockade) is often used as part of a general anaesthetic for one of three reasons:
- To facilitate intubation of the trachea.3)
- To facilitate ventilation by paralysing the diaphragm and muscles of the chest wall.
- To facilitate surgery by improving operating conditions.
Although modern muscle relaxants have very reliable kinetics4) that allow an anaesthetist to estimate the duration of action of a relaxant, it is still very important to monitor the degree of paralysis in a patient. This is particularly important in situations where repeat doses are given, or older, less predictable agents are used. In many developing countries, cost and access limit practise to these older agents where offset kinetics are much less reliable, being influenced by disease states, other drugs and individual idiosyncrasies.
Use of a PNS allows an anaesthetist to assess:
- The adequacy of muscle relaxation for intubation.
- The ongoing adequacy of muscle relaxation for surgery.
- The degree to which muscle relaxation has worn-off, allowing the remaining paralysis to be reversed.
- The presence of residual paralysis.
The cost of residual muscle paralysis
It is by assessing the reversibility of paralysis and alerting the anaesthetist to the presence of residual parlaysis that a PNS offers it greatest benefit.
While a post-operative patient with residual paralysis may be able to breath on his or her own, the residual muscle weakness exposes a patient to an increased risk of several problems. These include basal collapse, pneumonia and hypercapnoea due to the compromise of respiratory function, and resultant morbidity, extended hospital stay and increased mortality.
From a historical perspective, it is thought that the use of earlier, less-predictable relaxants in 1950s anaesthesia, along with the absence of neuromuscular monitoring, is one of the main reasons for the high rate of peri-operative mortality and morbidity during this period when compared with modern day anaesthesia.5)
In a developing country, access to modern muscle relaxants can often be limited, restricting use to older, less predictable agents where residual paralysis is more likely. Similarly, access to reversal agents (such as neostigmine) may also be limited, making accurate assesment of the degree of residual parlaysis and the need for reversal more important in considering the use of these limited resources.
Finally, the cost of residual paralysis is greater. Local health infrastructure may be less able to manage subsequent complications without effecting the provision of health care to the rest of the community.
Why an open source PNS?
Commercial PNS devices typically cost from US$400 upwards to several thousand dollars. In a setting where there are limited funds available for even essential drugs and basic medical supplies, it is clearly unreasonable to expect such expenditure on a single device. Even if access were available, the absence of local language instructions and access to consumables or maintenance makes a commercial peripheral nerve stimulator a questionable utility.
The existence of a free peripheral nerve stimulator design lowers the bar of access to this simple, yet important, piece of medical equipment. By enabling developing country health professionals access to such a device, it is hoped that the costs and complications of residual muscle paralysis in the post-anaesthetic patient can be reduced.