Wires and damned Wires vII by fjzhxb


									Wires, wires and damned wires
At Stoke Mandeville we have a six bedded Intensive Care caring for adults and some paediatrics specialising in burns, spinal, surgical, trauma and medical patients. A variety of equipment is used, our base ventilator is the Puritan Bennett 7200ae, with Fisher & Paykel 720 series dula servo heated wire humidifiers always available. Normally an Intersurgical Heat & Moisture Filter (HME) is used. However septic patients are inevitably transferred onto a heated humidifier to sustain optimum respiratory performance and often a ‘low-air’ loss bed also. These patients often suffer pyrexia, especially tetraplegic spinal patients, who have lost their own regulatory temperature mechanism. The Nurses often supply laminar air-flow devices (fans) to maintain a cool flow of air over the, sometimes conscious, pyrexial patient. Herein lies the problem. As heated fully saturated gases cool during delivery they lose a substantial measure of water vapour via condensation in the circuit. This phenomena can be often seen in our daily environment as the dew deposited over everything in the early (cooler) morning after a hot and humid evening. The dew-point effect. Fisher & Paykel have offered a work around. In order to deliver optimum saturated gases, the distal temperature at the wye-piece needs to be raised to 40oC and the primary humidifier bowl re-set to -3oC lower. Thus the gas is heated actively during transport avoiding the dew-point dropout effect. The temperature then drops 3oC between the wye-piece and the unheated end of the patients ET Tube; maintaining a fully saturated 37oC in the patients airway. Unfortunately in certain situations the heated wires cannot maintain this heat output and fail after about 5 to 12 hours usage, frustrating in continuous ITU care. In order to resolve this problem an F&P circuit was obtained with helix shaped heated wires in both inspiratory and expiratory hoses offering more heat in the inspiratory hose. The same problem persisted but the ventilator expiratory block also became flooded with condensation leading to severe ventilator service problems. Examination of the Intersurgical circuits in use threw up some problematic similarities which occurred when some or all of the following factors were present:  The Puritan 7200ae was in pressure controlled mode with ‘flow-by1’ enabled.  The patient had a fan circulating air over their upper body (cooling the circuit Wyepiece, containing the temperature probe).  A low-air-loss bed was in use (more cooling).  The isolation side-rooms were used (air-conditioning).


Flow-by, a mode where the ventilator is flow rather than pressure triggered via a continuous flow of gas through the patient circuit. The additional gas cooled the heated wires. Page 1

The F&P 730 humidifier monitors temperature at the patient wye-piece and at the humidifier dome itself via a dual servo feedback mechanism, the temperature being set as previously described. After a period of 3-5 hours the temperature difference between the humidifier dome and the patients wye-piece became much greater than the specified settings. (Measurements of circuit temperatures were made with an external portable thermistor.) The wire had started to ‘slip’ back away from the wye-piece as it softened under its own heat. The circuit temperature was therefore ‘read’ by the wye-piece thermistor as lower than the set temperature differential and consequently the humidifier supplied more compensating heat to the wire, but with diminishing effect. After a set period, the F&P humidifier is (correctly) programmed to regard this temperature differential as a fault and consequently the power to the unit is terminated and the whole humidifier and circuit cools to room temperature. Only a power interruption will re-set the controls. However busy nurses ignoring the constant humidifier alarm would sometimes detrimentally miss the fact that the humidifier became cold and ineffective. With help from Intersurgical engineers, we found that the heated wire coil had expanded with the increased power during the correction period and had become less efficient as the coil expansion caused a loss of impedance, thus making the device mechanically less than able to cope with the demands of the humidifier. We experimented by fixing the wire at the wye-piece, not allowing it to slip back. Unfortunately this still did not cope with the external cooling effect from the circulating fan or the low-air loss bed in close proximity to the wye-piece. The air-conditioning of the isolation side-rooms also exacerbated the problem. The answer proved quite simple, that weekend I had been insulating heating pipes in my new house extension, which just goes to prove that there’s nothing new under the Sun! Back at work on Monday, when faced with trying to keep a heated circuit warm, in a cool environment I simply resorted to wrapping the ventilator inspiratory circuit with an insulating layer of Velband orthopaedic wool, normally used under plaster casts. Wrapping the Intersurgical inspiratory circuit alone resulted in immediate gratifying success proving a good cheap and above all else - an effective solution! Again Intersurgical engineers kindly produced a more manageable prototype circuit insulator from a simple sleeve of plastic, which was secured with ‘Tye-bands’ at either end of the circuit. Crude but very effective. Measuring the circuit temperature proved the circuit a success. Previously the humidifier bowl was noticeably much hotter to the touch and had been measured at 43oC on a few occasions! With the new sleeve the whole circuit is much cooler and complies with the 39oC distal & -2oC bowl settings comfortably and is quite capable of the newly suggested target of 40oC and -3oC setting too!

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A trial on longevity proved the circuit could maintain the temperature for two weeks, which is somewhat more economical than 5 hours! In use we discovered that no-one found the additional sleeve objectionable, in fact its presence usually had to be pointed out to observers. Both Intersurgical and Fisher & Paykel now offer forms of insulated patient circuits for sale, however the later is considerably more expensive than the former. In keeping with modern advertising trends I have suggested that Intersurgical add a ‘go-faster’ racing stripe down the sleeves length - watch this space!
All trademarks are acknowledged.

Chris Wheatley MIOT (Copyright by author) Senior Medical Technician Life Support Services, ITU Stoke Mandeville Hospital

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