Shackleton Department of Anaesthesia, Southampton University Hospitals NHS Trust, Southampton General Hospital, Tremona Road, Southampton SO16 6YD, UK*Corresponding author
Accepted for publication: February 13, 2001
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Abstract |
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Br J Anaesth 2001; 87: 2379
Keywords: measurement techniques, transthoracic impedance; pregnancy; heart, defibrillation
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Introduction |
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Resuscitation may involve defibrillation for treatment of ventricular arrhythmias. The transthoracic current generated during defibrillation must depolarize a critical mass of myocardium to achieve successful reversion to sinus rhythm. Transmyocardial current is only about 4% of the total transthoracic current,2 so any factors that reduce transthoracic current may reduce transmyocardial current with an adverse effect on the success of defibrillation.
Physiological changes during pregnancy may change transthoracic impedance (TTI) and therefore affect transthoracic current during defibrillation. Increased intra- and extracellular fluid, increased blood volume, structural changes in the heart,36 changes in functional residual capacity of the lungs and increased thoracic volume may all change TTI and current pathways with unpredictable results.
We examined TTI at term and 68 weeks after delivery, once pregnancy-related physiological changes had resolved, to establish whether changes in TTI could affect defibrillation energy requirements at term.
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Materials and methods |
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TTI was measured using MultiScan 5000 impedance-measuring equipment (Bodystat Ltd, Isle of Man, UK). Self-adhesive defibrillation pads (Hewlett Packard M1749B) were placed according to the current guidelines of the European Resuscitation Council in an anterior-apical position.7 We avoided placing the apical pad over the breast. Impedance was measured using a low-current monophasic wave at 30 kHz averaged over a 3 s period. The subjects felt no electrical current or pain during measurements. This technique measures total transthoracic impedance as seen by the defibrillator (impedance of skinelectrode interface plus impedance of intrathoracic pathways). Three TTI measurements were made at end-expiration while breath was held for the duration of the measurement and the average measurement was recorded. All measurements were made with the subject reclining at 45° to the horizontal.
Statistical analysis
Calculation of sample size for paired data requires an estimate of the SD of the difference between measurements. We had no data on which to base estimates but assumed that 1 SD of the difference would be no greater than the SD of TTI of a population of normal subjects, which we know from previous studies to be approximately 7 in a similar age group.8 We estimated, therefore, that to detect a difference of >10% change in TTI, assuming a baseline TTI of approximately 66
as found with previous studies,8 a total of 40 patients would be required for the study to give a power >0.80 at P<0.05.9 TTI is know to have a parametric distribution and results were analysed using a paired two-way t-test. Significance was taken as P<0.05.
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Results |
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Discussion |
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Physiological changes during pregnancy may affect TTI and, therefore, transthoracic current during defibrillation. During normal pregnancy, total body water increases by 68 litres, of which 46 litres is extracellular and at least 23 litres interstitial.3 A 4050% increase in blood volume occurs, most of which is in the venous system and increases thoracic blood volume and pulmonary blood volume. The heart remodels early, and by term has increased in end-diastolic volume6 and undergone mild ventricular hypertrophy and dilation.5 Reduced plasma oncotic pressure may increase the water content of the lungs.4 These factors may decrease TTI by improving electrical conductivity through the tissues. Conversely, increased thoracic volume may increase TTI by increasing current pathways between paddles. Overall, the effects on TTI are difficult to predict.
Physiological changes resolve rapidly after delivery. Most changes in blood volume and fluid shifts have returned to normal by 2 weeks.5 We assumed that the physiological state would have returned to that of a non-pregnant woman by 68 weeks after delivery. Although resolution of mild ventricular hypertrophy seen at term takes 5 months,10 we considered that 68 weeks after delivery, its effects on overall TTI would be negligible.
We found no significant change in TTI during pregnancy, and so conclude that current guidelines for energy requirements for adults are appropriate for use during pregnancy. Although an unchanged TTI does not necessarily mean that current pathways and, in particular, transmyocardial current, are unchanged, we believe that transmyocardial current is unlikely to be altered significantly if overall TTI is unchanged.
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Acknowledgements |
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References |
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