British Journal of Anaesthesia, 2001, Vol. 87, No. 2 237-239
© 2001 The Board of Management and Trustees of the British Journal of Anaesthesia
Clinical Investigations |
Do physiological changes in pregnancy change defibrillation energy requirements?
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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Resuscitation during pregnancy is uncommon, with approximately 70 deaths occurring during pregancy in the UK per annum. Physiological changes during pregnancy may affect transthoracic impedance (TTI), which determines transmyocardial current. Increased blood volume, cardiomegaly, haemodilution, changes in lung volume and changes in thoracic volume may alter impedance in ways that are difficult to predict. We measured TTI at term and after delivery once physiological changes had resolved. Mean (SD) TTI was 91.3 (15.8)
at term and 91.6 (11.8) 6–8 weeks after delivery; the difference was not statistically significant. We conclude that current energy requirements for adult defibrillation are appropriate for use during pregnancy. Br J Anaesth 2001; 87: 237–9
Keywords: measurement techniques, transthoracic impedance; pregnancy; heart, defibrillation
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Introduction |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Deaths in pregnancy are relatively rare but have a particularly significant impact in terms of the young age of the mother, the mortality of the unborn child and the long-term effects on the family. In the UK, deaths during pregnancy exceed 70 per annum.1
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,3–6 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 6–8 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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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Following Ethics Committee approval and written informed consent, we sequentially recruited 45 women with uncomplicated pregnancy within 2 weeks of estimated date of delivery who were attending the Princess Anne Hospital, Southampton. TTI was measured before delivery and 6–8 weeks after delivery.
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 skin–electrode 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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Forty-five women consented to measurement of TTI at term. Post partum measurements were made 6–8 weeks later on 42 of these women; the remaining three patients declined to be measured for a second time. Patient characteristics are given in Table 1. The mean (SD) TTI was 91.3 (15.8)
at term (range 63.0–137.7 ) and 91.6 (11.5) 6–8 weeks after delivery (range 69.3–118.3 ). There was no statistically significant difference between the two groups (P>0.80).
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Examining for a 10% difference in TTI between the two groups, the standardized difference9 was 0.64, giving the study a power of >0.80 at P<0.05.
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Discussion |
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Cardiac arrest during pregnancy is a rare event: <1% of deaths in the female population aged 15–44 yr occur during pregnancy. In the UK, this equates to approximately 70 direct and indirect deaths per annum.1 The proportion of these cases requiring defibrillation for ventricular arrhythmias is unknown. Pulseless electrical activity (PEA) is likely to be a common presenting arrhythmia in deaths from pulmonary embolism, hypovolaemia and amniotic fluid embolus but may often degenerate to ventricular fibrillation.
Physiological changes during pregnancy may affect TTI and, therefore, transthoracic current during defibrillation. During normal pregnancy, total body water increases by 6–8 litres, of which 4–6 litres is extracellular and at least 2–3 litres interstitial.3 A 40–50% 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 6–8 weeks after delivery. Although resolution of mild ventricular hypertrophy seen at term takes 
5 months,10 we considered that 6–8 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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We are very grateful to Bodystat for their loan of the ‘BodyStat 5000’ impedance-measuring equipment. We also thank Dr Graham Petley for his technical support of this study.
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References |
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1 Department of Health. Why Mothers Die. Report on Confidential Enquiries into Maternal Deaths in the United Kingdom 1994–6. London: HMSO, 1998
2 Bossaert L, Koster R. Defibrillation: methods and strategies. Resuscitation 1992; 24: 2111–225
3 Davison JM. Edema in pregnancy. Kidney Int 1997; 51: S90–96
4 Hytten F. Blood volume changes in normal pregnancy. Clin Haematol 1985; 14: 601–12[Web of Science][Medline]
5 Hunter S, Robson S. Adaptation of the maternal heart in pregnancy. Br Heart J 1992; 68: 540–3
6 Thornberg KL, Jacobson SL, Giraud GD, Morton MJ. Hemodynamic changes in pregnancy. Semin Perinatol 2000; 24: 11–14[Web of Science][Medline]
7 Advanced Life Support Working Group of the ERC. Guidelines for advanced life support. Resuscitation 1992; 24: 111–132[Web of Science][Medline]
8 Deakin CD, McLaren RM, Petley G, Dalrymple-Hay M, Clewlow F. Effects of positive end-expiratory pressure on transthoracic impedance—implications for defibrillation. Resuscitation 1998; 37: 9–12[Web of Science][Medline]
9 Altman D. Statistics and ethics in medical research: how large a sample? Br Med J 1980; 281: 1336–8
10 Duvekot J, Peeters L. Maternal cardiovascular haemodynamic adaptation to pregnancy. Obstet Gynecol Surv 1994; 49: S1–14[Medline]
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