Reporting standard for describing first responder systems, smartphone alerting systems, and AED networks.

Standardized reporting of data is crucial for out-of-hospital cardiac arrest (OHCA) research. While the implementation of first responder systems dispatching volunteers to OHCA is encouraged, there is currently no uniform reporting standard for describing these systems. A steering committee established a literature search to identify experts in smartphone alerting systems. These international experts were invited to a conference held in Hinterzarten, Germany, with 40 researchers from 13 countries in attendance. Prior to the conference, participants submitted proposals for parameters to be included in the reporting standard. The conference comprised five workshops covering different aspects of smartphone alerting systems. Proposed parameters were discussed, clarified, and consensus was achieved using the Nominal Group Technique. Participants voted in a modified Delphi approach on including each category as a core or supplementary element in the reporting standard. Results were presented, and a writing group developed definitions for all categories and items, which were sent to participants for revision and final voting using LimeSurvey web-based software. The resulting reporting standard consists of 68 core items and 21 supplementary items grouped into five topics (first responder system, first responder network, technology/algorithm/strategies, reporting data, and automated external defibrillators (AED)). This proposed reporting standard generated by an expert opinion group fills the gap in describing first responder systems. Its adoption in future research will facilitate comparison of systems and research outcomes, enhancing the transfer of scientific findings to clinical practice.

Incidence and costs of severe hypoglycaemia requiring attendance by the emergency medical services in South Central England.

AIMS: The aim was to estimate the incidence of severe hypoglycaemia requiring emergency ambulance assistance, its management and associated costs. METHODS: A retrospective observational study used routinely collected data for a 1-year period from December 2009 to November 2010 from the South Central Ambulance Service National Health Service Trust, UK. The main outcome was episodes reported by ambulance personnel and costs were estimated from published data. RESULTS: During the 1-year study period, 398,409 emergency calls were received, of which 4081 (1.02%) were coded as hypoglycaemia. The overall numbers (and annual rate) of hypoglycaemia recorded among people ≥ 15 years with presumed diabetes was 3962 (2.1%), but for those aged 15-35 years was 516 (7.5%) and for those aged ≥ 65 years was 1886 (1.9%). Of those attended, 1441 (35.3%) were taken to hospital. The estimated total cost of initial ambulance attendance and treatment at scene was £553,000; if transport to hospital was necessary, the additional ambulance costs were £223,000 plus emergency department costs of £140,000; and the cost of primary care follow-up was estimated as £61,000. The average cost per emergency call was £263. The estimated annual cost of emergency calls for severe hypoglycaemia is £13.6m for England. CONCLUSIONS: Our estimates suggest prevalence of severe hypoglycaemia attended by the emergency services is high in younger age groups and lower for older age groups, although the absolute numbers of severe events in older age groups contribute substantially to the overall costs of providing emergency assistance for hypoglycaemia.

Prehospital advanced airway management by ambulance technicians and paramedics: is clinical practice sufficient to maintain skills?

INTRODUCTION: Ambulance paramedics are now trained routinely in advanced airway skills, including tracheal intubation. Initial training in this skill requires the insertion of 25 tracheal tubes, and further ongoing training is attained through clinical practice and manikin-based practice. In contrast, training standards for hospital-based practitioners are considerably greater, requiring approximately 200 tracheal intubations before practice is unsupervised. With debate growing regarding the efficacy of paramedic intubation, there is a need to assess current paramedic airway practice in order to review whether initial training and maintenance of skills provide an acceptable level of competence with which to practice advanced airway skills. METHODS: All ambulance patient report forms (anonymised) for the period 1 January 2007 to 31 December 2007 were reviewed, and data relating to airway management were collected. Paramedic and technician identification codes were used to determine the number of airway procedures undertaken on an individual basis. RESULTS: Of the 269 paramedics, 128 (47.6%) had undertaken no intubation and 204 (75.8%) had undertaken one or less intubation in the 12-month study period. The median number of intubations per paramedic during the 12-month period was 1.0 (range 0-11). A total of 76 laryngeal mask insertion attempts were recorded by 41 technicians and 30 paramedics. The median number of laryngeal mask insertions per paramedic/technician during the 12-month period was 0 (range 0-2). A survey of ongoing continuing professional development across all ambulance trusts demonstrated no provision for adequate training to compensate for the lack of clinical exposure to advanced airway skills. CONCLUSION: Paramedics use advanced airway skills infrequently. Continuing professional development programmes within ambulance trusts do not provide the necessary additional practice to maintain tracheal intubation skills at an acceptable level. Advanced airway management delivered by ambulance crews is likely to be inadequate with such infrequent exposure to the skill.

Is ambulance telephone triage using advanced medical priority dispatch protocols able to identify patients with acute stroke correctly?

BACKGROUND AND PURPOSE: As many as half the patients presenting with acute stroke access medical care through the ambulance service. In order to identify and triage these patients effectively as life-threatening emergencies, telephone-based ambulance software must have high sensitivity and specificity when using verbal descriptions to identify such patients. Software-based clinical coding was compared with the patient's final clinical diagnosis for all patients admitted by ambulance to North Hampshire Hospital (NHH) emergency department (ED) over a 6-month period to establish the ability of telephone-based triage to identify patients with likely stroke accurately. METHODS: All emergency calls to South Central Ambulance Service over a 6-month period resulting in a patient being taken to NHH ED were reviewed. The classification allocated to the patient by ambulance advanced medical priority dispatch software (AMPDS version 11.1) was compared with the final clinical diagnosis made by a doctor in the ED. RESULTS: 4810 patients were admitted to NHH during the study period. Of these, 126 patients were subsequently diagnosed as having had a stroke. The sensitivity of AMPDS software for detecting stroke in this sample was 47.62%, specificity was 98.68%, positive predictive value was 0.49 and negative predictive value was 0.986. CONCLUSIONS: Fewer than half of all patients with acute stroke were identified using telephone triage on the initial emergency call to the ambulance service. Less than one quarter received the highest priority of ambulance response. This first link in the chain of survival needs strengthening in order to provide prompt and timely emergency care for these patients.

A prospective infant manikin-based observational study of telephone-cardiopulmonary resuscitation.

INTRODUCTION: Bystander cardiopulmonary resuscitation (CPR) has been shown to significantly improve outcome in sudden cardiac arrest in children. In view of this, most emergency medicine services deliver telephone instructions for carrying out CPR to laypeople who call the emergency services. Little is known as to whether laypeople carrying out these instructions deliver effective CPR. METHODS: Adult volunteers who had no previous experience of CPR were recruited. They were presented with a scenario and asked to perform CPR for 3 min on a training manikin according to the instructions they were given by telephone. Tidal volume, compression rate and depth, time to the beginning of CPR and hand positioning were recorded. RESULTS: Fifty-five volunteers were recruited; three were excluded (two had previous CPR training and one refused to perform CPR). None of the subjects identified correctly that the manikin was not breathing and achieved a level of CPR performance that was consistent with all of the current guidelines. Median tidal volume of rescue breaths was 38 mL. Only 23% of subjects delivered rescue breaths of optimal volume (40-50 mL) and 23% delivered no effective breaths at all. Chest compressions were performed at a median rate of 95 min(-1) with 37% delivering compressions at the optimum rate of 90-110 min(-1). CONCLUSION: None of our volunteers performed telephone-CPR at a level consistent with current guidelines. Further investigation is necessary to determine whether the instructions can be improved to optimise CPR performance.

Prehospital clearance of the cervical spine: does it need to be a pain in the neck?

Prehospital cervical spine (c-spine) immobilisation is common, despite c-spine injury being relatively rare. Unnecessary immobilisation results in a significant burden on limited prehospital and emergency department (ED) resources. This study aimed to determine whether the incidence of unnecessary c-spine immobilisation by ambulance personnel could be safely reduced through the implementation of an evidence-based algorithm. Following a training programme, complete forms on 103 patients were identified during the audit period, of which 69 (67%) patients had their c-spines cleared at scene. Of these, 60 (87%) were discharged at scene, with no clinical adverse events reported, and 9 (13%) were taken to the local ED with non-distracting minor injuries, all being discharged home the same day. 34 (33%) patients could not have their c-spines safely cleared at scene according to the algorithm. Of these, 4 (12%) patients self-discharged at scene and 30 (88%) were conveyed to an ED as per the normal procedure. C-spine clearance at scene by ambulance personnel may have positive impacts on patient care, efficient use of resources and cost to healthcare organisations.

Post-admission outcomes of participants in the PARAMEDIC trial: A cluster randomised trial of mechanical or manual chest compressions.

BACKGROUND: The PARAMEDIC cluster randomised trial evaluated the LUCAS mechanical chest compression device, and did not find evidence that use of mechanical chest compression led to an improvement in survival at 30 days. This paper reports patient outcomes from admission to hospital to 12 months after randomisation. METHODS: Information about hospital length of stay and intensive care management was obtained through linkage with Hospital Episode Statistics and the Intensive Care National Audit and Research Centre. Patients surviving to hospital discharge were approached to complete questionnaires (SF-12v2, EQ-5D, MMSE, HADS and PTSD-CL) at 90days and 12 months. The study is registered with Current Controlled Trials, number ISRCTN08233942. RESULTS: 377 patients in the LUCAS arm and 658 patients in the manual chest compression were admitted to hospital. Hospital and intensive care length of stay were similar. Long term follow-up assessments were limited by poor response rates (53.7% at 3 months and 55.6% at 12 months). Follow-up rates were lower in those with worse neurological function. Among respondents, long term health related quality of life outcomes and emotional well-being was similar between groups. Cognitive function, measured by MMSE, was marginally lower in the LUCAS arm mean 26.9 (SD 3.7) compared to control mean 28.0 (SD 2.3), adjusted mean difference -1.5 (95% CI -2.6 to -0.4). CONCLUSION: There were no clinically important differences identified in outcomes at long term follow-up between those allocated to the mechanical chest compression compared to those receiving manual chest compression.

Does telephone triage of emergency (999) calls using Advanced Medical Priority Dispatch (AMPDS) with Department of Health (DH) call prioritisation effectively identify patients with an acute coronary syndrome? An audit of 42,657 emergency calls to Hampshire Ambulance Service NHS Trust.

INTRODUCTION: The National Service Framework for Coronary Heart Disease requires identification of patients with an acute coronary syndrome (ACS) to enable prompt identification of those who may subsequently require pre-hospital thrombolysis. The Advanced Medical Priority Dispatch System (AMPDS) with Department of Health (DH) call prioritisation is now the common triage tool for emergency ('999') calls in the UK. We retrospectively examined patients with ACS to identify whether this triage tool had been able to allocate an appropriate emergency response. METHODS: All emergency calls to Hampshire Ambulance Service NHS Trust (HAST) from the Southampton area over an 8 month period (January to August 2004) were analysed. The classification allocated to the patient by AMPDS (version 10.4) was specifically identified. Data from the Myocardial Infarct National Audit Project) were obtained from the receiving hospital in Southampton to identify the actual number of patients with a true ACS. RESULTS: In total, 42 657 emergency calls were made to HAST from the Southampton area. Of these, 263 patients were subsequently diagnosed in hospital as having an ACS. Of these 263 patients, 76 presented without chest pain. Sensitivity of AMPDS for detecting ACS in this sample was 71.1% and specificity 92.5%. Positive predictive value was 5.6% (95% confidence interval 4.8 to 6.4%), and 12.5% (33/263) of patients with confirmed ACS were classified as non-life threatening (category B) incidents. CONCLUSION: Only one of approximately every 18 patients with chest pain has an ACS. AMPDS with DH call prioritisation is not a tool designed for clinical diagnosis, and its extension into this field does not enable accurate identification of patients with ACS.

Characteristics of patients who are not resuscitated in out of hospital cardiac arrests and opportunities to improve community response to cardiac arrest.

AIM: This study explores why resuscitation is withheld when emergency medical staff arrive at the scene of a cardiac arrest and identifies modifiable factors associated with this decision. METHODS: This is a secondary analysis of unselected patients who sustained an out of hospital cardiac arrest attended by ambulance vehicles participating in a randomized controlled trial of a mechanical chest compression device (PARAMEDIC trial). Patients were categorized as 'non-resuscitation' patients if there was a do-not-attempt-cardiopulmonary-resuscitation (DNACPR) order, signs unequivocally associated with death or resuscitation was deemed futile (15min had elapsed since collapse with no bystander-CPR and asystole recorded on EMS arrival). RESULTS: Emergency Medical Services attended 11,451 cardiac arrests. Resuscitation was attempted or continued by Emergency Medical Service staff in 4805 (42%) of cases. Resuscitation was withheld in 6646 cases (58%). 711 (6.2%) had a do not attempt resuscitation decision, 4439 (38.8%) had signs unequivocally associated with death and in 1496 cases (13.1%) CPR was considered futile. Those where resuscitation was withheld due to futility were characterised by low bystander CPR rates (7.2%) and by being female. CONCLUSIONS: Resuscitation was withheld by ambulance staff in over one in ten (13.1%) victims of out of hospital cardiac arrest on the basis of futility. These cases were associated with a very low rate of bystander CPR. Future studies should explore strengthening the 'Chain of Survival' to increase the community bystander CPR response and evaluate the effect on the numbers of survivors from out of hospital cardiac arrest.

Securing the prehospital airway: a comparison of laryngeal mask insertion and endotracheal intubation by UK paramedics.

OBJECTIVES: The recent introduction of a disposable laryngeal mask airway has provided paramedics with an alternative to endotracheal intubation. Time taken to secure the airway with each device was compared in patients undergoing elective surgery. METHODS: Patients undergoing general anaesthesia were studied. Paramedics trained in laryngeal mask use and endotracheal intubation participated in the study. A Portex disposable laryngeal mask was inserted and removed, followed by a Portex endotracheal tube. Time taken from beginning of the procedure to ventilation of the patient was recorded. RESULTS: Laryngeal mask insertion and endotracheal intubation was attempted on 52 patients. Median age was 63.5 years (range 39-83). Laryngeal mask insertion was successful in 88.5% (46 of 52) patients; endotracheal intubation was successful in 71.2% (37 of 52) patients (after no more than two attempts), p = 0.049. Intubation success was related to laryngoscopic view (87.5% grade 1, 56.3% grade 2, 0.0% grade 3. p<0.0001). When laryngeal mask/endotracheal tube insertion were both successful (n = 35 of 52), there was no significant difference in median time to secure the airway (laryngeal mask 47.0 seconds (range 24-126) compared with endotracheal tube 52.0 seconds (range 27-148) p = 0.22). Laryngeal mask insertion was successful in 80.0% (12 of 15) patients in whom endotracheal intubation had failed. CONCLUSIONS: Even under optimal conditions, 30% of attempts at intubation by paramedics were unsuccessful. A disposable laryngeal mask has a higher success rate in securing the airway and overall, secures the airway more reliably than endotracheal intubation.

Haemodynamic effects of descending aortic occlusion during cardiopulmonary resuscitation.

We describe two patients who both suffered a cardiac arrest whilst maintained on an intra-aortic balloon pump. In an attempt to improve coronary and cerebral blood flow during cardiopulmonary resuscitation, the intra-aortic balloon was inflated to occlude the descending aorta and preferentially direct blood to the cerebral and coronary circulation. In case 1, mean radial artery pressure rose from 71/14 mmHg (mean = 33 mmHg) to 92/24 mmHg (mean = 47 mmHg). Diastolic right atrial pressure was 16 mmHg both with the balloon deflated and inflated. In patient 2, mean radial artery pressure rose from 48/21 mmHg (mean = 25 mmHg) to 62/26 mmHg (mean = 36 mmHg). Right atrial pressure was 90/6 mmHg (mean 34 mmHg) with the balloon deflated and 104/8 mmHg (mean = 40 mmHg) with the balloon inflated. Coronary artery perfusion pressure in case 1 increased from -2 to 8 mmHg and in case 2 increased from 15 to 18 mmHg. These results suggest that occlusion of the descending aorta during cardiac massage may improve coronary and cerebral perfusion pressures. Animal studies are consistent with these findings and show that aortic occlusion may significantly improve outcome from cardiac arrest.

Does paddle force applied during defibrillation meet advanced life support guidelines of the European Resuscitation Council?

OBJECTIVES: to determine whether paddle force applied during defibrillation meets the 12 kg (approximately 120 N) force recommended by the advanced life support (ALS) guidelines of the European Resuscitation Council (ERC). MATERIALS AND METHODS: an adult mannequin was "defibrillated" using standard defibrillation paddles instrumented to measure paddle force. Paddle force was recorded at the time the discharge buttons on the paddle handles were depressed. RESULTS: 54 doctors and nurses performed simulated defibrillation on a mannequin. Median sternal paddle force was 60.6 N (range 26.1-132.8 N) and median apical paddle force was 59.5 N (range 18.6-118.5 N). Only 3/54 operators (5.6%) applied sternal paddle force equal to or in excess of ERC recommendations. No operator applied apical paddle force equal to or in excess of ERC recommendations. CONCLUSIONS: force applied to defibrillation paddles does not meet guidelines of the European Resuscitation Council. Greater emphasis during advanced life support training should be placed on the importance of firm paddle force during defibrillation.

Evidence-based guidelines for the use of defibrillation pads.

OBJECTIVE: Defibrillation pads are used routinely at both cardiac arrests and cardioversion procedures. There are currently no evidence-based guidelines on how often pads should be replaced, although it has been suggested that they should be changed as often as every three shocks to maintain optimal performance. Previously, we have shown that on exposure to air, pad mass diminishes over time due to evaporation--an effect likely to lead to poorer conduction between skin and paddle. This prospective study was designed to determine if evaporation is accelerated by the passage of a defibrillation current and to formulate evidence-based guidelines for defibrillation pad replacement. MATERIALS AND METHODS: 3M defibrillation pads (2346N) were collected from acute wards and emergency departments in two hospitals in the UK over a 2 month period. The duration of exposure to air, number and energy of shocks, and type of procedure were recorded. When no longer required, pad masses were determined and the loss of pad mass due to evaporation calculated. RESULTS: 26 pairs of pads were collected from 14 cardiac arrests and 12 cardioversions. The total defibrillation energy used ranged from 150 to 5080 J and evaporative drying time from 4 to 38 min. The rate of evaporation from these pads (86.1 mg x min(-1)) was not significantly different from pads previously studied on volunteers in the absence of a defibrillation current (99.4 mg x min(-1)). Of the defibrillation pads exposed to air for less than 30 min, in only one of 49 pads was the loss of mass due to evaporation consistent with a significant increase in transthoracic impedance (TTI). Correspondingly, of two pads used for more than 30 min, both attained a mass consistent with a significant increase in TTI. CONCLUSIONS: Defibrillation pads can be used for up to 30 min without evaporation causing a clinically significant increase in TTI. The passage of a defibrillation current across pads does not further accelerate water loss.

Are European Resuscitation Council recommendations for paddle force achievable during defibrillation?

Transthoracic impedance (TTI) is an important determinant of success in defibrillation. Low TTI increases transmyocardial current and therefore increases the chance of depolarising a critical mass of myocardium. A major component of TTI occurs at the paddle-skin interface and is minimised by pressure applied to the defibrillation paddles. The International Liaison Committee on Resuscitation (ILCOR) 2000 guidelines recommend that 'firm force' should be applied to both paddles, whereas previous European Resuscitation Council (ERC) 1992 guidelines were more precise, recommending that 12 kg of force should to be applied. We assessed whether defibrillator operators are capable of achieving 12 kg paddle force. Fifty advanced life support-trained doctors and nurses attempted to achieve 12 kg paddle force while simulating defibrillation on a resuscitation doll. The median value of the maximum pressures obtainable was 10.1 (max 16.0; min 5.0) kg force. Only 14% could achieve > or =12 kg force on both paddles for defibrillation. Men achieved more force than women (10.7 vs. 8.1 kg force; P<0.01), and there was a correlation between maximum force achieved and operator height (r2=0.27) and dominant hand-grip strength (r2=0.34). The ERC recommendation of 12 kg paddle force is not achievable by the majority of defibrillator operators.

How often should defibrillation pads be changed?: the effect of evaporative drying.

OBJECTIVE: In order to minimise transthoracic impedance (TTI) during defibrillation, water-based pads are used to improve conductivity between metal defibrillation paddles and skin. Subjectively, these pads appear to dry very quickly; an effect that may lead to an increase in TTI due to poorer conduction between paddles and skin. This study was carried out to assess the effect of evaporative drying of defibrillation pads on TTI. MATERIALS AND METHODS: TTI was measured at 5-10 min intervals in 20 adult male volunteers across 3M defibrillation pads (2346N) placed in the anterior-apical position. Measurements of TTI were made at 30 kHz using a Bodystat MultiScan 5000 monitor at end-expiration. A third pad was placed on the left precordium and its mass recorded each time a TTI measurement was made. RESULTS: The median age of subjects was 22 years (range 21-52 years) and their median body mass index was 23.1 kg m(-2) (range 18.4-42.8 kg m(-2)). Median room temperature was 23.0 degrees C (range 19.0-24.0 degrees C) and the median humidity was 31.0% (range 28.0-48.0%). 3M defibrillation pads had an initial mean mass of 25.14 g (S.D. +/- 0.41 g). Changes in defibrillation pad mass due to evaporative loss occurred immediately and rapidly, with all measurements being significantly lower than the baseline value. Mean baseline TTI was 63.6 ohms (S.D. +/- 13.7 ohms). After 30 min a statistically significant (P = 0.012) rise of 1.4 ohms (2.2%), was observed corresponding to a 12.6% decrease in pad mass, after which TTI continued to increase in a linear fashion. CONCLUSION: In the absence of a defibrillation current. 3M defibrillation pads can safely be left on the chest wall for at least 25 min in a typical hospital environment before evaporative drying results in a significant increase in transthoracic impedance.

A comparison of transthoracic impedance using standard defibrillation paddles and self-adhesive defibrillation pads.

The success of defibrillation is related to transmyocardial current. This current is inversely proportional to transthoracic impedance (TTI). A similar TTI between different pads and paddles is important to deliver a consistent therapeutic dose to all subjects. Failure to do so may result in either insufficient or excessive transmyocardial current leading to a risk of failed defibrillation or tissue damage respectively. Several different types of defibrillation paddles and self-adhesive defibrillation pads are currently available for clinical use but the TTI achieved with each type has not been established. We measured TTI using two types of commonly used paddles and self-adhesive pads to establish whether any significant differences exist between the products. TTI in 40 adult males was measured using defibrillation paddles and self-adhesive defibrillation pads placed in the antero-apical position. Measurements were made using a 30 kHz low amplitude AC current and taken at end-expiration. Mean TTI (omega)+/-S.D. was 68.2+/-16.1 (Hewlett Packard paddles; A), 62.8+/-13.2 (Hewlett Packard pads; B), 64.6+/-14.3 (PhysioControl paddles; C) and 95.6+/-22.3 (PhysioControl pads; D). Significant differences existed between all groups (P < 0.05) except between B and C. Differences in TTI between A, B and C were small and probably of no clinical significance. TTI in group D is significantly larger. Although transmyocardial current is related to TTI, the relationship is complex and differences in TTI alone cannot predict the outcome from defibrillation.

Effects of positive end-expiratory pressure on transthoracic impedance--implications for defibrillation.

The success of defibrillation is determined by trans-myocardial current. This current is inversely proportional to transthoracic impedance (TTI). We proposed that increasing lung volume using positive end-expiratory pressure (PEEP) would increase TTI. 12 healthy subjects aged 21-37 years (6 male) were recruited. TTI between self-adhesive defibrillation pads placed in the standard antero-apical position was measured at 30 kHz at end-expiration using Bodystat MultiScan 5000 equipment. PEEP at 2.5, 5.0, 10.0, 20.0 and 40.0 cm H2O was generated using a standard continuous positive airways pressure (CPAP) circuit. Data were analysed using linear regression and paired t-tests comparing baseline TTI with that at a given PEEP. Mean (+/- S.D.) baseline TTI was 65.7 +/- 6.9 omega. TTI increased linearly with increasing PEEP (r2 = 0.99; P < 0.001). Mean percentage increases in TTI were 0.6% at 2.5 cm H2O PEEP (P = 0.05), 1.5%, at 5.0 cm H2O PEEP (P < 0.001), 3.1% at 10.0 cm H2O PEEP (P < 0.001), 5.6% at 20.0 cm H2O PEEP (P < 0.001) and 10.4% at 40.0 cm H2O PEEP (P < 0.001). PEEP increases TTI, proportionately reducing transthoracic current during defibrillation. Clinically, high levels of PEEP (20-40 cm H2O) may occur during ventilation of patients at cardiac arrest and in acute asthmatics. To maximise peak defibrillation current, PEEP should either be minimised prior to defibrillation or consideration should be given to earlier use of high energy levels for defibrillation.

Effects of respiratory gas composition on transthoracic impedance.

The transthoracic current generated during defibrillation comprises a cardiac and extracardiac component. Changes in impedance of transthoracic pathways will alter the transmyocardial current and may thus affect the outcome from defibrillation. The lungs comprise a large component of extracardiac tissue but the effects of different respiratory gases on transthoracic impedance has not been documented. We therefore measured transthoracic impedance (TTI) using different respiratory gas mixtures. TTI across self-adhesive defibrillation pads placed in the antero-apical position was measured at end-expiration using a 30 kHz low amplitude AC current. Ten healthy subjects aged 22-34 years (eight male) were studied whilst breathing alternate mixtures of air, 100% oxygen. 70% helium in 30% oxygen, and 70% nitrous oxide in 30% oxygen administered in a random sequence. There was no significant difference in TTI between any of the four respiratory gases. Therefore transthoracic current during defibrillation is unlikely to be affected by different respiratory gases. This is the first study to document that these respiratory gases do not change the impedance of transthoracic pathways.