Face immersion in cold water induces prolongation of the QT interval and T-wave changes in children with nonfamilial long QT syndrome

Drowning incidents precipitated by unusual causes (DIPUCs): A narrative review of their diagnoses, evaluation and management

Drowning is a cause of significant morbidity and mortality worldwide. In most circumstances, the proximate cause is attributable to human factors, such as inexperience, fatigue, intoxication, or hazardous water conditions. The phenomenon of drowning incidents precipitated by unusual circumstances (DIPUCs) - either fatal or nonfatal - involving otherwise healthy individuals under generally safe conditions has not been comprehensively addressed in the medical and drowning literature to date. In this review, we discuss etiologies of DIPUCs, diagnostic clues, suggested workup, suggested postmortem testing, and implications for surviving patients and families. Identifying the cause of a drowning incident can be extremely challenging for the initially treating physician, relying perforce on historical context, environmental clues, physical exam, medical history, eyewitness accounts or video recordings. If no clear explanation for a drowning incident emerges despite a thorough investigation, clinicians should consider some of the less common diagnoses we describe in this paper, and, when appropriate, refer for an autopsy with postmortem molecular genetic testing. While time-consuming, these efforts can prove life-saving for some non-fatal drowning victims and the families of all victims of DIPUCs.

Cardiac response to water activities in children with Long QT syndrome type 1

BACKGROUND

Swimming is a genotype-specific trigger in long QT syndrome type 1 (LQT1).

OBJECTIVE

To examine the autonomic response to water activities in children and adolescents with LQT1.

METHODS

In this cross-sectional study, LQT1 patients were age and sex matched to one healthy control subject. Electrocardiograms (ECGs) were recorded during face immersion (FI), swimming, diving, and whole-body submersion (WBS). Heart rate (HR) and heart rate variability (HRV) was measured. The high frequency (HF) component of HRV was interpreted to reflect parasympathetic activity, while the low frequency (LF) component was interpreted as reflecting the combined influence of sympathetic and parasympathetic activity on autonomic nervous modulation of the heart.

RESULTS

Fifteen LQT1 patients (aged 7-19 years, all on beta-blocker therapy) and fifteen age and sex matched non-medicated controls were included. No significant ventricular arrhythmias were observed in the LQT1 population during the water activities. Out of these 15 matched pairs, 12 pairs managed to complete FI and WBS for more than 10 seconds and were subsequently included in HR and HRV analyses. In response to FI, the LQT1 group experienced a drop in HR of 48 bpm, compared to 67 bpm in the control group (p = 0.006). In response to WBS, HR decreased by 48 bpm in the LQT1 group and 70 bpm in the control group (p = 0.007). A significantly lower PTOT (p < 0.001) and HF (p = 0.011) component was observed before, during and after FI in LQT1 patients compared with the controls. Before, during and after WBS, a significantly lower total power (p < 0.001), LF (p = 0.002) and HF (p = 0.006) component was observed in the LQT1 patients.

CONCLUSION

A significantly lower HR decrease in response to water activities was observed in LQT1 subjects on beta-blocker therapy, compared to matched non-medicated controls. The data suggests an impaired parasympathetic response in LQT1 children and adolescents. An aberrant autonomic nervous system (ANS) response may cause an autonomic imbalance in this patient group.

Provocation testing in congenital long QT syndrome: A practical guide

Congenital long QT syndrome (LQTS) is a hereditary cardiac channelopathy with an estimated prevalence of 1 in 2500. A prolonged resting QT interval corrected for heart rate (QTc interval) remains a key diagnostic component; however, the QTc value may be normal in up to 40% of patients with genotype-positive LQTS and borderline in a further 30%. Provocation of QTc prolongation and T-wave changes may be pivotal to unmasking the diagnosis and useful in predicting genotype. LQTS provocation testing involves assessment of repolarization during and after exercise, in response to changes in heart rate or autonomic tone, with patients with LQTS exhibiting a maladaptive repolarization response. We review the utility and strengths and limitations of 4 forms of provocation testing-stand-up test, exercise stress test, epinephrine challenge, and mental stress test-in diagnosing LQTS and provide some practical guidance for performing provocation testing. Ultimately, exercise testing, when feasible, is the most useful form of provocation testing when considering diagnostic sensitivity and specificity.

Utility of Provocative Testing in the Diagnosis and Genotyping of Congenital Long QT Syndrome: A Systematic Review and Meta-Analysis

Background Diagnosis is particularly challenging in concealed or asymptomatic long QT syndrome (LQTS). Provocative testing, unmasking the characterization of LQTS, is a promising alternative method for the diagnosis of LQTS, but without uniform standards. Methods and Results A comprehensive search was conducted in PubMed, Embase, and the Cochrane Library through October 14, 2021. The fixed effects model was used to assess the effect of the provocative testing on QTc interval. A total of 22 studies with 1137 patients with LQTS were included. At baseline, QTc interval was 40 ms longer in patients with LQTS than in controls (mean difference [MD], 40.54 [95% CI, 37.43-43.65]; P<0.001). Compared with the control group, patients with LQTS had 28 ms longer ΔQTc upon standing (MD, 28.82 [95% CI, 23.05-34.58]; P<0.001), nearly 30 ms longer both at peak exercise (MD, 27.31 [95% CI, 21.51-33.11]; P<0.001) and recovery 4 to 5 minutes (MD, 29.85 [95% CI, 24.36-35.35]; P<0.001). With epinephrine infusion, QTc interval was prolonged both in controls and patients with QTS, most obviously in LQT1 (MD, 68.26 [95% CI, 58.91-77.60]; P<0.001) and LQT2 (MD, 60.17 [95% CI, 50.18-70.16]; P<0.001). Subgroup analysis showed QTc interval response to abrupt stand testing and exercise testing varied between LQT1, LQT2, and LQT3, named Type Ⅰ, Type Ⅱ, and Type Ⅲ. Conclusions QTc trend Type Ⅰ and Type Ⅲ during abrupt stand testing and exercise testing can be used to propose a prospective evaluation of LQT1 and LQT3, respectively. Type Ⅱ QTc trend combined epinephrine infusion testing could distinguish LQT2 from control. A preliminary diagnostic workflow was proposed but deserves further evaluation.

The association between body temperature and electrocardiographic parameters in normothermic healthy volunteers

Background

Previous studies reported that hypo‐ and hyperthermia are associated with several atrial and ventricular electrocardiographical parameters, including corrected QT (QTc) interval. Enhanced characterization of variations in QTc interval and normothermic body temperature aids in better understanding the underlying mechanism behind drug induced QTc interval effects. The analysis’ objective was to investigate associations between body temperature and electrocardiographical parameters in normothermic healthy volunteers.

Methods

Data from 3023 volunteers collected at our center were retrospectively analyzed. Subjects were considered healthy after review of collected data by a physician, including a normal tympanic body temperature (35.5‐37.5°C) and in sinus rhythm. A linear multivariate model with body temperature as a continuous was performed. Another multivariate analysis was performed with only the QT subintervals as independent variables and body temperature as dependent variable.

Results

Mean age was 33.8 ± 17.5 years and mean body temperature was 36.6 ± 0.4°C. Body temperature was independently associated with age (standardized coefficient [SC] = −0.255, P < .001), female gender (SC = +0.209, P < .001), heart rate (SC = +0.231, P < .001), P‐wave axis (SC = −0.051, P < .001), J‐point elevation in lead V4 (SC = −0.121, P < .001), and QTcF duration (SC = −0.061, P = .002). In contrast, other atrial and atrioventricular (AV) nodal parameters were not independently associated with body temperature. QT subinterval analysis revealed that only QRS duration (SC = −0.121, P < .001) was independently associated with body temperature.

Conclusion

Body temperature in normothermic healthy volunteers was associated with heart rate, P‐wave axis, J‐point amplitude in lead V4, and ventricular conductivity, the latter primarily through prolongation of the QRS duration.

Dynamic QT response to cold-water face immersion in long-QT syndrome type 3

BACKGROUND

Abnormal dynamics of QT intervals in response to sympathetic nervous system stimulation are used to diagnose long-QT syndrome (LQTS). We hypothesized that parasympathetic stimulation with cold-water face immersion following exercise would influence QT dynamics in patients with LQTS type 3 (LQT3).

METHODS

Study participants (n = 42; mean age = 11.2 years) comprised 20 genotyped LQTS children and 22 healthy children. The LQTS group was divided into LQT3 (n = 12) and non-LQT3 (n = 8) subgroups. Provocative testing for assessing QT dynamics comprised a treadmill exercise followed by cold-water face immersion. The QT intervals were automatically measured at rest and during exercise, recovery, and cold-water face immersion. The QT/heart rate (HR) relationship was visualized by plotting beat-to-beat confluence of the data.

RESULTS

The QT/HR slopes, determined by linear regression analysis, were steeper in the LQTS group than in the control group during exercise and immersion tests: -2.16 ± 0.63 versus -1.21 ± 0.28, P < 0.0001, and -2.02 ± 0.76 vs -0.75 ± 0.24, P < 0.0001, respectively. The LQT3 patients had steeper slopes in the immersion test than did non-LQT3 and control individuals: -2.42 ± 0.52 vs -1.40 ± 0.65, P < 0.0001, and vs -0.75 ± 0.24, P < 0.0001.

CONCLUSIONS

The QT dynamics of LQT3 patients differ from those of other LQTS subtypes during the post-exercise cold-water face immersion test in this study. Abnormal QT dynamics during the parasympathetic provocative test are concordant with the fact that cardiac events occur when HRs are lower or during sleep in LQT3 patients.

The Brain-Heart Connection in Sympathetically Triggered Inherited Arrhythmia Syndromes

Sympathetically triggered inherited arrhythmia syndromes, including the long QT syndrome (LQTS) and catecholaminergic polymorphic ventricular tachycardia (CPVT), can cause sudden cardiac death in young individuals with structurally normal hearts. With cardiac events typically triggered by physical or emotional stress, not surprisingly, two of the most common treatments are neuromodulators, including mainstay beta blocker pharmacotherapy, and surgical sympathetic cardiac denervation. This review updates the clinician on the relevant anatomy and physiology of the cardiac autonomic nervous system, outlines neurocardiac arrhythmia mechanisms, and discusses the latest rationale for a neurocardiac therapeutic approach to manage sympathetic-induced arrhythmia in patients with inherited cardiac disease.

Sudden Unexpected Death and the Mammalian Dive Response: Catastrophic Failure of a Complex Tightly Coupled System

In tightly coupled complex systems, when two or more factors or events interact in unanticipated ways, catastrophic failures of high-risk technical systems happen rarely, but quickly. Safety features are commonly built into complex systems to avoid disasters but are often part of the problem. The human body may be considered as a complex tightly coupled system at risk of rare catastrophic failure (sudden unexpected death, SUD) when certain factors or events interact. The mammalian dive response (MDR) is a built-in safety feature of the body that normally conserves oxygen during acute hypoxia. Activation of the MDR is the final pathway to sudden cardiac (SCD) in some cases of sudden infant death syndrome (SIDS), sudden unexpected death in epilepsy (SUDEP), and sudden cardiac death in water (SCDIW, fatal drowning). There is no single cause in any of these death scenarios, but an array of, unanticipated, often unknown, factors or events that activate or interact with the mammalian dive reflex. In any particular case, the relevant risk factors or events might include a combination of genetic, developmental, metabolic, disease, environmental, or operational influences. Determination of a single cause in any of these death scenarios is unlikely. The common thread among these seemingly different death scenarios is activation of the mammalian dive response. The human body is a complex tightly coupled system at risk of rare catastrophic failure when that "safety feature" is activated.

Long-QT Syndrome and Competitive Sports

Long QT syndrome (LQTS) is an inherited channelopathy which exposes athletes to a risk of sudden cardiac death. Diagnosis is more difficult in this population because: the QT interval is prolonged by training; and the extreme bradycardia frequently observed in athletes makes the QT correction formula less accurate. Based on limited clinical data which tend to demonstrate that exercise, especially swimming, is a trigger for cardiac events, participation in any competitive sports practice is not supported by 2005 European guidelines. However, based on recent retrospective studies and adopting a different medical approach, involving the patient-athlete in shared decision making, the 2015 US guidelines are less restrictive, especially in asymptomatic genotype-positive/phenotype-negative athletes. These guidelines also consider giving medical clearance to competitive sport participation in asymptomatic athletes with appropriate medical therapy.

Exercise participation and shared decision-making in patients with inherited channelopathies and cardiomyopathies

Sports eligibility and disqualification of patients with cardiac diseases are important considerations for adult and pediatric cardiologists. The 2005 guidelines that addressed this issue have recently been revised and updated, and the new guidelines advocate for a shared decision-making approach in which the well-informed athlete and family participate in the discussion. In this review, we focus on the benefits of sports participation and review the revised guidelines related to sports participation in patients with channelopathies and cardiomyopathies.

Citalopram-Induced Long QT Syndrome and the Mammalian Dive Reflex

While SCUBA diving, a 44-year-old Caucasian patient had an abnormal cardiac rhythm, presumably Torsade de Pointes (TdP), during the initial descent to depth. Upon surfacing, she developed ventricular fibrillation and died. The patient had been treated for mild depression for nearly a year with citalopram 60 mg per day, a drug known to cause prolonged QT interval. She had also been treated with two potentially hepatotoxic drugs. Liver impairment causes selective loss of cytochrome P450 (CYP) 2C19 activity, the major pathway for metabolism of citalopram. The post mortem blood level of citalopram was 1300 ng/mL. The patient was found to be an intermediate metabolizer via CYP2D6, the major pathway for metabolism of desmethylcitalopram; the level of which was also abnormally high. It is suggested that drug-induced long QT syndrome (DILQTS), caused by citalopram, combined with the mammalian dive reflex triggered malignant ventricular rhythms resulting in the patient’s death. It is further suggested that, in general, the dive reflex increases the risk of fatal cardiac rhythms when the QT interval is prolonged by drugs.

Swimming and the heart

Exercise training is accepted to be beneficial in lowering morbidity and mortality in patients with cardiac disease. Swimming is a popular recreational activity, gaining recognition as an effective option in maintaining and improving cardiovascular fitness. Swimming is a unique form of exercise, differing from land-based exercises such as running in many aspects including medium, position, breathing pattern, and the muscle groups used. Water immersion places compressive forces on the body with resulting physiologic effects. We reviewed the physiologic effects and cardiovascular responses to swimming, the cardiac adaptations to swim training, swimming as a cardiac disease risk factor modifier, and the effects of swimming in those with cardiac disease conditions such as coronary artery disease, congestive heart failure and the long-QT syndrome.

'Autonomic conflict': a different way to die during cold water immersion?

Cold water submersion can induce a high incidence of cardiac arrhythmias in healthy volunteers. Submersion and the release of breath holding can activate two powerful and antagonistic responses: the 'cold shock response' and the 'diving response'. The former involves the activation of a sympathetically driven tachycardia while the latter promotes a parasympathetically mediated bradycardia. We propose that the strong and simultaneous activation of the two limbs of the autonomic nervous system ('autonomic conflict') may account for these arrhythmias and may, in some vulnerable individuals, be responsible for deaths that have previously wrongly been ascribed to drowning or hypothermia. In this review, we consider the evidence supporting this claim and also hypothesise that other environmental triggers may induce autonomic conflict and this may be more widely responsible for sudden death in individuals with other predisposing conditions.

Drowning

PURPOSE OF REVIEW

To summarize current knowledge on pathophysiology and treatment of drowning accidents. Studies and case reports were searched using the keywords drowning, near-drowning, asphyxia, hypoxia and hypothermia in conjunction with organ systems and specific treatment options.

RECENT FINDINGS

Drowning is defined as death by suffocation in a liquid. In contrast, near-drowning is defined as survival beyond 24 h after a drowning accident. Drowning is a frequent preventable accident with a significant morbidity and mortality in a mostly healthy population. In the majority of patients the primary injury is pulmonary, resulting in severe arterial hypoxemia and secondary damage to other organs. Damage to the central nervous system is most critical in terms of patient survival and subsequent quality of life. Therefore, prompt resuscitation and aggressive respiratory and cardiovascular treatment are crucial for optimal survival. Immediate interruption of hypoxia, aggressive treatment of hypothermia and cardiovascular failure are the cornerstones of correct medical treatment. Unfortunately, accurate neurologic prognosis cannot be predicted from initial clinical presentation, laboratory, radiological or electrophysiological examinations.

SUMMARY

Several case studies have convincingly demonstrated that drowning victims may survive neurologically intact even after prolonged submersion times, in particular in cold water. Therefore, aggressive initial therapeutic efforts are indicated in most near-drowning victims.

Ventricular Repolarization and Heart Rate Responses During Cardiovascular Autonomic Function Testing in LQT1 Subtype of Long QT Syndrome

BACKGROUND

In the most prevalent LQT1 form of inherited long QT syndrome symptoms often occur during abrupt physical or emotional stress. Sympathetic stimulation aggravates repolarization abnormalities in experimental LQT1 models. We hypothesized that autonomic function tests might reveal the abnormal repolarization in asymptomatic LQT1 patients.

METHODS

We measured heart rates (HRs) and QT intervals in nine asymptomatic carriers of a C-terminal KCNQ1 mutation and 8 unaffected healthy subjects using an approach of global QT values derived from 28 simultaneous electrocardiographic leads on beat-to-beat base during Valsalva maneuver, mental stress, sustained handgrip, and light supine exercise.

RESULTS

LQT1 patients exhibited impaired shortening of both QTpeak and QTend intervals during autonomic interventions but exaggerated lengthening of the intervals--a QT overshoot--during the recovery phases. The number of tests with a QT overshoot was 2.4 +/- 1.7 in LQT1 patients and 0.8 +/- 0.7 in unaffected subjects (P = 0.02). Valsalva strain prolonged T wave peak to T wave end interval (TPE) in LQT1 but not in unaffected patients. LQT1 patients showed diminished HR acceleration in response to adrenergic challenge whereas HR responses to vagal stimuli were similar in both groups.

CONCLUSIONS

Standard cardiovascular autonomic provocations induce a QT interval overshoot during recovery in asymptomatic KCNQ1 mutation carriers. Valsalva maneuver causes an exaggerated fluctuation of QT and TPE intervals partly explaining the occurrence of cardiac events during abrupt bursts of autonomic activity in LQT1 patients.

Respiratory responses to cold water immersion: neural pathways, interactions, and clinical consequences awake and asleep

The ventilatory responses to immersion and changes in temperature are reviewed. A fall in skin temperature elicits a powerful cardiorespiratory response, termed “cold shock,” comprising an initial gasp, hypertension, and hyperventilation despite a profound hypocapnia. The physiology and neural pathways of this are examined with data from original studies. The respiratory responses to skin cooling override both conscious and other autonomic respiratory controls and may act as a precursor to drowning. There is emerging evidence that the combination of the reestablishment of respiratory rhythm following apnea, hypoxemia, and coincident sympathetic nervous and cyclic vagal stimulation appears to be an arrhythmogenic trigger. The potential clinical implications of this during wakefulness and sleep are discussed in relation to sudden death during immersion, underwater birth, and sleep apnea. A drop in deep body temperature leads to a slowing of respiration, which is more profound than the reduced metabolic demand seen with hypothermia, leading to hypercapnia and hypoxia. The control of respiration is abnormal during hypothermia, and correction of the hypoxia by inhalation of oxygen may lead to a further depression of ventilation and even respiratory arrest. The immediate care of patients with hypothermia needs to take these factors into account to maximize the chances of a favorable outcome for the rescued casualty.

Fosphenytoin may cause hemodynamically unstable bradydysrhythmias

The prodrug fosphenytoin (FOS) was recently introduced as an alternative to phenytoin (PTN) and has since become a first line therapy for status epilepticus. Unlike PTN, FOS generally has been considered to be safe from cardiac toxicity. To better characterize cardiac toxicity associated with FOS administration, we performed a review of the Food and Drug Administration's Adverse Event Reporting System databank for reports of possible FOS toxicity from 1997-2002. There were 29 applicable reports of adverse cardiac events likely related to FOS infusion, including 10 cardiac deaths. Among survivors, there were four cases of high-grade atrioventricular block, and five cases of transient sinus arrest. Our data suggest that FOS may produce more cardiac toxicity than previously thought. Clinicians should consider administering intravenous FOS in a monitored setting for selected high-risk patients.

Cold-Water Face Immersion Per Se Elicits Cardiac Parasympathetic Activity

BACKGROUND

Cold-water face immersion (FI) is known to produce physiological changes, including bradycardia, by stimulating the parasympathetic system. However, other factors such as sympathetic activity, intrapleural pressures, and changes in chemical mediators may also contribute to these changes.

METHODS AND RESULTS

Eight healthy volunteers underwent a series of experiments designed to observe the effects of FI on heart rate and its variability, as detected using wavelet transformation. Each subject was instructed to bend over and put the entire face into an empty basin with and without breathing (protocols 1 and 2, respectively), and then perform FI in warm-water (protocols 3 and 4, respectively) and cold-water (protocols 5 and 6, respectively) while breathing and breath holding. Change in the R-R interval with FI was only significantly greater for protocol 6 than for the control procedure (protocol 1). Also, changes in the natural logarithm of high-frequency power with FI were significantly greater for protocols 5 and 6 than the protocol 1.

CONCLUSIONS

Bradycardia associated with cold-water FI is mainly attributed to cardiac vagal activity, which is independent of both the change in body position caused by bending over a basin and breath holding.

Interval Representative of Transmural Dispersion of Repolarization in Children and Young Adolescents With Congenital Long QT Syndrome

BACKGROUND

It has been shown experimentally that the interval from the nadir of the initial negative T wave to the end of the T wave is representative of transmural dispersion of repolarization (TDR) when complex T waves are present. In the clinical setting, however, the interval representative of TDR in patients with long QT syndrome (LQTS) is a controversial subject.

METHODS AND RESULTS

Five symptomatic patients (3 boys, 2 girls; 3 LQT1, 2 LQT2) were evaluated by a face immersion test before and after treatment to compare the configuration of the T wave. When the notch disappeared after treatment, the single peak of the T wave after treatment coincided with the nadir of the notch before treatment. When the notch remained the same after treatment as before treatment and when the QTc decreased, the corrected interval from the nadir of the notch to the end of the T wave was for the most part shortened.

CONCLUSIONS

The present study showed that the interval representative of the TDR in the clinical surface electrocardiogram can be obtained from the nadir of the notch to the end of the T wave in children and adolescents with LQTS, as was shown in the experimental study.

Spectrum and frequency of cardiac channel defects in swimming-triggered arrhythmia syndromes

BACKGROUND

Swimming is a relatively genotype-specific arrhythmogenic trigger for type 1 long-QT syndrome (LQT1). We hypothesize that mimickers of concealed LQT1, namely catecholaminergic polymorphic ventricular tachycardia (CPVT), may also underlie swimming-triggered cardiac events.

METHODS AND RESULTS

Between August 1997 and May 2003, 388 consecutive, unrelated patients were referred specifically for LQTS genetic testing. The presence of a personal and/or family history of a near-drowning or drowning was determined by review of the medical records and/or phone interviews and was blinded to genetic test results. Comprehensive mutational analysis of the 5 LQTS-causing channel genes, KCNQ1 (LQT1), KCNH2 (LQT2), SCN5A (LQT3), KCNE1 (LQT5), and KCNE2 (LQT6), along with KCNJ2 (Andersen-Tawil syndrome) and targeted analysis of 18 CPVT1-associated exons in RyR2, was performed with the use of denaturing high-performance liquid chromatography and direct DNA sequencing. Approximately 11% (43 of 388) of the index cases had a positive swimming phenotype. Thirty-three of these 43 index cases had a "Schwartz" score (> or =4) suggesting high clinical probability of LQTS. Among this subset, 28 patients (85%) were LQT1, 2 patients (6%) were LQT2, and 3 were genotype negative. Among the 10 cases with low clinical probability for LQTS, 9 had novel, putative CPVT1-causing RyR2 mutations.

CONCLUSIONS

In contrast to previous studies that suggested universal LQT1 specificity, genetic heterogeneity underlies channelopathies that are suspected chiefly because of a near-drowning or drowning. CPVT1 and strategic genotyping of RyR2 should be considered when LQT1 is excluded in the pathogenesis of a swimming-triggered arrhythmia syndrome.

Who Is at Risk for Cardiac Events in Young Patients With Long QT Syndrome?

The objective of this study was to determine who is at risk for cardiac events among young patients with long QT syndrome (LQTS) with or without a past history of LQTS-related cardiac events. The subjects were young patients with LQTS who had visited one of 36 hospitals from January 1997 to August 2000 in Japan. To predict the risk factors for cardiac events, stepwise regression analyses were performed for a total of 197 cases. There were 7 of 129 cases (5%) without a past history and 32 of the 68 (47%) cases with a past history of LQTS-related cardiac events that experienced new events after diagnosis (p<0.0001). Patients with a family history showed a higher incidence of symptoms both before and after diagnosis than patients with sporadic occurrence. Analyses revealed that noncompliance with medication and a lower age at diagnosis were significant predictors for the group with a past history. A negative predictive value <4 points was 100% in the group without a past history. To prevent future cardiac events, compliance with medication must be improved in those with a past history. A total LQTS score <4 points was useful to predict the absence of cardiac events in the group without a past history.

Atrial supersensitivity to noradrenaline in stressed female rats

Stress can change the responses to catecholamines in many tissues. The aim of this study was to investigate the influence of the estrous cycle on the sensitivity of right atria to noradrenaline in female rats subjected to acute swimming stress. Female Wistar rats in proestrus, estrus, metestrus or diestrus were submitted to a 50 min-swimming session. Immediately after the exercise, the rats were killed and their right atria were mounted for isometric recording of the spontaneous beating rate. Concentration-effect curves to noradrenaline were obtained before and after the inhibition of neuronal uptake with phenoxybenzamine (10 microM) and of extraneuronal uptake with estradiol (5 microM). Acute swimming stress did not change the right atrial sensitivity to noradrenaline in rats in estrus, metestrus and diestrus. However, swimming stress produced supersensitivity to noradrenaline in proestrus (pD(2) control: 7.14 +/- 0.03 vs. pD(2) swimming: 7.55 +/- 0.04; p<0.05). This supersensitivity was still observed after uptake inhibition. When catecholamine uptake was inhibited, the concentration-effect curve to noradrenaline was shifted to the left 2.5-fold in the proestrus control group and 1.7-fold in the proestrus stress group (p<0.05). In conclusion, the estrous cycle influenced the acute stress-induced atrial supersensitivity to noradrenaline.

Effect of age and overweight on the QT interval and the prevalence of long QT syndrome in children

The change in QT interval with age during childhood of normal children and children with long QT syndrome (LQTS) and the effects of body mass index on the QT interval have not been studied in detail. The prevalence of LQTS in children is not well known. We measured 3 consecutive QT and RR intervals in 4,655 children. Their electrocardiograms along with their height and weight were recorded when they were in the first grade in 1994 and again when they were in the seventh grade in 2000. The QT interval was corrected by Bazett's formula. The longer corrected QT intervals in female subjects than male subjects start at elementary school age, earlier than previously reported. Overweight did not have an impact on the uncorrected or corrected QT interval. None of the 4 children diagnosed with LQTS in the seventh grade had characteristic electrocardiographic findings of LQTS in the first grade. All 4 are nonfamilial cases. The prevalence of LQTS in children was found to be 1 of 1,164. These data suggest that abnormal electrocardiographic phenotypes in children with nonfamilial LQTS may appear during the elementary school year. The longer QT intervals in female subjects than male subjects start at the same period. No correlation was found between obesity and length of the QT interval. Finally, the prevalence of LQTS in children is greater than previously suspected.

Facial immersion bradycardia in teenagers and adults accustomed to swimming

We compared heart rate and breath-hold duration during facial immersion in teenagers, 11-14 years (N = 6), 15-18 years (N = 6) and adults, 33-48 years (N = 11). The subjects were members of a competitive swimming club, and were familiar with facial immersion. In contrast to the results of a previous study (J. Appl. Physiol. 63 (1987) 665) in which naïve subjects were used, the 11-14 group were able to breath-hold as long as adults (mean +/- SE, 47+/-6 vs. 46+/-4 s). This allowed time for the full development of bradycardia. Pre-immersion heart rate was significantly higher in young teens than in adults (100+/-4 vs. 78+/-3 b.p.m.). Heart rate after 30 s of head immersion was statistically identical (young teens, 65+/-5 b.p.m.; adults, 64+/-3 b.p.m.). Therefore, both the percentage reduction from pre-immersion rate and rate of fall in heart rate were greater in 11-14-year-olds than in adults. Oxygen loading increased breath-hold time in all groups, and slowed the onset of bradycardia in adults and older teens, but not in the 11-14-year-old group, during the first 10 s after immersion. We conclude that breath-hold time in teenagers is influenced by familiarity with underwater breath-holding. The resulting cardiovascular adjustments in 11-14-year-olds are intrinsically at least as intense as those in adults and seem to have a faster onset.

Swimming, a gene-specific arrhythmogenic trigger for inherited long QT syndrome

OBJECTIVE

To determine the genetic basis for long QT syndrome (LQTS) in a cohort of patients with a personal history or an extended family history of a swimming-triggered cardiac event.

PATIENTS AND METHODS

After review of the Mayo Clinic unit medical record system, blood samples or archived autopsy tissue samples were obtained from a retrospective cohort of 35 cases diagnosed as having autosomal dominant LQTS. Exon-specific amplification by polymerase chain reaction and direct sequence analyses were performed on the entire KVLQT1 gene.

RESULTS

Six cases had a personal history or an extended family history of a near drowning or drowning. In all 6 cases, LQTS-causing mutations in KVLQT1 gene were identified: 3 deletion mutations, 2 donor splice site mutations, and 1 missense mutation. One of the mutations, a novel donor splicing defect, was determined by postmortem molecular analysis of a paraffin-embedded tissue block from a 12-year-old girl who died in 1976. Distinct KVLQT1 mutations were demonstrated in 3 of the remaining 29 cases. The overall frequency of KVLQT1 defects in LQTS was 100% (6/6) in those with and 10% (3/29) in those without a personal history or an extended family history of drowning or near drowning (P<.001).

CONCLUSION

Swimming appears to be a gene-specific (KVLQT1) arrhythmogenic trigger for LQTS. This study provides proof of principle that an unexplained drowning or near drowning may have a genetic basis.