Johnson, Bruce D.

Cold‐water swimming is a recreational activity that may become a future Winter Olympic event. However, unintentional cold‐water exposure can lead to arrhythmias, changes in electrocardiogram (ECG) appearance, and death. This study sought to characterize cardiac responses to recreational one‐kilometer swims performed in normal and cold‐water. Competitive swimmers (2 male 2 female; age 44±17 years; BMI 24±2) completed a one‐kilometer swim (22 laps × 75‐foot lengths) under free‐living indoor (25.5°C water and 25.0°C air) and cold outdoor (10.9°C water and 6.5°C) conditions. Continuous ECGs were obtained with a waterproof recording system (Actiwave, CamNtech Inc., Boerne, TX). ECG PR‐, QT‐, and RR‐intervals (sec), ventricular diastole (sec), and heart rate (beats/minute) were measured 8.7±1.7 minutes before the swim, then 0.6±0.2, 3.0±0.0 and 6.4±2.1 minutes after the swim. Data (mean±SD) was analyzed with repeated measures ANOVA with significance if P<0.05. Swimmers completed the indoor and outdoor swim events in 20.7±1.7 and 21.8±1.1 minutes, respectively. For the indoor swim, PR‐interval (sec) at −8.7, +0.6, +3, and +6.4 minutes was 0.12±0.01, 0.12±0.01, 0.11±0.01, and 0.12±0.01, and PR‐interval for the outdoor swim at −8.7, +0.6, +3.0 and +6.4 minutes was 0.13±0.01, 0.13±0.01, 0.13±0.01, and 0.14±0.01 (sec). PR‐interval was significantly longer at +6.4 minutes post cold‐water swim. For the indoor swim, ventricular diastole (sec) at −8.7, +0.6, +3, and +6.4 minutes was 0.37±0.06, 0.22±0.04, 0.30±0.01, and 0.32±0.05 (sec), and for the outdoor swim at −8.7, +0.6, +3.0 and +6.4 minutes ventricular diastole was 0.35±0.08, 0.19±0.02, 0.30±0.04, and 0.24±0.06 (sec). Ventricular diastole was significantly shorter at +6.4 minutes post cold‐water swim. No significant changes were observed for QT‐interval, RR‐interval, and heart rate between swim conditions. These parameters were also assessed for the ECG recordings collected during the 20.7±1.7‐minute normal and 21.8±1.1‐minute cold‐water swims to characterize the physiological response during swim exercise. This study of normal and cold‐water one‐kilometer recreational swimmers determined that a waterproof ECG could generate a continuous record of changes in cardiac function. During cold‐water swim recovery (+6.4 minutes) PR interval was lengthened and ventricular diastole was shortened suggesting an influence of temperature on cardiac function.

The pattern of glycogen utilization was used to determine whether various muscle fiber types in the rat diaphragm are differentially susceptible to neuromuscular transmission failure. Muscle segments from the midcostal region were repetitively stimulated directly or via the phrenic nerve at 10 or 75 Hz. Muscle fiber types were classified histochemically as type I, IIa, or IIb. The amount of muscle fiber glycogen depletion with direct stimulation depended on stimulation rate (75 Hz > 10 Hz) and fiber type (IIb > IIa > I). However, with nerve stimulation, muscle fiber glycogen depletion did not display the same dependency on stimulation rate (10 Hz > 75 Hz), although with stimulation at 10 Hz, the same rank order of fiber depletion was observed (IIb > IIa > I). This rank order of depletion was reversed (I > IIa > IIb) during repetitive stimulation of the nerve at 75 Hz. By intermittently stimulating the muscle directly during continuous nerve stimulation, we determined that neuromuscular transmission failure contributed significantly to the force decline after 2 min of stimulation at 75 Hz but relatively little to the force decline after 2 min of stimulation at 10 Hz. A significantly greater fraction of the force decline could be attributed to neuromuscular transmission failure with repetitive bouts of stimulation at 10 Hz. We conclude that neuromuscular transmission failure causes a significant portion of the force decline after 8 min of stimulation at 10 and 75 Hz, that all diaphragm fiber types are susceptible to neuromuscular transmission failure, but that type IIb fibers are particularly susceptible at higher frequencies of stimulation.

Abstract Aims Traditionally, VO2peak has been used to determine prognosis in heart failure; however, this measure has limitations. Hence, other exercise and gas exchange parameters measured submaximally, e.g. breathing efficiency (VE/VCO2), end-tidal CO2 (PETCO2), oxygen uptake efficiency slope (OUES), and circulatory power [ systolic blood pressure (SBP)], have been investigated. The aim of this study was to investigate the prognostic relevance of submaximal exercise gas exchange in heart failure patients. Method and results One hundred and thirty-two consecutive heart failure patients (mean age 56 ± 12 years, ejection fraction 29 ± 11%) performed peak treadmill testing. Gas exchange and haemodynamic variables were measured continuously. Gas exchange data obtained from the first 2 min of exercise and at a respiratory exchange ratio (RER) of 0.9 were the measurements of interest. Over a median follow-up period of 62.4 (range 0–114) months, there were 44 endpoints (death or transplant). Univariate analysis demonstrated submaximal predictors of survival, which included VE/VCO2 slope and ratio, PETCO2, OUES, and circulatory power (P≤ 0.01). When these and additional submaximal variables were included together in the multivariable analysis, the strongest submaximal exercise predictive model (C-statistic 0.75) comprised data from the first stage of exercise (VE and circulatory power) and at an RER of 0.9 (VE/VCO2 ratio). The inclusion of VO2peak and demographic data, with submaximal data (VE/VCO2 ratio at an RER = 0.9), increased the predictiveness of the model (C-statistic 0.78). Conclusion Submaximal exercise measures provide useful prognostic information for predicting survival in heart failure. This form of testing is logistically easier, cheaper, and safer for patients compared with maximal exercise.