Association between RR interval and high-frequency heart rate variability acquired during short-term, resting recordings with free and paced breathing

High-frequency (HF) oscillations in RR interval from 0.15-0.40 Hz are widely accepted as a measure of cardiac vagal outflow but the HF/RR relationship appears complex, particularly with longer RR intervals. The aim of this study was to evaluate the HF/RR interval relationship during free and paced breathing. HF power and mean RR interval length were recorded in 150 men and 120 women (mean age 34.5 +/- 11.4) during 5 min of supine rest with either free or paced (12 cycles min(-1)) breathing. Linear and quadratic models were used to assess the relationship between RR interval and the natural logarithm of HF power (lnHF). The RR interval length at which there was no further increase in lnHF was determined as the deflection point. ANCOVA was used to determine differences in the linear regression slopes for lnHF/RR with paced or free breathing. With free breathing (n = 131), the adjusted R(2) was similar between linear (15.3%) and quadratic (17.5%) fits and saturation of lnHF occurred within the recorded RR interval range (1326 ms). With paced breathing (n = 139), adjusted R(2) values were again similar between linear (22.4%) and quadratic (23.2%) fits. The deflection point was outside the range of recorded RR intervals at 1458 ms. ANCOVA showed a significant difference in the slope of the lnHF/RR regression lines between free and paced breathing. The lnHF/RR relationship is weaker when derived from between-subject recordings than from repeated within-subject samples. lnHF/RR showed evidence of saturation at approximately 45 bpm with free breathing. With paced breathing, a deflection in lnHF was found outside the recorded RR interval range ( approximately 41 bpm). Paced breathing creates a stronger lnHF/RR relationship. The slope of the lnHF/RR regression line with paced breathing is significantly different from that observed with free breathing. It appears that lnHF is a valid index of vagal outflow, except in subjects with very low heart rates. Paced breathing data collection protocols appear preferable.

Modelling the lactate response to short-term all out exercise

Background

The maximum post exercise blood lactate concentration (BLC_max) has been positively correlated with maximal short-term exercise (MSE) performance. However, the moment when BLC_max occurs (TBLC_max) is rather unpredictable and interpretation of BLC response to MSE is therefore difficult.

Methods

We compared a 3- and a 4-parameter model for the analysis of the dynamics of BLC response to MSEs lasting 10 (MSE10) and 30 s (MSE30) in eleven males (24.6 ± 2.3 yrs; 182.4 ± 6.8 cm; 75.1 ± 9.4 kg). The 3-parameter model uses BLC at MSE-start, extra-vascular increase (A) and rate constants of BLC appearance (k₁) and disappearance (k₂). The 4-parameter model includes BLC at MSE termination and amplitudes and rate constants of increase (A₁, y₁) and decrease (A₂, y₂) of post MSE-BLC.

Results

Both models consistently explained 93.69 % or more of the variance of individual BLC responses. Reduction of the number of parameters decreased (p < 0.05) the goodness of the fit in every MSE10 and in 3 MSE30. A (9.1 ± 2.1 vs. 15.3 ± 2.1 mmol l^-1) and A₁ (7.1 ± 1.6 vs. 10.9 ± 2.0 mmol l^-1) were lower (p < 0.05) in MSE10 than in MSE30. k₁ (0.610 ± 0.119 vs. 0.505 ± 0.107 min^-1), k₂ (4.21 10^-2 ± 1.06 10^-2 vs. 2.45 10^-2 ± 1.04 10^-2 min^-1), and A₂ (-563.8 ± 370.8 vs. -1412.6 ± 868.8 mmol l^-1), and y₁ (0.579 ± 0.137 vs. 0.489 ± 0.076 min^-1) were higher (p < 0.05) in MSE10 than in MSE30. No corresponding difference in y₂ (0.41 10^-2 ± 0.82 10^-2 vs. 0.15 10^-2 ± 0.42 10^-2 min^-1) was found.

Conclusion

The 3-parameter model estimates of lactate appearance and disappearance were sensitive to differences in test duration and support an interrelation between BLC level and halftime of lactate elimination previously found. The 4-parameter model results support the 3-parameter model findings about lactate appearance; however, parameter estimates for lactate disappearance were unrealistic in the 4-parameter model. The 3-parameter model provides useful information about the dynamics of the lactate response to MSE.

Anaerobic performance and metabolism in boys and male adolescents

Short-term maximum intensity performance, absolute and related to body mass, is lower in children than adolescents. The underlying mechanisms are not clear. We analysed Wingate Anaerobic Test (WAnT) performance and metabolism in ten boys (mean (SD); age 11.8 (0.5) years, height 1.51 (0.05) m, body mass 36.9 (2.5) kg, muscle mass 13.0 (1.0) kg) and 10 adolescents (16.3 (0.7) years, 1.81 (0.05) m, 67.3 (4.1) kg, 28.2 (1.7) kg). Related to body mass, power of flywheel acceleration (6.0 (1.6) vs. 8.1 (1.1) W kg(-1)), peak power (10.8 (0.7) vs. 11.5 (0.6) W kg(-1)), average power (7.9 (0.5) vs. 8.9 (0.7) W kg(-1)), minimum power (6.1 (0.7) vs. 6.9 (0.9) W kg(-1)) and anaerobic lactic energy (687.6 (75.6) vs. 798.2 (43.0) J kg(-1)) were lower (P < 0.05) in boys than in adolescents. Related to muscle mass the change in lactate (0.69 (0.08) vs. 0.69 (0.04) mmol kg (MM) (-1) s(-1)) and PCr (0.60 (0.17) vs. 0.52 (0.10) mmol kg (MM) (-1) s(-1)) were not different. The corresponding oxygen uptake (1.34 (0.13) vs. 1.09 (0.13) ml kg (MM) (-1) s(-1)), total metabolic rate (132.4 (12.6) vs. 119.7 (8.5) W kg (MM) (-1) ) and PP (30.5 (2.6) vs. 27.5 (1.7 W) kg (MM) (-1) ) were higher (P < 0.01) in boys than in adolescents. The results reflect a lower relative muscle mass combined with no differences in muscular anaerobic but fascilitated aerobic metabolism in boys. Compared with adolescents, boys' performance seemed to be significantly impaired by flywheel inertia but supported by identical brake force related to body mass.

Cardiospecificity of the 3rd generation cardiac troponin T assay during and after a 216 km ultra-endurance marathon run in Death Valley

BACKGROUND

The reasons for the appearance of cardiacspecific troponin (cTnT) after strenuous exercise are unclear. The aim of the present study was to evaluate the cardiospecificity of the 3(rd) generation cardiac cTnT assay during and after an ultra-endurance race of 216 km at extreme environmental conditions in Death Valley.

STUDY DESIGN AND METHODS

We measured serially cTnT, creatine kinase (CK), activity and mass of the isoenzyme MB of CK (CK-MB(act) and CK-MB(mass)), and myoglobin in 10 well-trained athletes before, repeatedly during and after the race.

RESULTS

Six of 10 participants finished the race within a preset time of 60 hours. Postrace values of biochemical markers CK, CK-MB(act), CKMB(mass), and myoglobin were significantly increased compared to baseline (p<0.05). CK-MB(act) increased from (median (25(th)/ 75(th)percentile) 12 (10/13) U/L to 72 (32/110) U/L, CK-MB(mass) from 3.9 (2.9/5.6) U/L to 65 (18/80) U/L and CK increased from median 136 (98/ 228) U/L to 3,570 (985/6,884) U/L respectively. Pre-race myoglobin was 27 (22/31) microg/l compared to 530 (178/657) microg/l after the run. One runner developed significant exercise-induced rhabdomyolysis with spontaneous recovery. cTnT values remained below the 99(th) percentile reference limit in all athletes including the runner who developed significant rhabdomyolysis (peak CK 27,951 U/L).

CONCLUSIONS

Strenuous endurance exercise, even under extreme environmental conditions, does not result in structural myocardial damage in well-trained ultra-endurance athletes. We found no crossreactivity between cTnT and CK, neither in exercise-induced skeletal muscle trauma nor after rhabdomyolysis underscoring the excellent analytical performance of 3(rd) generation cTnT assay.

Exercise-induced hemolysis is caused by protein modification and most evident during the early phase of an ultraendurance race

Whether structural changes of the erythrocyte membrane increase the susceptibility to hemolysis particularly of the relatively older cell population during the early phase of a 216-km ultrarace was tested in six male runners (age 53.6 +/- 10.4 yr, height 175.8 +/- 11.1 cm, body mass 75.9 +/- 8.4 kg). Erythrocyte membrane spectrins were lowest (P < 0.001) after 42 km (75.59 +/- 5.25% of prerace) and increased (P < 0.001) toward 216 km (88.27 +/- 3.37%). Susceptibility to osmotic hemolysis was highest (P < 0.01) after 42 km (107.34 +/- 3.02 mOsm sodium phosphate buffer) with almost identical (P > 0.05) values prerace (97.98 +/- 3.41 mOsm) and postrace (98.61 +/- 3.26 mOsm). Haptoglobin indicated intravascular hemolysis of 9.27 x 10(9) cells/l (P < 0.05) during the initial 84 km. Changes in hematocrit and plasma proteins indicated an estimated total net erythrocyte loss of 3.47 x 10(11) cells/l (P < 0.05) after 21 km. This was compensated by a gain in erythrocytes (P < 0.05) of 3.31 x 10(11) cells/l during the final 132 km. A main effect (P < 0.05) on erythropoietin suggests increased erythropoiesis throughout the race. Exercise-induced hemolysis reflects alterations in erythrocyte membrane spectrins and occurs particularly in the early phase of an ultraendurance race because of a relative older cell population.

Explaining pH Change in Exercising Muscle: Lactic acid, Proton Consumption, and Buffering vs. Strong Ion Difference

The development of acidosis during intense exercise has traditionally been explained by the increased production of lactic acid, causing the release of a proton and the formation of the acid salt sodium lactate. On the basis of this explanation, if the rate of lactate production is high enough, the cellular proton buffering capacity can be exceeded, resulting in a decrease in cellular pH. These biochemical events have been termed lactic acidosis. The lactic acidosis of exercise has been a classic explanation of the biochemistry of acidosis for more than 80 years. This belief has led to the interpretation that lactate production causes acidosis and, in turn, that increased lactate production is one of the several causes of muscle fatigue during intense exercise. This review presents clear evidence that there is no biochemical support for lactate production causing acidosis. Lactate production retards, not causes, acidosis. Similarly, there is a wealth of research evidence to show that acidosis is caused by reactions other than lactate production. Every time ATP is broken down to ADP and Pi, a proton is released. When the ATP demand of muscle contraction is met by mitochondrial respiration, there is no proton accumulation in the cell, as protons are used by the mitochondria for oxidative phosphorylation and to maintain the proton gradient in the intermembranous space. It is only when the exercise intensity increases beyond steady state that there is a need for greater reliance on ATP regeneration from glycolysis and the phosphagen system. The ATP that is supplied from these nonmitochondrial sources and is eventually used to fuel muscle contraction increases proton release and causes the acidosis of intense exercise. Lactate production increases under these cellular conditions to prevent pyruvate accumulation and supply the NAD+ needed for phase 2 of glycolysis. Thus increased lactate production coincides with cellular acidosis and remains a good indirect marker for cell metabolic conditions that induce metabolic acidosis. If muscle did not produce lactate, acidosis and muscle fatigue would occur more quickly and exercise performance would be severely impaired.