Sympatho-vagal interaction in the recovery phase of exercise Mikko P. Tulppo1,2, Antti M. Kiviniemi1, Arto J. Hautala1, Mika Kallio3, Tapio Seppanen4, �� Suvi Tiinanen4, Timo H. Makikallio2 �� and Heikki V. Huikuri2 1Department of Exercise and Medical Physiology, Verve, Oulu, 2Institute of Clinical Medicine, Department of Internal Medicine, University of Oulu, Oulu, Departments of 3Clinical Neurophysiology, and 4Electrical and Information Engineering, University of Oulu, Oulu, Finland Correspondence Mikko P. Tulppo, PhD, Verve, Kasarmintie 13, PO Box 404, FI-90101 Oulu, Finland E-mail: mikko.tulppo@verve.fi Accepted for publication Received 8 October 2010 accepted 17 January 2011 Key words heart rate sympathetic activity vagal activity Summary Reciprocal autonomic regulation occurs during incremental exercise. We hypoth- esized that sympatho-vagal interplay may become altered after exercise because of the differences in recovery patterns of autonomic arms. The cardiac vagal activity was assessed by measurement of beat-to-beat R���R interval oscillations using a Poincare �� plot method (SD1), and muscle sympathetic nervous activity (MSNA) was measured from peroneus nerve by a microneurography technique during and after exercise in 16 healthy subjects. Autonomic regulation was compared between the rest and after exercise (3��5 �� 1��0 min after exercise) at equal heart rates (HR). SD1 was at the equal level at the recovery phase (40 �� 21 ms) compared to the resting condition (38 �� 16 ms, P = ns) at comparable HR (57 �� 10 for both). MSNA was higher at the recovery phase (40 �� 19 burst per 100 heartbeats) than at rest (25 �� 13 burst per 100 heartbeats, P0��0001). The difference of MSNA activity between rest and late recovery phase had a strong positive correlation with the difference in SD1 (r = 0��78, P0��001) at equal HRs. Subjects who have a higher sympathetic activity in the recovery phase of exercise have a more augmented cardiac vagal activity resulting in an accentuated sympatho-vagal outflow. The altered autonomic interaction observed here may partly explain the clustering of various cardiovascular events to the recovery phase of exercise. Introduction The interplay between sympathetic and vagal regulation of heart rate (HR) is usually organized ina reciprocal fashion, i.e. increased activity in one system is accompanied by decreased activity in the other. Such reciprocal changes in sympathetic and vagal activity occur during common autonomic challenges, such as dynamic exercise (Robinson et al., 1966 Maciel et al., 1986 Orizio et al., 1988 Yamamoto & Hughson, 1991 Tulppo et al., 1996) cold hand test (Vallbo et al., 1979 Tulppo et al., 2005b) and passive head-up tilt test (Montano et al., 1994 Furlan et al., 2000 Tulppo et al., 2001). Cold face immersion is a well-known intervention where a dual autonomic activation is observed in humans, expressed as simultaneous sympathetic and vagal outflow result- ing in abrupt and marked changes in beat-to-beat R���R interval dynamics, anincreaseinbloodpressureandabreakdown offractal organization of heart rate (Tulppo et al., 2005a,b). In animal studies, the dual autonomic activation is observed during a chemoreceptor activation of the carotid body (Koizumi et al., 1982), during upper airway stimulation by smoke (Peterson et al., 1983) andduring rapid eye movement sleep (Mancia et al., 1971). There is increasing evidence that the recovery phase after exercise may be a vulnerable phase for various cardiovascular events. Case-crossover studies have shown that exercise as a trigger of acute myocardial infarction is not limited to the time of exercise, but extends for a certain time period after cessation of physical activity (Siscovick et al., 1982, 1984 Albert et al., 2000 von Klot et al., 2008). Similarly, the risk of sudden cardiac death is transiently increased in the 30 min immediately after vigorous exercise, and atrial fibrillation episodes occur more commonly after rather than before exercise (Siscovick et al., 1982, 1984 Coumel, 1994 Albert et al., 2000 Huikuri, 2008). Measurement of autonomic function in the recovery phase of exercise has also provided some prognostic information. For example, delayed heart rate recovery 1���2 min after exercise has been shown to predict cardiovascular events in the general population and in various patient groups and animal studies (Cole et al., 1999 Lauer & Froelicher, 2002 Nissinen et al., 2003 Jouven et al., 2005 Smith et al., 2005). Despite these clinical observations, the exact behaviour of cardiovascular autonomic regulation in the recovery phase of exercise is not well known. Therefore, the present research was designed to Clin Physiol Funct Imaging (2011) 31, pp272���281 doi: 10.1111/j.1475-097X.2011.01012.x �� 2011 The Authors Clinical Physiology and Functional Imaging �� 2011 Scandinavian Society of Clinical Physiology and Nuclear Medicine 31, 4, 272���281 272

study the dynamics of cardiac autonomic regulation by HR variability techniques and peripheral sympathetic activity by muscle sympathetic nervous activity (MSNA) technique before, during and after exercise in healthy men. We hypothesized that dual autonomic activation may occur after exercise because of differences in the recovery patterns of vagal and sympathetic outflow in the cardiac or ��� and peripheral regions. Methods Subjects and study protocol All subjects were healthy male volunteers without any medica- tions. Thirty healthy recreational male runners were recruited from the local sports club by e-mail message. The candidates were interviewed with a standardized scheme to ascertain their medical history and levels of physical activity. All smokers, subjects with BMI 28 kg m)2, subjects who had performed regular physical exercise training less than twice a week during past 3 months, competing athletes and subjects with diabetes mellitus, asthma or cardiovascular disorders were excluded. The aim was to have at least 15 subjects with adequate recordings of MSNA activity during and after exercise. This calculation was based on previous experience (Tulppo et al., 2005a,b) of being able to get reliable MSNA measurements from 50% to 60% of the subjects during and after interventions. Therefore, we required 30 subjects to the study, and finally, we were able to record a high-quality MSNA signal during every phase of intervention in 16 subjects. The characteristics of the subjects are showing in Table 1. The protocol was approved by the Ethics Committee of the Northern Ostrobothnia Hospital District, Oulu, Finland, and all the subjects gave written informed consent and the investigation conforms with the principles outlined in the Declaration of Helsinki. The subjects were not allowed to eat or drink coffee or other caffeine drinks for 3 h before the tests. Vigorous exercise and alcohol were also forbidden for 48 h before the testing day. The subjects lie in a supine position in a quiet room for at least 15 min before data collection. Their breathing frequency was spontaneous throughout the protocol. The subjects per- formed incremental arm cranking in a supine position (Lido Uppercycle, Loredan Biomedical, Davis, CA, USA), starting at 20 W followed by 40, 60 and 80 W loads, 1 min each. We wanted to increase exercise intensity to as heavy load as possible during the test to have a clear sympathetic activation and vagal withdrawal at the end of exercise. However, based on our prior testing, the load of 80 W was the highest possible load where MSNA could be measured without artefacts, e.g. because of body movements. After the end of exercise, subjects continued to lie in a supine position for 10 min and they were not allowed to move or speak. The subjects performed a graded maximal exercise test on a treadmill 4���5 days before arm cranking protocol to document their fitness level (Telineyhtyma, �� Kotka, Finland), starting at 8��0 km h)1 and followed by an increase of 0��5 km h)1every minute until voluntary exhaustion. Ventilation (VE) and gas exchange (M909 ergospirometer Medikro, Kuopio, Finland) were measured and reported as the mean value per minute. The highest mean value of oxygen consumption was expressed as the peak oxygen consumption (VO2peak) because plateau of VO2 was not observed during the test in all cases although other criteria for VO2max given in the literature [i.e. respiratory exchange ratio (RER) 1��1 and maximum HR within 10 beats of the age-appropriate reference value] were fulfilled. Measurements An ECG was recorded using standard methods (Nihon Kohden TEC-7700, Nihon Kohden Corporation, Tokyo, Japan). Blood pressure was measured with an automatic blood pressure recorder at the baseline and during the recovery phase (Tango Sun-Tech, Raleigh, NC, USA). Respiration frequency and tidal volume were measured with a disposable screen flow transducer (Spirometer ADInstruments, Sydney, Australia). Gas exchange and the RER were measured with a Medikro M909 ergospi- rometer (Medikro, Kuopio, Finland). Multifiber recordings of MSNA were obtained with a tungsten microelectrode inserted into the peroneal nerve. A reference electrode was placed subcutaneously 2���3 cm from the recording electrode. The recording electrode was adjusted until a site was found in which muscle sympathetic bursts were clearly identified, using previously established criteria (Vallbo et al., 1979). The nerve signal was amplified (50 000 times), passed through a band- pass filter with a bandwidth of 700���2000 Hz and integrated with a time constant of 0��1 s. The nerve signal was also routed to an oscilloscope and a loudspeaker for monitoring throughout the study. Analogue signals were sampled at 1000 Hz. All the measurements were performed between 9 AM and 15 PM at the room temperature 20��. All the signals were analysed minute by minute at rest, during and after exercise. In particular, the HR variability and MSNA activity of each subject were analysed twice: (i) at rest (1-min period just before exercise for all subjects) and in the late recovery phase at equal HR for each subject as at resting condition (within �� 2 beats, 3��5 �� 1��0 min after exercise, range from 2���3 to 5���6 min) (ii) in the early recovery phase (a period from 30���90 s after exercise for all subjects) and during exercise at equal HR for each subject as at early recovery Table 1 Characteristics of the subjects (n = 16). Mean �� SD Range min���max Age, years 33 �� 6 22���45 Height, cm 182 �� 4 176���187 Weight, kg 82 �� 6 69���92 BMI, kg m)2 24 �� 1 22���27 Fat, % 15��0 �� 3��5 10��5���21��0 HR max, bpm 185 �� 10 169���202 VO2peak, ml kg)1 min)1 55 �� 6 40���66 RER max 1��14 �� 0��04 1��10���1��23 HR, heart rate RER, respiratory exchange ratio. Postexercise autonomic regulation, M. P. Tulppo et al. �� 2011 The Authors Clinical Physiology and Functional Imaging �� 2011 Scandinavian Society of Clinical Physiology and Nuclear Medicine 31, 4, 272���281 273