Persistence of baroreceptor control of cerebral blood flow velocity at a simulated altitude of 5000 m

Objective - To assess the effects of acute exposure to simulated high altitude on baroreflex control of mean cerebral blood flow velocity (MCFV). Patients and methods - We compared beat-to-beat changes in RR interval, arterial blood pressure, mean MCFV (by transcranial Doppler velocimetry in the middle cerebral artery), end-tidal CO2, oxygen saturation and respiration in 19 healthy subjects at baseline (Albuquerque, 1779 m), after acute exposure to simulated high altitude in a hypobaric chamber (barometric pressure as at 5000 m) and during oxygen administration (to achieve 100% oxygen saturation) at the same barometric pressure (HOX). Baroreflex control on each signal was assessed by univariate and bivariate power spectral analysis performed on time series obtained during controlled (15 breaths/min) breathing, before and during baroreflex modulation induced by 0.1-Hz sinusoidal neck suction. Results - At baseline, neck suction was able to induce a clear increase in low-frequency power in MCFV (P0.001) as well as in RR and blood pressure. At high altitude, MCFV, as well as RR and blood pressure, was still able to respond to neck suction (all P0.001), compared to controlled breathing alone, despite marked decreases in end-tidal CO2 and oxygen saturation at high altitude. A similar response was obtained at HOX. Phase delay analysis excluded a passive transmission of low-frequency oscillations from arterial pressure to cerebral circulation. Conclusions - During acute exposure to high altitude, cerebral blood flow is still modulated by the autonomic nervous system through the baroreflex, whose sensitivity is not affected by changes in CO2 and oxygen saturation levels

Effects of controlled breathing, mental activity and mental stress with or without verbalization on heart rate variability

AbstractOBJECTIVESTo assess whether talking or reading (silently or aloud) could affect heart rate variability (HRV) and to what extent these changes require a simultaneous recording of respiratory activity to be correctly interpreted.BACKGROUNDSympathetic predominance in the power spectrum obtained from short- and long-term HRV recordings predicts a poor prognosis in a number of cardiac diseases. Heart rate variability is often recorded without measuring respiration; slow breaths might artefactually increase low frequency power in RR interval (RR) and falsely mimic sympathetic activation.METHODSIn 12 healthy volunteers we evaluated the effect of free talking and reading, silently and aloud, on respiration, RR and blood pressure (BP). We also compared spontaneous breathing to controlled breathing and mental arithmetic, silent or aloud. The power in the so called low- (LF) and high-frequency (HF) bands in RR and BP was obtained from autoregressive power spectrum analysis.RESULTSCompared with spontaneous breathing, reading silently increased the speed of breathing (p < 0.05), decreased mean RR and RR variability and increased BP. Reading aloud, free talking and mental arithmetic aloud shifted the respiratory frequency into the LF band, thus increasing LF% and decreasing HF% to a similar degree in both RR and respiration, with decrease in mean RR but with minor differences in crude RR variability.CONCLUSIONSSimple mental and verbal activities markedly affect HRV through changes in respiratory frequency. This possibility should be taken into account when analyzing HRV without simultaneous acquisition and analysis of respiration

Autonomic Regulation of Heart Rate and Peripheral Circulation: Comparison of High Altitude and Sea Level Residents

1. To evaluate the activity of the autonomic nervous system on the heart and peripheral circulation in native high-altitude residents, during a Himalayan expedition we studied 12 men (age: 48 ± 4, mean ± SEM), life-long resident in a village at 4800 m (Sumdo village, Zanskar, India) and 7 healthy sea-level residents (age: 37 ± 4) after 7 days of acclimatization (acclimatized lowlanders) at the same altitude. Furthermore 25 sea level residents (age: 46 ± 2) underwent the same protocol at sea level. 2. R-R interval (RR), respiratory signal, non-invasive blood pressure, and skin arteriolar blood flow were evaluated in three different conditions: during free breathing in supine position and during controlled breathing (at 0.15 Hz), in supine and upright position, and analysed by autoregressive spectral analysis [low- (around 0.1 Hz) and high-frequency (respiratory) fluctuations, LF and HF, markers of sympathetic and vagal activity, respectively]. 3. High-altitude residents showed in supine position a higher RR than acclimatized lowlanders, similar to sea-level residents. RR variability was reduced in acclimatized lowlanders compared to both high-altitude residents and sea level residents. Systolic blood pressure (SBP) did not show significant differences between the three groups. High altitude residents showed in supine lower LF in RR signal compared to sea-level residents, and, compared to acclimatized lowlanders, higher HF and lower LF/HF ratio; high-altitude residents showed a reduction in skin microcirculation variability compared to sea-level residents, but this was eight fold greater than in acclimatized lowlander, thus indicating a much greater vasoconstriction in acclimatized lowlanders than in high-altitude residents. 4. In upright position, high-altitude residents showed the same behaviour as sea-level residents, with increase in LF-RR, and decrease in HF and LF-SBP. Acclimatized lowlanders showed similar directional trends though not significant changes for RR-LF. 5. After one week of acclimatization, lowlanders still manifested sympathetic activation and skin vasoconstriction; high-altitude residents did not show reduced vagal tone compared to sea-level residents, but a mild vasoconstriction appeared to be present. In conclusion, normal or enhanced vagal tone and preserved vasomotion are probably evidence of adaptation at high altitude hypoxia

Radiofrequency catheter ablation of concealed accessory pathways in two dogs with symptomatic atrioventricular reciprocating tachycardia

Radiofrequency catheter ablation (RFCA) is widely used as a curative therapeutic strategy in human beings with paroxysmal supraventricular tachycardia (SVT), but rarely applied in animals. This report describes successful RFCA of atrioventricular accessory pathways (AP) in two dogs with episodic weakness caused by frequent paroxysms of supraventricular tachycardia. METHODS AND RESULTS: Invasive electrophysiological studies (EPS) identified two APs in the 1st dog (right postero-septal, right posterior), and one in the 2nd dog (right posterior). Programmed electrical stimulation demonstrated that all APs had only retrograde unidirectional conduction, and played a role to maintain inducible atrioventricular reciprocating tachycardia (AVRT). The bypass tracts were successfully eliminated with RFCA, with consequent prevention of AVRT induction during post-ablation EPS. At 8months follow-up, the dogs were asymptomatic, and no reoccurrence of tachycardia was seen. CONCLUSION: Concealed APs responsible for AVRT and accompanied symptoms may be safely eliminated using RFCA in dog

Cardiovascular autonomic modulation and activity of carotid baroreceptors at altitude

1. To assess the effects of acute exposure to high altitude on baroreceptor function in man we evaluated the effects of baroreceptor activation on R-R interval and blood pressure control at high altitude. We measured the low-frequency (LF) and high-frequency (HF) components in R-R, non-invasive blood pressure and skin blood flow, and the effect of baroreceptor modulation by 0.1-Hz sinusoidal neck suction. Ten healthy sea-level natives and three high-altitude native, long-term sea-level residents were evaluated at sea level, upon arrival at 4970 m and 1 week later. 2. Compared with sea level, acute high altitude decreased R-R and increased blood pressure in all subjects [sea-level natives: R-R from 1002 ± 45 to 775 ± 57 ms, systolic blood pressure from 130 ± 3 to 150 ± 8 mmHg; high-altitude natives: R-R from 809 ± 116 to 749 ± 47 ms, systolic blood pressure from 110 ± 12 to 125 ± 11 mmHg (P < 0.05 for all)]. One week later systolic blood pressure was similar to values at sea level in all subjects, whereas R-R remained elevated in sea-level natives. The low-frequency power in R-R and systolic blood pressure increased in sea-level natives [R-R-LF from 47 ± 8 to 65 ± 10% (P < 0.05), systolic blood pressure-LF from 1.7 ± 0.3 to 2.6 ± 0.4 In-mmHg2 (P < 0.05)], but not in high-altitude natives (R-R-LF from 32 ± 13 to 38 ± 19%, systolic blood pressure-LF from 1.9 ± 0.5 to 1.7 ± 0.8 In-mmHg2). The R-R-HF decreased in sea-level natives but not in high-altitude natives, and no changes occurred in systolic blood pressure-HF. These changes remained evident 1 week later. Skin blood flow variability and its spectral components decreased markedly at high altitude in sea-level natives but showed no changes in high-altitude natives. Neck suction significantly increased the R-R- and systolic blood pressure-LF in all subjects at both sea level and high altitude. 3. High altitude induces sympathetic activation in sea-level natives which is partially counteracted by active baroreflex. Despite long-term acclimatization at sea level, high-altitude natives also maintain active baroreflex at high altitude but with lower sympathetic activation, indicating a persisting high-altitude adaptation which may be genetic or due to baroreflex activity not completely lost by at least 1 year's sea-level residence

Influence of Type of Surgery on the Occurrence of Parasympathetic Reinnervation After Cardiac Transplantation

Background —Cardiac autonomic reinnervation after human cardiac transplantation has been demonstrated frequently but to date only for sympathetic efferents. Standard surgical techniques leave many parasympathetic branches intact in the original atria and thus with less stimulus to reinnervate the donor atria. Methods and Results —We used changes in the RR-interval power spectrum induced by sinusoidal modulation of arterial baroreceptors by neck suction at different frequencies to detect both parasympathetic and sympathetic reinnervation in 79 subjects with “standard” and 10 “bicaval” heart transplants. In 24 subjects (17 standard and 7 bicaval), the protocol was repeated 6 and 11 months after transplantation. Neck suction at 0.20 Hz produced a component at 0.20 Hz in the RR-interval spectrum not due to respiration (fixed at 0.25 Hz), which suggested parasympathetic reinnervation, in 4 of 10 bicaval but in only 2 of 79 standard transplant subjects (whose recipient atria underwent &gt;50% resection to remove scars of previous interventions), P &lt;.001. In only 1 (bicaval) transplant subject was parasympathetic reinnervation present 6 months after transplantation (confirmed 3 months later); in 4 subjects, it was absent at 6 months but appeared after 11 months after transplantation. Atropine (0.04 mg/kg IV) abolished the response to fast (0.20 Hz) and reduced that to slow stimulation, confirming the presence of parasympathetic reinnervation (4 subjects). Conclusions —Parasympathetic reinnervation depends on the surgical technique: because bicaval surgery cuts all sympathetic and parasympathetic nerves, regeneration might be stimulated similarly in both branches. Standard surgery cuts only ≈50% of sympathetic fibers; most recipient parasympathetic axons remain intact, hence their regeneration might not be stimulated. </jats:p