Ventilatory response and arterial blood gases during exercise in children

To investigate the difference in ventilatory response to exercise between children and young adults, we administered a treadmill progressive exercise test to seven boys (aged 8 to 11 y [group A]) and six male young adults (aged 14 to 21 y [group B]), who had a history of Kawasaki disease without significant coronary arterial lesions, and analyzed their arterial blood gases. There was no significant difference in arterial PO2 or the end-tidal to arterial oxygen tension difference during exercise between groups A and B. The arterial PCO2 (PaCO2) at the ventilatory anaerobic threshold and at peak exercise was significantly lower in group A than in group B (p < 0.05). The arterial to end-tidal carbon dioxide tension difference at peak exercise was significantly greater in group B than in group A (p < 0.05), whereas there was no significant difference at rest or at the ventilatory anaerobic threshold level. The arterial to end-tidal carbon dioxide tension difference at peak exercise was correlated with tidal volume (p < 0.01) and carbon dioxide production (p < 0.05) at peak exercise in all subjects. Although improvement in the physiologic dead space/tidal volume ratio during exercise was smaller in group A than in group B, there was no significant difference in total alveolar ventilation during exercise. However, the total carbon dioxide production during exercise was significantly smaller in group A than in group B. These data suggest that PaCO2 during exercise is better estimated by end-tidal carbon dioxide tension in children than in young adults, that there is a significant difference in change of the PaCO2 during exercise between children and young adults, and that the decrease in PaCO2 in children is related to the mismatch between well-maintained alveolar ventilation and immature metabolic development in the working muscles during moderate-to-severe exercise.     

Cardiovascular autonomic regulation in preterm infants: the effect of atropine

To study cardiovascular autonomic control, we assessed the effect of atropine on heart rate (HR) and blood pressure (BP) variability in 12 preterm infants (range 26-32 wk) before intubation for respiratory insufficiency. Spectral power analysis of R-R interval and systolic BP (SBP) series were estimated in a low-frequency (LF; 0.04-0.15 Hz) and high-frequency (HF; 0.4-1.5 Hz) band and evaluated for a 10-min period before and a 10-min period after atropine sulfate (0.01 mg/kg). Baroreceptor reflex (BR) functioning was estimated using transfer function analysis at LF (coherence, gain, and phase). Atropine resulted in a significant 12% increase in steady-state HR (p < 0.01) and unchanged SBP. For R-R interval series, the total spectral power decreased 6-fold (p < 0.01), which was predominantly due to a reduction in the LF band (16-fold; p < 0.01). In contrast, we observed a significant increase (25%; p < 0.05) in total spectral power of SBP series partly as a result of an increase in HF power. The LF power of SBP series was not altered. The median LF transfer gain (BR sensitivity) between SBP and R-R interval decreased from 4.2 to 1.4 ms/mm Hg (p < 0.01) after atropine. The LF phase relationship (BP leads R-R interval fluctuations by approximately 4 s) was not changed after atropine. In conclusion, even in preterm infants in distress, atropine modulates HR and BP variability, suggesting that BR-mediated parasympathetic control of heart rate is of significance for cardiovascular control at that age.     

学龄前儿童氯胺酮麻醉深度监测的量化研究

目的 比较学龄前儿童氯胺酮(KTM)麻醉的数量化脑电图和心率变异各指标值差异并将其量化。方法 选择学龄前小儿74例,分成A组(4-5岁)、B组(5~6岁)、C组(6-7岁),均肌注KTM 5 mg／kg,记录各指标变化,体动时静注KTM 1 mg／kg加深麻醉。结果 麻醉前A组双频谱指数(BIS)最小,而低频／高频(LF／HF)、心率变异指数(HRVI)最大(P<0.05)。肌注KTM后95％边缘频率下降,其他指标先升高后下降。3组中A组BIS最小,C组LF／HF最大(P<0.05)。切皮前C组BIS较A、B组小(P<0.05)。A、B组BIS<42时,C组BIS<33时,切皮体动发生率<20％。切皮时无体动者BIS,A、B组较C组大(P<0.05)。切皮后C组BIS、LF/HF增大较A、B组明显。术中LF佃>10时,体动反应达80％;HRVI<30时,无体动反应。结论 BIS、HRVI、LF／HF可量化,联合使用可监测KTM麻醉深度。     

Cardiac Rehabilitation Improves Heart Rate Recovery Following Peak Exercise in Children with Repaired Congenital Heart Disease

We assessed heart rate (HR) recovery following peak exercise before and after a 12-week cardiac rehabilitation program in 14 children, 12.1 ± 1.8 years of age, with repaired complex congenital heart disease (CHD; 11 with Fontan surgery) and impaired exercise performance. Exercise testing using bicycle ergometry was performed at baseline, after completion of the rehab program and 1.0 ± 0.2 years after the baseline test. These data were compared to HR recovery in 15 controls (age, 12.7 ± 2.4 years) with CHD (13 with Fontan surgery) with two serial exercise tests at an interval of 1.1± 0.3 years. There was no change in peak HR between the two serial tests in either group. Peak VO₂ improved in the rehab group (26.3 ± 9.6 ml/kg/min at baseline vs 30.9 ± 9.6 ml/kg/min after rehab, p = 0.01) but remained unchanged in controls on serial testing. One-minute HR recovery (in beats per minute) improved significantly following completion of the rehab program (27 ± 15 at baseline vs 40 ± 23 after rehab, p = 0.01). Partial improvement in 1-minute HR recovery in the rehab group persisted 1 year later (1-minute HR recovery, 35 ± 19; p = 0.1 compared to baseline). There was no change in 1-minute HR recovery over time in the control group (37 ± 16 vs 40 ± 13, p = not significant). In conclusion, HR recovery following peak exercise improves in children with CHD after participation in a cardiac rehab program.     

小儿扩张型心肌病心率变异性分析

目的分析扩张型心肌病(DCM)儿童的心率变异性(HRV)。方法DCM儿童23例(研究组),匹配健康儿童23例为对照(对照组)。采用康泰TLC3000A12通道动态心电图(EKG)分析系统描记24hEKG,分析心率、HRV的时域指标和频域指标。应用SPSS11.0软件进行统计学处理。结果与对照组比较,研究组最低心率明显增高(P<0.05),最高心率稍增高(P>0.05);HRV时域指标SDNN、SDANN、pNN50明显降低(P<0.05),rMSSD稍降低(P>0.05);HRV频域指标TP、ULF明显降低(P<0.05),VLF、LF、HF、LF/HF稍增高(P>0.05)。结论DCM儿童自主神经功能明显受损。     

甲状腺功能亢进症儿童的甲状腺激素与心率减速力及心率变异性的相关性

目的 对甲状腺功能亢进症（简称“甲亢”）儿童的心率减速力（DC）、心率加速力（AC）、心率变异性（HRV）进行分析,并探讨甲亢儿童的血清甲状腺激素水平与DC、AC及HRV的关系。方法 选取甲亢儿童47例,另选取50例健康儿童为对照组,对所有受试者行24 h动态心电图检查,将甲亢组与对照组的DC、AC、心率（HR）及HRV各指标[RR间期总体标准差（SDNN）、RR间期平均值的标准差（SDANN）、相邻RR间期差值的均方根（RMSSD）、低频功率（LF）、高频功率（HF）]进行比较,并将甲亢儿童的甲状腺激素指标[游离三碘甲状腺原氨酸（FT3）、游离甲状腺素（FT4）]与DC、AC及HRV各指标进行相关性分析。结果 甲亢组的DC、SDNN、SDANN、RMSSD、LF、HF均低于对照组;AC和HR均高于对照组（P < 0.05）。甲亢儿童的血清FT3、FT4与DC、SDNN、SDANN、RMSSD、LF、HF均呈负相关（P < 0.05）;与AC和HR呈正相关（P < 0.05）。结论 甲亢儿童的心脏自主神经功能受损,表现为迷走神经张力降低。血清甲状腺激素水平越高,迷走神经张力越低,提示发生心血管疾病的危险性越大。     

川崎病患儿心率变异性与冠状动脉损害的相关性研究

目的 探讨川崎病(KD)患儿的心率变异性(HRV)指标与肌钙蛋白I(cTnI)和氨基末端脑钠肽前体(NT-proBNP)的相关性及其在预后中的应用价值。方法 将130 例KD 患儿分为冠状动脉损害组(n=47, CAL 组)和无CAL 组(n=83, NCAL 组), 同期选取110 例健康儿童为对照组, 29 例非心血管疾病恢复期患儿为非KD 组。各组儿童均行长程HRV 指标检测及分析。检测KD 组及非KD 组患儿血清NT-proBNP 及cTnI 水平。结果 同年龄性别KD 组患儿正常窦性N-N 间期标准差(SDNN)、相邻N-N 间期标准差的平均值(SDNNindex)、相邻N-N 间期之差>50 ms 的心搏数占心搏总数的百分数(PNN50)、极低频功率(VLF)、低频功率(LF)和高频功率(HF)值较对照组均明显下降, LF/HF 值较对照组升高(P<0.05)。CAL 组SDNN、全部记录中每5 min N-N 间期平均值的标准差(SDANN)、SDNNindex、相邻N-N 间期差值的均方根值(rMSSD)、PNN50、VLF、LF 和HF 值均低于对照组和非KD 组, LF/HF 值高于对照组(P<0.05)。CAL 组及NCAL 组的cTnI 和NT-proBNP 水平均高于非KD 组(P<0.05)。KD 患儿cTnI 与SDNN、HF 呈负相关, 与LF/HF 呈正相关(P<0.05);NT-proBNP 与SDNN、SDANN、HF 呈负相关(P<0.05)。结论 HRV 指标对KD 患儿的CAL 判断具有一定的临床意义。     

Cardiopulmonary exercise parameters in children with atrial septal defect and increased pulmonary blood flow: short-term effects of defect closure

Markedly increased pulmonary blood flow because of a relevant atrial septal defect (ASD) leads to impaired cardiopulmonary function during maximum exercise in adults. No comparative preoperative and postoperative data are available on the short-term effects of shunt closure on cardiorespiratory function at peak exercise in children. Pulmonary function testing at rest and cardiopulmonary exercise testing together with haemodynamic assessment was done prospectively in children with an ASD preoperatively and again after full recovery at 3-4 mo postoperatively and compared with a matched normal population. Sixteen children, aged 6.8-16.1 y, with a defect of 8-23 mm (median 15 mm) and a pulmonary/systemic flow ratio of 1.5-3.5 (median 2.2) were tested and compared with 15 healthy children. Preoperatively, baseline pulmonary function parameters and exercise capacity were no different from normals. At peak exercise, patients with a shunt had increased pulmonary resistance, especially of the distal airways (p = 0.04), with a significantly larger proportion of children having a paradoxical increase in total airway resistance during exercise (p < 0.05). Maximum serum lactate at peak exercise was elevated (p < 0.05) in patients. In patients, maximum oxygen uptake was impaired (p = 0.03) and remained so at repeat evaluation postoperatively. The same observation was made for chronotropic response to exercise. Conclusion: Cardiopulmonary exercise parameters in patients with ASD differed only slightly from those in normal children. The most important deviations were a lower maximum oxygen uptake and an increase in airway resistance at maximum exercise.     

Heart rate recovery from 1 minute of exercise in children and adults

Previous studies demonstrated that the time required for oxygen uptake, CO2 production, and minute ventilation to return to baseline levels after 1-min bursts of exercise is different in children compared with adults. To test the hypothesis that the heart rate (HR) recovery time after exercise is also different in children compared with adults, we examined HR in 10 children (range 7-11 y old) and 12 adults (26-42 y old) for 10 min after 1 min of cycle ergometer exercise. Each subject exercised at work rates corresponding to 80% of the lactate or anaerobic threshold (AT), 50% of the difference between AT and maximal O2 uptake (delta), 100% of maximal uptake, and 125% of maximal uptake. Gas exchange was measured breath by breath. In adults, the HR recovery time increased significantly with work intensity as judged by the time constant of a single exponential curve fit to postburst-exercise HR [23 +/- 8 (SD) s at 80% AT, 55 +/- 16 at 50% delta, 74 +/- 13 at 100% of maximal uptake, and 83 +/- 20 at 125% of maximal uptake]. HR recovery time tended to increase with work intensity in children (16 +/- 7, 20 +/- 4, 23 +/- 7, and 27 +/- 9; for 80% AT, 50% delta, 100% of maximal uptake, and 125% of maximal uptake respectively), but to a much smaller extent, and the HR recovery time was significantly smaller in children in the high-intensity (above AT) range of exercise (p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)     

Hypertension after heart and heart/lung transplantation in childhood--study on the evolution of short-term blood pressure regulation

Arterial hypertension complicates the follow-up of heart- and heart/lung-transplanted children. We investigated the evolution of BRS as short-time BP regulation mechanism and BP after heart and heart/lung transplantation. Twenty patients (15 males; mean age 15.1 ± 4.3 yr) were studied twice at intervals of 2.96 ± 0.87 yr. BRS was calculated using non-invasive beat-to-beat BP measurement system. HRV was calculated (LF, sympathetic influence; HF, parasympathetic influence). BRS increased in 10 patients (3.67 ± 1.43 ms/mmHg vs. 7.59 ± 3.40 mmHg, p = 0.005) (group 1). Six of 10 patients received antihypertensive medication. BRS decreased or remained unchanged in 10 patients (8.93 ± 7.9 ms/mmHg vs. 5.32 ± 6.6 ms/mmHg, p = 0.008) (group 2) with 9/10 patients necessitating antihypertensive medication. Group 1 showed LF/HF increase (LF/HF 1.03 ± 0.9 vs. 4.36 ± 2.32, p = 0.03); group 2 showed LF/HF decrease (LF/HF 3.7 ± 2.1 vs. 1.84 ± 1.1, p = 0.023). Evolution of BRS after heart and heart/lung transplantation in childhood seems to influence the necessity of antihypertensive medication. With time, increasing short-time BP regulation involving sympathetic reinnervation may improve BP.     

Effect of obesity status on heart rate peak in female youth

The purpose of this study was to compare heart rate peak in obese (n = 43) and normal weight (n = 45) female youth. Heart rate (HR) peak was significantly lower (p < or = 0.05) in the obese group as compared to the normal weight group (192.3 +/- 9.3, 203.4 +/- 7.6), and VO2 (L x min(-1)) peak similar between groups (1.77 +/- 0.53, 1.97 +/- 0.60). Bivariate correlations for heart rate peak with body weight, percent fat, and body mass index yielded the following: -0.53, -0.54, and -0.57. These findings agree with the adult data indicating low HR peak in obese individuals. Further research is needed to explore physiologic factors that may lead to reduced HR peak in obese female youth.     

С�����鲡���ʱ�����ָ���뼡�Ƶ��ע�Ͱ���ĩ��������ǰ������Է���

??Objective To analyze the correlation of heart rate variability??HRV?? parameters with troponin??cTn?? and N-terminal pro-B-type natriuretic peptide??NT-proBNP?? in children with Kawasaki disease??KD??. Methods According to presence or absence of coronary artery lesion??CAL????the enrolled 118 children with KD as KD group were divided into CAL group??n??43?? and non-CAL??NCAL?? group??n??75??. Meanwhile??32 children without cardiovascular disease in convalescent period and 106 healthy children at the same term were respectively selected as non-KD group and control group. Long-term HRV parameters were detected in each group??and levels of cTn?? and NT-proBNP were detected in KD group and non-KD group. Results HRV parameters in KD group were all higher during recovery phase than those during acute phase??except for LF/HF value lower than that during acute phase ??P??0.05 or P??0.01??. HRV parameters in KD group were all lower than those in control group??except for LF/HF value higher than that in control group ??P??0.05 or P??0.01??. Each index in time domain of HRV and high frequency ??HF????low frequency ??LF?? and very low frequency ??VLF?? in CAL group were all lower than those in control group and non-KD group ??P??0.05 or P??0.01????and SDANN??SDNN and PNN50 were also lower than those in NCAL group ??P??0.01??. Besides??the LF/HF value in CAL group was higher than that in control group??non-KD group and NCAL group ??P??0.05 or P??0.01??. The levels of cTn?? and NT-proBNP in CAL group and NCAL group were significantly higher than those in non-KD group??in which the level of cTn?? in CAL group was also higher than that in NCAL group ??P??0.01??. The level of cTn?? was negatively correlated with SDNN and HF in children with KD??while positively correlated with LF/HF value ??P??0.01??. The level of NT-proBNP had a significantly negative correlation with SDANN??SDNN and HF ??P??0.01??. Conclusion The heart autonomic nervous function is damaged in children with KD. HRV parameters have a certain clinical value for assessment of CAL in children with KD.     

不同体重指数血管迷走性晕厥心脏抑制型儿童心率变异性的差异

目的探讨不同体重指数(BMI)血管迷走性晕厥心脏抑制型(VVS-CI)儿童心率变异性(HRV)的差异。方法回顾性分析2012年1月至2019年12月因晕厥或晕厥先兆在中南大学湘雅二医院儿童晕厥专科门诊诊断为VVS-CI的34例儿童的临床资料。根据身高、体重计算体重指数(BMI),分为偏瘦组(BMI≤18.4 kg/m²,n=19)和正常组(BMI 18.5-23.9 kg/m²,n=15)。对24 h动态心电图HRV进行分析。HRV分析采用线性分析法,时域指标SDNN、SDANN、rMSSD和pNN50,频域指标总功率(TP)、超低频功率(ULF)、极低频功率(VLF)、低频功率(LF)、高频功率(HF)和LF/HF。结果偏瘦组与正常组比较SDNN、SDANN和rMSSD差异无统计学意义(P>0.05),pNN50升高(P<0.05)。两组间TP、ULF、LF、HF和LF/HF差异无统计学意义(P>0.05),偏瘦组较正常组VLF降低(P<0.05)。偏瘦组、正常组不同性别之间时域指标、频域指标差异无统计学意义(P>0.05)。偏瘦组<12岁较≥12岁SDNN、SDANN、LF升高(P<0.05),rMSSD、pNN50、TP、ULF、VLF、HF和LF/HF差异无统计学意义(P>0.05)。正常组<12岁较≥12岁ULF升高、LF降低(P<0.05),SDNN、SDANN、rMSSD、pNN50、TP、VLF、HF和LF/HF差异无统计学意义(P>0.05)。结论低BMI与正常BMI的VVS-CI儿童自主神经调节功能不同,引起HRV存在差异。相同BMI<12岁与≥12岁儿童之间HRV也存在差异。     

Effects of caffeine on physiological responses to exercise: boys versus men

We compared the influence of caffeine on physiological responses to exercise between boys and men. Fifty-two participants (26 boys and 26 men) participated in a double blind, randomized, double crossover study. Each participant received the caffeinated (5 mg/kg) drink (CAF) and placebo (PL) twice each on 4 separate days. One hour after drink consumption preexercise heart rate (HR) and blood pressure (BP) were measured. Then while the participants rode stationary cycle ergometers at two different exercise intensities, HR, BP, and oxygen consumption (VO2) were measured. Blood pressure was not significantly affected by CAF, although on average it was always higher in boys for diastolic BP (3 mmHg) and systolic BP (3-4 mmHg) and men for diastolic BP (2-3 mmHg) and systolic BP (1-6 mmHg) both at rest and during exercise. HR was significantly (p < .05) lower at rest, 25W and 50W in CAF versus PL in boys, with no change in adults. During exercise, VO2 and respiratory exchange ratio (RER) were not different in CAF versus PL in either group. In conclusion, metabolism is not affected by a moderate caffeine dose in children or adults. The same dose has a similar effect on BP in both groups. The effect on HR was different, however, with a significant (p < .05) lowering in children in CAF versus PL, with no adult effects.     

特发性室性期前收缩患儿心率变异性和心率减速力的研究

目的探讨不同起源特发性室性期前收缩患儿的心率变异性(HRV)与心率减速力(DC)各项指标的关联性。方法回顾性分析155例特发性室性期前收缩患儿的临床资料,按不同年龄组分为婴幼儿期(3岁),学龄前期组(3~6岁)和学龄期(~16岁),并在不同年龄组内按室性期前收缩来源部位不同,分为右室型组和左室型组;比较分析不同年龄组、不同来源期前收缩组之间HRV与DC各指标的差异。结果三个不同年龄组的DC和HRV时域分析各项参数的差异均有统计学意义(P0.05)。婴幼儿组中,右室型组和左室型组间相邻RR间期差值的均方根(RMSSD)、高频功率(HF)、低频功率(LF)/HF、DC、相邻RR间期差值50 ms的百分数(PNN50)差异均有统计学意义(P均0.05);学龄前期组中,右室型组和左室型组间的RMSSD、LF、HF、LF/HF、DC差异有统计学意义(P均0.05),学龄期组中,右室型组和左室型组间的RMSSD、HF、LF/HF、DC差异均有统计学意义(P均0.05)。结论特发性室性期前收缩患儿自主神经平衡调节受损,以迷走神经张力的降低为主;起源于优势心室(婴幼儿期及学龄前期患儿以右心室占优势,学龄期患儿左心室为优势心室)的频发室性期前收缩,增加了恶性心律失常发生的风险。     

Exercise capacity following pediatric heart transplantation: A systematic review

Pediatric HTs account for 13% of all HTs with >60% of recipients surviving at least 10 years post‐HT. The purpose of this systematic review is to synthesize the literature on exercise capacity of pediatric HT recipients to improve understanding of the mechanisms that may explain the decreased exercise capacity. Six databases were searched for studies that compared the exercise capacity of HT recipients ≤21 years old with a control group or normative data. Sixteen studies were included. Pediatric HT recipients, as compared to controls or normative data, exhibit significantly higher resting HR, and at peak exercise exhibit significantly decreased HR, VO₂, power, work, minute ventilation, and exercise duration. Peak VO₂ appears to improve within the first 2.5 years post‐HT; peak work remains constant; and there is inconclusive evidence that peak HR, HR recovery, and HR reserve improve with time since HT. These results are discussed in the context of the mechanisms that may explain the impaired exercise capacity of pediatric HT recipients, including chronotropic incompetence, graft dysfunction, side effects of immunosuppression therapy, and deconditioning. In addition, the limited literature on rehabilitation after pediatric HT is summarized.     

Serial measurements of exercise performance in pediatric heart transplant patients using stress echocardiography

Heart transplantation is an increasingly acceptable therapeutic option for children with end-stage and complex congenital heart disease. With advances in surgery, immunosuppression, and follow-up care, functional outcomes need to be evaluated. We report the results of serial exercise testing performed using stress echocardiography in a cohort of pediatric HTP. HTP (n = 7) exercised on a semi-recumbent ergometer to volitional fatigue. Echocardiography-Doppler measurements, HR, and blood pressure were taken at rest and during staged exercise. Results were compared with healthy CON (n = 12). HTP did significantly less work during exercise (940 vs. 1218 J/kg, p < 0.03). Their SVI (33 vs. 49 mL/m(2), p < 0.003), CI (5.16 vs. 9.25 L/min/m(2), p < 0.0005), and HR (162 vs. 185 bpm, p < 0.02) were lower at peak exercise. HTP had a lower SF at peak exercise (48% vs. 52%, p < 0.03) and an abnormal relationship between the MVCFc and σPS. During follow-up, hemodynamics and left ventricular function remained relatively constant in HTP. HTP are able to exercise safely; however, their exercise tolerance is reduced, and hemodynamics and contractility are diminished. Over time, their hemodynamics and left ventricular function have remained relatively constant.     

Plasma lactate and plasma volume recovery in adults and children following high-intensity exercises

AIM: To compare plasma lactate concentration recovery kinetics when measured and corrected for variations in plasma volume between children and adults. METHODS: Nine boys (11.3 +/- 1.1 y) and 8 men (21.9 +/- 1.9 y) performed a maximal and a supramaximal exercise until exhaustion. Plasma lactate concentrations, haemoglobin and haematocrit were measured at rest, immediately on completion of exercise and after the 2nd, 5th, 12th and 30th minute of recovery. The plasma lactate concentrations and the rate of recovery were corrected for variations in plasma volume. RESULTS: The maximal decreases in plasma volume were significantly higher in adults than in children for maximal exercise (-18.7 +/- 2.6% vs -14.5 +/- 3.2%; p < 0.05), but similar for the supramaximal exercise (-16.9 +/- 3.4% vs -15.2 +/- 3.4%). During recovery, measured and corrected plasma lactate concentrations were significantly higher in adults. The rate of plasma lactate recovery was higher in adults for maximal exercise only. The same results were obtained when the rates of plasma lactate decrease were calculated from corrected plasma lactate concentrations. CONCLUSION: The correction of the plasma lactate concentrations for variations in plasma volume did not influence the comparison of the concentrations obtained in adults and children, or their rate of recovery.     

Development of baroreflex influences on heart rate variability in preterm infants

To investigate developmental changes in autonomic cardiovascular reflexes in preterm infants, we used autoregressive power spectral analysis to analyze the effect of upright tilting on heart rate variability in preterm infants. Twenty-eight infants were studied in a longitudinal fashion beginning at 28-32 weeks postconceptional age (postnatal age 1-5 weeks). Each week, heart rate variability in the supine position and after 45 degrees head-up tilt was analyzed by spectral analysis. With the initial study of each infant, there was no significant change in heart rate following head-up tilt compared with baseline (-0.5+/-0.9 bpm). However, linear regression analysis revealed that with increasing postnatal age, the change in heart rate in response to tilting became more positive (mean slope of regressions 0.45+/-0.12 bpm/week, P<0.005). The power spectral density of R-R interval variability in the low-(LF; 0.02-0.15 Hz) and high-(HF; 0.15-1.5 Hz) frequency ranges were obtained and the values normalized by dividing each component by the total power. For measurements obtained in the supine position, the LF/HF ratio progressively decreased with increasing postnatal age, indicating a maturational change in sympathovagal balance. We used the difference in the LF/HF ratio between tilt and the recumbent position as a measure of the change in autonomic input to the heart in response to unloading of the arterial baroreceptors. No significant change in these ratios were observed when infants were first studied between 28 and 32 weeks postconceptional age, suggesting that the cardiac baroreflex is poorly developed at this stage of development. However, with postnatal maturation, the LF component of the power spectrum became progressively larger with tilt relative to the basal state, such that the difference between LF/HF(tilt) and LF/HF(base) became progressively more positive (P <0.006). These findings suggest that in premature infants, cardiac baroreceptor reflexes become more functional with postnatal development.