Cacciatori V, Gemma M L, Bellavere F, Castello R, De Gregori M E, Zoppini G, Thomaseth K, Moghetti P, Muggeo M
Division of Endocrinology and Metabolic Diseases, Azienda Ospedaliera di Verona and University of Verona, Verona, Italy.
Eur J Endocrinol. 2000 Sep;143(3):327-33. doi: 10.1530/eje.0.1430327.
The aim of the present study was to evaluate the impact of hypothyroidism on the autonomic regulation of the cardiovascular system by analysing separately sympathetic and parasympathetic influences on the heart.
In seven newly diagnosed untreated hypothyroid patients we analysed power spectral density of heart rate cyclic variations at rest, while lying, and while standing. The same protocol was repeated after the induction of stable euthyroidism by levothyroxine (L-T(4)) treatment. The results were also compared with those obtained from seven age-, sex- and body mass index-matched control subjects.
Heart rate variability was evaluated by autoregressive power spectral analysis (PSA). This method allows reliable quantification of low frequency (LF) and high frequency (HF) components of the heart rate power spectral density. These are considered to be under mainly sympathetic and purely parasympathetic control respectively. In addition, heart rate variations during deep breathing, lying to standing, and Valsalva's manoeuvre were assessed.
PSA showed a sharp reduction in the HF (parasympathetic) component in hypothyroid subjects compared with controls (lying, 29.4+/-5.4 vs 47.7+/-6.3 normalized units (NU) (means +/- s.e.m.), P<0.05; standing, 14.0+/-3.5 vs 32.1+/-3.6NU, P<0.005). Conversely, the LF (mainly sympathetic) component was higher in hypothyroid subjects than in controls (lying, 61.6+/-6.4 vs 45.4+/-6.7 NU; standing, 71.7+/-8.0 vs 53.1+/-5.6NU), this difference being significant in the standing position. Hence, the LF/HF ratio, which is considered an index of sympathovagal balance, was increased in hypothyroid subjects while both lying (2.75+/-0.6 vs 1.16+/-0.3; P<0.05) and standing (10.0+/-3.7 vs 1.85+/-0.3; P<0. 02). Total heart rate variability, expressed as total power spectral density, was lower in hypothyroid patients than in control subjects, this difference being significant in the lying position (574+/-126 vs 2302+/-994ms(2), P<0.05). In patients re-examined after L-T(4) therapy, complete normalization of cardiovascular parameters was observed (LF/HF ratio, lying, 1.26+/-0.4; standing, 2.56+/-0.8, both P<0.01 vs baseline values). The response to conventional cardiovascular autonomic tests was not significantly different between hypothyroid patients and healthy controls, and did not change in patients after therapy.
These results suggest that, contrary to the clinical picture, thyroid hormone deficiency is associated with an increased sympathetic influence on the autonomic cardiovascular system. The changes in sympathetic function could be explained by a secondary adaptation to an altered cardiovascular responsiveness.
本研究旨在通过分别分析交感神经和副交感神经对心脏的影响，评估甲状腺功能减退对心血管系统自主调节的影响。
在7例新诊断的未经治疗的甲状腺功能减退患者中，我们分析了静息、卧位及站立时心率周期性变化的功率谱密度。在通过左旋甲状腺素（L-T4）治疗诱导达到稳定的甲状腺功能正常状态后，重复相同方案。结果还与7例年龄、性别和体重指数匹配的对照受试者的结果进行比较。
通过自回归功率谱分析（PSA）评估心率变异性。该方法可可靠地量化心率功率谱密度的低频（LF）和高频（HF）成分。这些成分分别主要受交感神经和纯副交感神经控制。此外，评估了深呼吸、卧位到站立及瓦尔萨尔瓦动作期间的心率变化。
PSA显示，与对照组相比，甲状腺功能减退受试者的HF（副交感神经）成分急剧降低（卧位，29.4±5.4对47.7±6.3归一化单位（NU）（均值±标准误），P<0.05；站立位，14.0±3.5对32.1±3.6 NU，P<0.005）。相反，甲状腺功能减退受试者的LF（主要为交感神经）成分高于对照组（卧位，61.6±6.4对45.4±6.7 NU；站立位，71.7±8.0对53.1±5.6 NU），该差异在站立位显著。因此，被视为交感迷走神经平衡指标的LF/HF比值在甲状腺功能减退受试者卧位（2.75±0.6对1.16±0.3；P<0.05）和站立位（10.0±3.7对1.85±0.3；P<0.02）时均升高。以总功率谱密度表示的总心率变异性在甲状腺功能减退患者中低于对照受试者，该差异在卧位时显著（574±126对2302±994ms²，P<0.05）。在L-T4治疗后复查的患者中，观察到心血管参数完全恢复正常（LF/HF比值，卧位，1.26±0.4；站立位，2.56±0.8，两者与基线值相比P均<0.01）。甲状腺功能减退患者与健康对照之间对传统心血管自主神经测试的反应无显著差异，且治疗后患者的反应未改变。
这些结果表明，与临床表现相反，甲状腺激素缺乏与自主心血管系统交感神经影响增加有关。交感神经功能的变化可通过对改变的心血管反应性的继发适应来解释。
