Effects of insulin resistance on systemic haemodynamics and autonomic cardiovascular regulation in normotensive healthy adults

Hemodynamic effects of insulin resistance (IR) are thought to be largely dependent on its relationship with body mass index (BMI) and blood pressure (BP) levels. The first part of the present thesis was aimed at exploring whether IR is associated with hemodynamic indices of cardiovascular function in a large sample of non-diabetic individuals from the general population (n=731) and if so, to explore if such relationship is continuous across different categories of BMI (lean, overweight and obese), and BP (normal BP, high-normal BP and hypertension). IR was assessed with the homeostasis model assessment of IR (HOMA-IR). Based on a value of HOMA-IR of 2.09 (75th percentile of distribution curve), subjects were classified as insulin-sensitive (IS, HOMA<2.09) or insulin-resistant (IR, HOMA≥2.09). Synchronized beat-to-beat recordings of stroke volume (impedance cardiography) and R-R interval (ECG), along with repeated BP measurements were performed over 5 minutes. Stroke index (SI), cardiac index (CI), systemic vascular resistance index (SVRI), left cardiac work index (LCWI), pre-ejection period (PEP) and left ventricular ejection time (LVET) were computed and averaged. In analysis of co-variance allowing for confounders, IR subjects showed significantly higher BP levels and SVRI, and reduced R-R interval, SI, CI, LCWI, PEP and LVET. These differences remained significant when analyses were performed within each BMI and BP category. Overall, these results indicate that effects of IR on hemodynamic indices of cardiovascular function are continuous across different BMI and BP categories, reinforcing the importance of IR in the pathogenesis of cardiovascular alterations beyond its association with obesity and hypertension. The finding of a significant association between IR and hemodynamic alterations even in lean and normotensive subjects was the rationale to explore potential mechanisms for these alterations in this selected group of subjects. Specific objectives of this second part of the thesis were: 1) To explore the relationship between insulin resistance and systemic hemodynamics, cardiac baroreflex sensitivity and indices of autonomic CV modulation. 2) To explore the relationship of insulin resistance with 24h heart rate, average blood pressure levels and blood pressure variability over the 24h; and 3) To explore the relationship of insulin resistance with central blood pressure levels and with measures of large artery stiffness and wave reflections. The study population for these analyses was constituted by subjects who were below the 30th percentile of diastolic blood pressure (DBP) distribution curve (DBP ≤72 mmHg) and who had no elevation in systolic BP levels. In addition, subjects were excluded in case of diabetes mellitus (fasting blood glucose ≥126 mg/dL or use of medications for previously diagnosed type 2 diabetes) obesity (BMI≥30) or taking medications with effects on BP. A total of 90 subjects fulfilling inclusion criteria were considered for the present analysis and underwent further assessments. Insulin resistance was assessed with HOMA-index and subjects classified into IR tertiles, based on the distribution of HOMA-index values. 24h Ambulatory BP monitoring was performed. Mean SBP and DBP were averaged for the day, night and 24h, and the respective day-to-night dipping was calculated. BPV was assessed for SBP and DBP as 24h standard deviation (SD), weighted 24h SD (wSD), daytime and night-time SD. Recordings of pulse waveform were obtained by means of a previously validated oscillometric device for ambulatory BP monitoring with in-built transfer-function like method. Aortic pulse wave velocity (PWV, m/s) and other measures derived from pulse wave analysis such as augmentation index (AIx, %), central SBP (cSBP), central DBP (cDBP) and central pulse pressure (cPP) were computed. Peripheral SBP and DBP, and heart rate (HR) were recorded and pulse pressure (PP) calculated as the difference between SBP and DBP. Non-invasive assessment of beat-to-beat BP, R-R interval (ECG) and stroke volume (by means of impedance cardiography) were performed during 10 min in supine position and specific hemodynamic indices associated with their measurement were computed and averaged: RRI (msec), heart rate (HR, bpm), stroke volume index (SI, mL/beat/m2), cardiac index (CI, L/min/m2), SBP (mmHg) and DBP (mmHg), systemic vascular resistance index (SVRI, dyn/sec/cm-5/m2), left cardiac work index (LCWI, Kg/m/m2), pre-ejection period (PEP, msec), left ventricular ejection time (LVET, msec) and PEP/LVET ratio were calculated. Cardiac autonomic modulation was assessed by computer analysis of 10 min beat-to-beat BP and ECG recordings in resting supine position. Cardiac baroreflex sensitivity (BRS) was estimated by sequence method. Total variance, low-frequency (LF) and high-frequency (HF) spectral components of HR variability (HRV) were assessed by autoregressive analysis. LF/HF ratio was calculated. After multiple regression analysis, adjusting for common confounders such as age, sex, HR and BMI, increasing values of HOMA-IR were associated with reduced RRI, SI, CI, and with increased SVRI, SBP and DBP. IR was also associated with reduced BRS (up, down, and total slopes), decreased parasympathetic indices of autonomic CV modulation (SDRRI, HF-power, total power) and a predominance of sympathetic component of HRV (increased LF/HF ratio). Increasing values of HOMA-IR were also associated with increased HR and average SBP levels (during day, night and 24-h period), with augmented BP variability (Day SBP SD, and SBP wSD) and with a reduced dipping of HR. Finally, insulin resistance was shown to be associated with increasing values of aortic PWV, and with higher central and peripheral SBP and DBP levels. Overall, these results support significant associations between insulin resistance and changes in hemodynamic and autonomic indices of cardiovascular function, even after accounting for common confounders. These findings suggest that in normotensive healthy adults, increases in insulin resistance may promote alterations in autonomic cardiovascular modulation, in systemic hemodynamics and in arterial stiffness, all of which are known contributors to the pathogenesis of hypertension.