Continuous noninvasive blood pressure measurement in the head-up tilt test enabled us to observe rapid changes in blood pressure from the supine to the tilted upright position and to study underlying haemodynamic mechanisms in OH patients16, 17. A total of 45 typical OH patients (mean age 57.18 ± 15.94) were enrolled in our retrospective study, including 36 males and 9 females. This study aimed to gain insight into the clinical features and haemodynamic mechanisms underlying short-term orthostatic blood pressure decline patterns and the factors affecting blood pressure regulation.
Impaired increase in SVR
In patients with OH, we observed an impaired increase in SVR from the supine to the tilted upright position (shown in Table 1), and linear regression analysis showed that an increase in SVR was the main determinant of the maintenance of SBP from the supine to the tilted upright position (shown in Table 3).
Physiologically, during active standing, the muscles of the lower limbs and abdomen contract to compress the blood vessels of the lower limbs and abdominal cavity, which increases the blood return to the heart and maintains blood pressure. However, for the OH patients in the current study, the blood pressure decrease in the tilted upright position was based on a supply-demand mismatch between increasing CO and an impaired increase in SVR, with an even pronounced decrease possible1, 5. In general, the aetiology of OH is diverse and divided into nonneurogenic and neurogenic causes3. Herein, different causes of OH had different mechanisms for the impairment of SVR increase18–20. In nonneurogenic OH patients, SVR did not increase because of intravascular hypovolemia and consequently decreased CO, which was usually accompanied by compensatory tachycardia. In neurogenic OH patients, sympathetic noradrenergic failure resulted in an inadequate SVR increase in the tilted upright position. In addition, vasoactive and psychoactive drugs could influence SVR. Therefore, it was particularly important to identify and clarify the aetiology of OH for patient treatment and prognosis. A noninvasive head-up tilted test could be used as an important detection method for evaluation of autonomic function12, 21. Based on the changing trend of HR and SVR, further autonomic function was able to be evaluated. Based on the important role of SVR in the maintenance of tilted upright blood pressure, the use of leg stretch socks and repeated upright tilt training may be effective in improving the symptoms of OH patients22–24.
Pattern of haemodynamic changes in the upright position
In typical OH patients after tilting upright, we compared the amplitude and rate of change of haemodynamic parameters at each minute after tilting upright (shown in Table 1 and Table 2). It was found that the amplitude and rate of change of haemodynamic parameters were the greatest and the fastest in the first minute after tilting upright and that the changes gradually slowed down in the following two minutes. This pattern of haemodynamic changes was distinct from initial OH and delayed OH25. Previous studies25, 26 in healthy teenagers and young adult subjects using beat-to-beat measurement of SV with calculation of CO and SVR have established that CO actually increases with the onset of tilting upright, whereas SVR falls markedly. Herein, the initial fall in BP upon standing was due to a mismatch of this increase in CO and a decrease in SVR and did not occur or was far less pronounced in a passive change of posture. In delayed OH patients, a previous study25 showed that the SVR increase was significantly lower, whereas changes in CO were not different. These findings7, 27 indicated that delayed OH was associated with an impaired SVR response, which can point towards impaired sympathetic vasoconstrictor function or the use of vasodilators.
The influence of age on SV and HR changes
In OH patients younger than 60 years of age, HR increased significantly while SV decreased significantly in the first 3 minutes after tilting upright, both in terms of rates, absolute values and percentage of changes, compared with OH patients older than 60 years of age and older.
The significant decrease in SV in patients 60 years of age and older was due to decreased venous sclerosis and reduced compliance in older patients, leading to reduced cardiac volume and subsequent significant decrease in SV. Two factors may be involved28, 29. First, in older patients, more blood accumulated in the abdominal cavity due to a decrease in the ability of blood vessels to constrict in upright position.. On the other hand, in older patients, a decrease in myocardial compliance resulted in a greater need for cardiac preload to maintain left ventricle filling. Thus, with increasing age, myocardial compliance decreased, diastolic left ventricular filling was limited, and cardiac preload decreased. .This may be the most important reason for the significant decrease in SV in the upright position. In the case of increased heart rate, the relative shortening of diastolic period was more pronounced.
Although SV decreased significantly after standing, there was no corresponding increase in HR. This can be ascribed to diminished HR regulation with increasing age, which was further supported in this study by the decrease in the regulation of autonomic function in the elderly. In the mean time, the obvious fall in SV was not adequately compensated by a significant increase in SVR., All of these conditions indicated that orthostatic hypotension was intrinsically dependent on sympathetic vasomotor responses30, 31.
This study was conducted at a single centre, hence limiting its generalizability. Importantly, however, our study was truly retrospective of a real-world population of syncope or dizziness patients with typical OH. In linear regression analysis, a parsimonious model was generated because of a small sample size; this may have an impact on the strength of the association of predictors and main adverse events. Large-scale, multicentre, and prospective studies should be conducted in the future, and subgroup analyses should be performed in groups of typical OH.