Cold Pressor Test Induces Significant Changes in Internal Jugular Vein Flow Dynamics in Healthy Young Adults

Introduction

The venous system contains 70% of all blood in the human body, and the veins are under the autonomic control of the sympathetic nervous system [1–3]. The paraventricular nucleus in the hypothalamus is the main brain region that provides autonomic control of increased flow in the peripheral veins, increased stressed volume in the central veins, and thus the maintenance of cardiovascular functions in stressful situations [4,5]. In addition, the medullary lateral tegmental field has been found to have a considerable effect on venopressor responses and venous return [6]. Thus, the excitatory response in the efferent neurons of the sympathetic nervous system increases the blood volume returning from the peripheral veins to the right atrium [7]. The increase in peripheral venous return is correlated with increased cardiac output [8].

The cold pressor test (CPT), which has long been used to test autonomic functions by causing sympathetic excitation, increases systolic blood pressure (SBP) and diastolic blood pressure (DBP), increases heart rate (HR), and causes coronary vasodilation within physiological limits in healthy individuals [9,10]. The sympathetic excitatory effect of CPT is mediated by catecholamines such as norepinephrine, whose concentrations in the systemic circulation increase, as well as endothelin, prostaglandins, and angiotensin II [11,12]. CPT has been used in many experimental studies in the past to test human circulatory physiology parameters in healthy individuals and there is an increasing body of knowledge on this subject today; in addition, CPT has been used many times in various clinical research studies to predict the future risk of developing potential pathologies such as coronary heart disease and essential hypertension [10]. In short, CPT is used both in experimental physiology studies and in clinical research studies.

Clinical studies aimed at recording changes in human circulatory physiology induced by CPT using various radiological methods are relatively new. In recent years, numerous Doppler ultrasound (USG) studies have been published on the physiological changes in arterial system parameters caused by CPT. Zvan et al reported that CPT increased the mean velocity of the middle cerebral artery (MCA) [13]. However, this finding was not supported by other studies. One study found that CPT decreased the mean velocity of the MCA [14], but another study found the mean velocity did not change [15]. Flück et al found that the internal carotid artery diameter did not change, but there was increased volumetric flow [16]. In the study of Liu et al, the right common carotid artery diameter slightly increased with CPT, while the right common femoral artery diameter did not change [17]. Hendriks-Balk et al showed that CPT increased the renal perfusion index [18].

Doppler USG studies on the physiological changes caused by CPT have mostly focused on the venous system and there have been relatively fewer arterial studies. Young et al found that CPT did not change popliteal vein compliance [19]. On the other hand, Oue et al reported that CPT reduced the cross-sectional area in the superficial and deep veins of the upper extremity but did not significantly change their compliance [20]. Another study showed that the left popliteal vein’s peak velocity and volumetric flow increased significantly with CPT [10]. However, the splanchnic veins, which are important compliance veins of the human body [21], and the central venous system, which determines the right atrial filling pressure, have not been evaluated with Doppler USG during CPT. In other words, ultrasonographic data on the responses of flow parameters in these veins to sympathetic activation are still lacking.

The IJV is a large vein that collects venous flow from the brain, face, neck, and retromandibular and lingual regions and transfers it to the right atrium [23]. Although there is sufficient information in the literature about IJV pressure parameters [24], there is still no potentially reproducible radiology study using non-invasive techniques to investigate the response of central venous physiology to CPT. Measurement of the flow parameters in this vein to sympathetic activation triggered by CPT using Doppler USG will increase understanding of the physiological effects of the autonomic nervous system on blood circulation and cardiac function. Moreover, since this method is non-invasive, its results can be easily compared with future studies. The results of this study will also provide insight into autonomic control over venous drainage of the brain and other cranial structures. Within the scope of this research, we aimed to elucidate and record the response of the internal jugular vein (IJV), frequently used in central venous catheterization, to CPT by using Doppler USG [22]. Therefore, this study aimed to evaluate IJV flow parameters using the CPT with SBP, DBP, and HR in 40 healthy volunteers aged 18–40 years.

Material and Methods

SUBJECTS:

All experiments were conducted in VM Medicalpark Ankara Hospital from June to July 2024. All procedures followed were in accordance with the ethical standards of the responsible committee on human experimentation (institutional and national) and with the Helsinki Declaration of 1975, as revised in 2008. Ethics committee approval was granted from Kayseri City Hospital ethics committee on 06/04/2024 with protocol number 6/4/2024-120, and informed consent was obtained from all participants.

Healthy volunteers aged 18–40 were included in the study. We excluded patients with essential hypertension, diabetes mellitus, coronary artery disease, valvular heart disease, heart failure, peripheral artery disease, cerebrovascular disease, Reynaud phenomenon, liver disease of any etiology, renal failure, hyperthyroidism or hypothyroidism, malignancy in any organ, multiple sclerosis and other demyelinating diseases, polyneuropathy of any etiology, panic attacks, or generalized anxiety disorder. We also excluded patients who were underweight, overweight, or obese according to body mass index (BMI) calculated with calibrated devices, pregnant and breastfeeding mothers, those who were addicted to cigarettes, alcohol, or substances according to DSM-5 criteria, athletic individuals who regularly participated in sports, those with high white blood cell count, C-reactive protein, blood sugar, creatinine, high or low sodium and potassium values in fasting blood samples taken on the morning of the study, those with hyperthermia or hypothermia on the day of the study, those with any infection in the last 2 weeks, and/or those currently under antibiotic treatment.

EXPERIMENTAL DESIGN:

A neurologist verbally explained the experiment’s details and the expected physiological effects to the volunteers 24 hours before the study began and written informed consent was obtained. In the last 12 hours before the experiment, patients were told to avoid nicotine, alcohol, caffeine, tea, and physical exercise. However, they were allowed to eat a light meal 2 hours before the experiment.

The subjects were taken to the neurophysiology laboratory, maintained at 24–25ºC and free from noise for the last hour, and asked to rest as much as possible. At this stage, subjects with anxiety were evaluated by a clinical psychologist and anxious volunteers whose hemodynamic parameters could influence the study results were again excluded. The subjects were invited to lie down on a stretcher in a supine position for the last 10 minutes. After resting for a few more minutes, SBP and DBP were measured from the left arm by an intensive care nurse with a Riester R1 SHOCK PROOF R1250-107 manual measurement device, and pulse measurements were also made at this stage with continuous electrocardiogram monitoring using the GE Healthcare MAC 2000 device. Those with high SBP (>139 mmHg) and DBP (>89 mmHg), tachycardia (>100/min), or bradycardia (<60/min) were also excluded from the study.

After 10 minutes in supine position, baseline (CPT-0) SBP, DBP, and HR measurements were taken again and left IJV parameters were measured by a radiologist with 20 years of experience in use of Doppler USG using a 9-Hz linear probe of a General Electric LOQIC P9 ultrasound Doppler machine. The probe was placed in the longitudinal plane, and the insonation angle was below 60° [25] (Figure 1). The subjects’ IJV diameter, blood flow peak velocity, and volumetric flow values were recorded (Figure 2). In measuring blood volumetric flow value, cross-sectional area (square diameter [D2]×0.785) multiplied by mean blood velocity was used. Then, the subjects’ right hands were immersed in a bucket of cold water maintained at 1ºC up to the wrist [26] (Figure 3). The water temperature was checked with a UNI-T Ut 306S infrared laser thermometer. At the end of the first minute of CPT (CPT-1), we again measured SBP, DBP, HR, IJV diameter, peak velocity, and volumetric flow. The differences between baseline (CPT-0) and CPT-1 were determined.

STATISTICAL ANALYSES:

We performed statistical analyses using SPSS software (Version 22, SPSS, Inc., Chicago, IL, USA). Data visualization was conducted using R Studio’s open source ‘Rain Cloud Plots’ package (Version 2023.06.2 Build 561). Rain Cloud Plots is a data visualization approach that displays probability density, median, means, and associated confidence intervals for repeated pre- and post-measurements. This method provides a comprehensive view by combining traditional box and density plots [27]. Categorical data were summarized with frequencies (n) and percentages (%). Descriptive statistics for normally distributed numerical data are presented as mean±standard deviation (SD), while non-normally distributed data are reported as median (min-max). The normality of numerical data was assessed using the Shapiro-Wilk test, complemented by histograms and Q-Q plots. The paired t test was applied when parametric test assumptions were met; otherwise, the Wilcoxon signed-rank test was used. Pearson or Spearman correlation coefficients were computed based on parametric test assumptions for correlation analyses between numerical variables. The results were considered statistically significant when the P value was less than 0.05.

Results

CHANGES IN HEMODYNAMIC AND FLOW PARAMETERS:

The statistical findings regarding the comparison of hemodynamic parameters measured at CPT-0 and CPT-1 are demonstrated (Table 1). SBP and DBP values had significantly increased at CPT-1 compared to baseline values (P<0.001, P<0.001, respectively) (Figure 4). Heart rate and velocity also had significantly increased at CPT-1 compared to baseline values (P<0.001, P=0.001, respectively) (Figure 5). While diameter values showed a significant decrease compared to baseline, flow rates had significantly increased at CPT-1 (P=0.003, P<0.001, respectively) (Figure 6).

CHANGES ACCORDING TO SEX:

The statistical findings regarding the comparison of hemodynamic parameters measured at CPT-0 and CPT-1 for males and females separately are shown in Table 2. Both males and females showed a significant increase in SBP, DBP, and HR values at CPT-1 compared to baseline values (P<0.001 for all comparisons). Velocity values also showed a significant increase at CPT-1 compared to CPT-0 in males and females (P=0.049, P=0.014, respectively). While the mean diameter values showed a significant decrease in females compared to baseline (P=0.004), the change in the mean diameter in males was not significantly different (P=0.120). Both males and females showed a significant increase in volumetric flow rate values at CPT-1 compared to CPT-0 (P<0.001, P<0.001, respectively).

CHANGES ACCORDING TO AGE:

The findings of the correlation analysis between the ages of the individuals and the changes in hemodynamic parameters before and after CPT are shown in Table 3. A weak negative significant correlation was found between age and changes in DBP values from baseline to CPT-1 (r=−0.315, P=0.048). No statistically significant correlation was found between age and changes in SBP and HR values from baseline to CPT-1 (P=0.710, P=0.753, respectively). No statistically significant correlation was found between ages and changes in velocity, diameter, and flow rate values from baseline to CPT-1 (P=0.995, P=0.602, P=0.686, respectively).

Discussion

The current study found that SBP, DBP, HR, left IJV volumetric flow, and peak flow velocity increased with CPT-1, while venous diameter decreased. However, the change in venous diameter was statistically significant in women, but no significant change was found in men. Our findings are consistent with the hypothesis that the sympathetic nervous system determines cardiac preload by increasing venous return.

The effect of CPT on the venous system has been investigated in a few Doppler USG studies designed for the upper and lower extremities. Young et al found that compliance of the popliteal vein did not change during CPT [19], and Oue et al reported that compliance of the deep and superficial veins of the upper extremities did not change [20]. However, these 2 studies did not focus on the main abdominal compliance veins of the human body, the flow parameters of the central venous system, or on cardiac output [28], so they did not evaluate the effect of the autonomic nervous system on overall venous return, and the parameters of the veins of the extremities in terms of velocity and volumetric flow were not measured by Doppler USG. Another study showed that the right popliteal vein’s volumetric flow and peak systolic velocity increased with CPT-1 [10]. A later study showed that sympathetic activation promotes venous return in the extremities. Still, since no simultaneous measurements were made from a central vein, it cannot be determined whether this result affected the flow parameters in the central veins.

To the best of our knowledge, this is the first study to directly investigate the effect of CPT on IJV or central venous system flow parameters with Doppler USG. However, some studies have investigated the responses of central venous system flow parameters to sympathetic activation with other methodologies. For example, according to Ogoh et al, phenylephrine increases IJV volumetric flow and diameter measured ultrasonographically; however, the researchers reported no significant change in flow rate [29].

Moreover, our study underscores the need for further research in this area. Some studies have investigated the effect of sympathetic nervous system activation on central venous pressure (CVP). Wilson et al reported that sympathetic activation triggered by CPT decreased cerebral blood volume but did not change CVP [30]. Martin et al showed that CVP increased with intravenous bolus administration of phenylephrine, a sympathomimetic [31]. Similarly, in an animal study, sympathetic activation triggered by increasing cerebrospinal fluid (CSF) sodium (Na) concentration increased CVP [32]. Kirsch et al reported that, paradoxically, CVP decreased with heat stress while cardiac output increased [33]. However, it should be noted that heat stress causes sympathetic activation more in the muscles and skin [34], so the responses obtained may not fully reflect the physiology of the central venous system. However, a study reported that heat stress increased lower-extremity venous return [35], so further studies are urgently needed to investigate the relationship between heat stress and cardiac output. Some studies evaluated CVP with head-up and head-down tilt tests. Still, these studies have methodologies that affect the flow in the central venous system not only autonomically but also physically since they create sympathetic activation with the effect of gravity and baroreflex mechanisms. Therefore, it is not correct to compare our findings with theirs.

The few studies in the literature and the methodological heterogeneity in these studies suggest the need for further research on autonomic control of the venous system. In recent years, some studies have been designed to investigate the effects of the autonomic nervous system on venous blood parameters. Mitchell et al reported that norepinephrine concentration in the IJV increases with sympathetic nervous system activation [36]. Luft et al showed that venous adrenaline and norepinephrine concentrations increase in healthy individuals with CPT, but only adrenaline concentration increases in diabetic patients as an autonomic dysfunction [37]. However, we do not know whether adrenaline and norepinephrine concentrations in the veins correlate with flow velocity and volumetric flow. Further studies are needed to solve this problem.

Our findings showed that volumetric flow and velocity increased, but the diameter response was somewhat inconsistent. This may be because the sympathetic venopressor mechanism may not be the sole factor determining the venous return. One study has shown that the effect of inspiration and expiration on jugular and vertebral venous return is complex, but cardiac contractility is always the determining factor [38]. Observations indicate that venous return always correlates with cardiac output [39]. Therefore, the increase in heart rate due to sympathetic activation may be an important factor determining volumetric flow in the central veins. However, advanced Doppler USG studies in the main compliance veins would also be valuable in evaluating the sympathetic veno-constrictive response.

This study has some limitations. Methodologically, excessive pressure of the Doppler probe on the vein wall may change the flow parameters [40]. To avoid this effect, care was taken to apply as little pressure as possible to the IJV, and the Doppler probe was not removed from the skin during the experiments. Another limitation is that we could not judge the late effects of sympathetic nervous system activation, such as in the second or third minute. Further studies with different designs of materials and methods are needed to evaluate the later responses. Finally, the effect of expiration and inspiration on venous return was not investigated, and possible effects were not considered. Physiological studies examining the relationship between CPT and respiratory functions are needed to evaluate the extent to which respiratory parameters affect IJV venous flow.

Conclusions

Sympathetic nervous system activation triggered by CPT increases IJV volumetric flow and flow velocity in healthy young individuals. These findings are consistent with the physiological hypothesis that sympathetic nervous system activation affects IJV venous parameters and increases cardiac preload. This study is the first Doppler research on this topic. Furthermore, we have shown that sympathetic nervous system activation causes a venoconstrictive effect in the IJV. Still, since this effect is limited, especially in males, and further studies investigating the venoconstrictive effect on cardiac preload are needed. A Doppler USG study, especially on the splanchnic veins, which are the main compliance veins, will help to answer remaining questions.