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Corresponding author at: Department of Occupational Therapy, School of Nursing and Rehabilitation Sciences at Odawara, International University of Health and Welfare, 1-2-25 Shiroyama, Odawara, Kanagawa 250-8588, Japan. Tel.: +81 465 21 6626; fax: +81 465 21 6744.
We analyzed how three trunk postures in Fowler's position affected hemodynamics.
We measured heart rate and circulatory volume in ten healthy male volunteers.
An upright upper trunk had a higher stroke volume than an upright whole trunk.
An upright upper trunk had a lower heart rate than an upright whole trunk.
Slight differences in trunk posture might affect hemodynamics in Fowler's position.
We speculated that stroke volume would be higher and heart rate would be lower when the head and upper trunk were mainly upright in the Fowler's position. We therefore analyzed the effects of three trunk postures in Fowler's position on heart rate, blood pressure and circulatory volume.
Heart rate (HR), blood pressure (BP), stroke volume (SV), cardiac output (Q), systemic vascular resistance (SVR), ejection time (ET) and pre-ejection period (PEP) were measured in 10 healthy male volunteers (mean age ± SEM, 20.7 ± 0.5 y; range, 19–23 y) while in three trunk postures in Fowler's position. Stroke volume and Q were measured using impedance cardiography. The three trunk postures were 30° of lower and upper trunk inclination (WT30°), 30° and 60° of lower and upper trunk inclination (UT 60°), respectively and 60° of upper and lower trunk inclination (WT60°).
Both SV and ET were significantly higher and HR and PEP were lower at UT60° than at WT60° (p < 0.01) whereas these values did not significantly differ between WT30° and UT60° (p > 0.05). None of Q, SVR and BP significantly differed among the three conditions (p > 0.05).
These findings suggested that SV and preload are higher when the upper trunk is upright (UT60°) than when the entire trunk is upright (WT60°) while in Fowler's position. In addition, Q might be maintained without increasing HR through vagal withdrawal when only the upper trunk is upright in healthy young males in Fowler's position.
Studies of cardiovascular regulation in humans with the head tilted up (HUT) and down (HDT) have found that posture influences hemodynamics and the autonomic nervous system. Standing or HUT decreases ventricular filling pressure and stroke volume (SV) according to a shift in the distribution of blood volume to the lower extremities. Consequently, sympathetic nerve activity is activated, heart rate (HR) increases and vascular contraction maintains circulatory volume. Conversely, SV increases, sympathetic nerve activity and HR decrease and vagal nerve activity increases in the supine, compared with an upright position (
Fowler's position or the semi-seated position is often clinically applied as well as standing and supine positions. Fowler's position is achieved by inclining the backrest of a bed upwards from the supine position with flexed or straight knees unlike HUT and HDT which the whole body is inclined (
). The upright head and trunk in Fowler's position are more essential for the quality of life of patients who are confined to bed or frail and it is clinically applied most frequently at inclinations between 30° and 60° (
). Patients who are confined to bed or frail are frequently placed in Fowler's position instead of remaining supine to assist ambulation, monitor hemodynamics and facilitate breathing as well as routine activities such as eating or conversation (
). Thus, to understand the most effective posture required to counteract the downward fluid shift while in Fowler's position should be clinically meaningful. Some studies have described a relationship between the angle of Fowler's position and the accuracy of hemodynamic measurements among patients in intensive care units (
reported that cardiac output is decreased in patients under intensive care who are in the Fowler's position, compared with those who are supine. A study of young healthy individuals has shown that blood pressure in Fowler's position is intermediate between the seated and supine positions (
). However, the most appropriate posture required to counteract the downward fluid shift while in Fowler's position has not been investigated as far as we can ascertain.
We speculated that stroke volume would be higher and heart rate would be lower when the head and upper trunk are mainly upright compared with when the head and the whole trunk are upright in Fowler's position, because most body segment positions remain lower when the head and upper trunk are upright. We tested this hypothesis in a fundamental study of the effects of three trunk postures in Fowler's position on heart rate, blood pressure and circulatory volume in healthy young men.
2.1 Study participants
We assessed hemodynamics in 10 male university students (mean age ± SEM, 20.7 ± 0.5 y; range, 19–23 y; weight, 56.7 ± 1.2 kg; height, 170.6 ± 1.5 cm). All were free of chronic or acute cardiovascular, respiratory or other chronic diseases. Beverages containing caffeine or alcohol were not consumed for 24 h before starting the study. All participants refrained from eating and drinking after 2200 h on the evening before the experiments that started in the morning or consumed a light breakfast before experiments that proceeded in the afternoon. All participants were clothed only in shorts.
All experiments were implemented between 1100 and 1400 h. The Ethics Commission of the International University of Health and Welfare approved the study and all recruits provided voluntary written consent to participate after being fully informed about the procedure, risks and protocol.
The participants rested in a thermoneutral room at 28 °C for 15 min and were then prepared for electrocardiography (ECG), continuous measurements of arterial blood pressure and impedance cardiography (ICG). After 5 min resting, data were recorded for 5 min under each condition in all experiments.
The participants were placed in Fowler's positions on a bed at 30° of whole trunk inclination (WT30°), 30° of lower trunk inclination and 60° of upper trunk inclination (UT60°) and 60° of whole trunk inclination (WT60°) (Fig. 1). The upper and lower segments at UT60° were defined by the spinous process of the 10th thoracic vertebra. The height of the bottom of the upper and lower trunk was adjusted according to individual trunk size. All seated positions allowed slight hip and knee joint flexion. All analyses proceeded randomly and were repeated three times in all positions on the same day.
Continuous arterial blood pressure was measured at the radial artery by tonometry using a noninvasive arterial blood pressure monitor at the level of the heart. Continuous arterial blood pressure was calibrated using oscillometric sphygmomanometry to measure intermittent cuff blood pressure. Eight Vitrode M spot electrodes (Nihon Kohden, Tokyo, Japan) were attached to the neck and lower thorax of each participant for ICG based on a previous study (
). Data from an ECG 100C electrocardiographic lead II (BIOPAC Systems, Goleta, CA, USA) and from impedance cardiography using a NICO 100C instrument (BIOPAC) as well as continuous arterial blood pressure measured using a BP-608EV (Omron Colin, Tokyo, Japan) were recorded on a personal computer using the MP150 data acquisition system (BIOPAC) at a sampling rate of 1 kHz throughout all experiments.
2.4 Data analysis
Heart rate was determined from ECG data in all experiments. Systolic (SBP) and diastolic (DBP) blood pressure was determined from continuous arterial blood pressure. Mean blood pressure (MBP) was calculated as [1/3(SBP − DBP) + DBP)].
We assessed SV, stroke index (SI), cardiac output (Q), cardiac index (CI) and systemic vascular resistance (SVR) using ICG that calculates SV based on thoracic bioimpedance as described (
). Q was calculated from product result of SV and HR. Both SI and CI were normalized by SV and Q for the body surface area of each participant (SI = SV/body surface area; CI = Q/body surface area) and SVR was calculated as 80 MBP/Q. Five-minute means of all values were calculated.
We determined the pre-ejection period (PEP) and left ventricular ejection time (ET) from the ECG findings and the derivative signal of ICG for systolic interval analysis (
). The PEP was determined as the interval between the Q waves on ECG to point B which is associated with opening of the aortic valve at the first derivative thoracic bioimpedance. Ejection time was determined as the time from point B to point X which is associated with closure of the aortic valve at the first derivative thoracic bioimpedance.
2.5 Statistical analysis
We determined the effects of three postures in Fowler's position upon physiological variables using a repeated measures multivariate analysis of variance (MANOVA) with Pillai's trace statistic. Differences between the two conditions were evaluated using a paired t-test with Bonferroni correction when a main group effect was significant. Statistical significance was established at p < 0.05. All values are expressed as means ± SEM. Data were statistically analyzed using R for Windows version 2.13 (www.r-project.org) and the car 2.0-11 package for MANOVA.
Table 1 shows the results of all experiments. Although HR and SV were equal at WT30° and UT60°, HR was lower at WT30° and UT60° than at WT60° and SV was higher at WT30° and UT60° than at WT60°. Cardiac output was equal under all tested conditions. Significant main effects were found in HR, SV, SI, PEP and ET (HR, F (2,8) = 197.0, p < 0.01; SV, F (2,8) = 94.0, p < 0.01; SI, F (2,8) = 86.5, p < 0.01; PEP, F (2,8) = 13.4, p < 0.01 ET, F (2,8) = 85.1, p < 0.01 MANOVA) at all three angles. Multiple comparisons showed that HR was significantly lower at WT30° and UT60° than at WT60°. All of SV, SI and ET were significantly higher at WT30° and UT60° than at WT60° (p < 0.01) and PEP was significantly lower at UT60° than at WT60° (p < 0.01). None of HR, SV, SI or ET significantly differed between WT30° and UT60° (p > 0.05) and PEP did not significantly differ between WT30° and UT60° or WT60° (p > 0.05).
Table 1Hemodynamic values associated with all conditions.
The effects of Q, CI, SBP, MBP, DBP and SVR did not significantly differ (Q, F (2,8) = 0.45, p > 0.05; CI, F (2,8) = 0.42, p > 0.05; SBP, F (2,8) = 0.77, p > 0.05; MBP, F (2,8) = 0.41, p > 0.05; DBP, F (2,8) = 0.35, p > 0.05; SVR, F (2,10) = 0.52, p > 0.05; all MANOVA) among the three angles.
The results of multiple comparisons of conditions between two angles that significantly differed for each physiological variable in each individual tended to be the same.
Heart rate and PEP were lower and SV, SI and ET were higher at UT60° than at WT60° (Table 1) and HR, SV, SI, PEP and ET were equivalent at UT60° and WT30°. These findings suggested that UT60° did not increase HR or decrease SV and preload compared with WT60° and that hemodynamics and left ventricular function were equal at UT60° and WT30° in young healthy adult males.
The distribution of blood volume generally shifts to the lower extremities according to orthostatic stress and stroke volume decreases as described above. Thus, HR increases through autonomic regulation to maintain circulatory volume (
). The present study found that SV and SI were lower at WT60° than at UT60° and WT30°, meaning that the effect of gravity was larger at WT60° than at UT60° and WT30° because the lower trunk at UT60° was not upright, whereas the entire trunk was upright WT60°. Heart rate was faster at WT60° than at UT60° and WT30°, indicating that HR adjusted the circulatory volume through autonomic regulation. The HR was <100 bpm at all three angles, and was probably caused by vagal withdrawal (
Cardiac output at WT60° did not significantly decrease compared with the other two positions. Cardiac output decreases while upright compared with the supine position in the same manner as SV in healthy persons even if HR increases. Dynamic changes between supine and upright positions such as standing or HUT have been investigated in detail (
). However, we evaluated hemodynamics at slight angular differences of the trunk in Fowler's position. The difference in SV among three trunk angles was about 10 mL (between WT30° or UT60° and WT60°) which was smaller than that previously reported. One study found a mean difference in SV of about 30 mL between the supine position and 60° HUT in healthy male and female individuals (
). Furthermore, Q was maintained since the decreases in SV compensated for the increase in HR at WT60°.
At UT60° the upper trunk was upright from WT30° although the inclination of the lower trunk was the same. Values for HR, SV, SI, Q and CI did not significantly differ between WT30° and UT60°. Our previous preliminary findings suggested that SV and Q do not differ between WT30° and UT60° under fixed breathing in five healthy individuals (
) and the present results supported our findings. At UT60°, the upper trunk was raised to center at the bottom of the dorsal thorax from WT30°. Since the heart is located close to the rotational center of the bottom of the dorsal thorax, even if the inclination angle of the upper trunk increases, the level of heart at UT60° hardly changed compared with WT30°. Therefore, the effect of gravity on circulatory volume did not essentially differ between the WT30° and UT60° angles of the upper trunk and the blood volume distribution at UT60° probably did not clearly differ from that at WT30°.
Thus, blood volume probably did not shift to the lower body and HR did not increase through vagal withdrawal when only the upper trunk, rather than the entire trunk, was raised in Fowler's position.
4.2 Systolic time intervals
Blood volume shifts from the heart to the lower extremities and venous return and ventricular filling pressure decrease; hence, preload and stroke volume diminish in the upright position (
). Here, PEP was significantly shorter at UT60° than at WT60° and ET was significantly longer at UT60° and WT30° than at WT60°. The prolonged PEP, shortened ET and reduced SV at WT60° would reflect reduced preload compared with UT60° and WT30° because orthostatic stress might be higher at WT60° than at UT60° and WT30° (WT60° being an upright upper and lower trunk).
Although the upper trunk was upright at UT60° compared with that at WT30°, PEP and ET did not significantly differ between the two positions. The upright upper trunk angle of UT60° might be insufficient to affect blood volume compared with WT30° because the level of heart at UT60° and WT30° was essentially the same and gravity did not differentially affect circulatory volume between the two positions; that is, ventricular filling probably did not differ, and PEP and ET in fact did not significantly differ between WT30° and UT60°.
These findings suggested that preload is higher at UT60° than at WT60° and similar at WT30°.
All of our participants were young and healthy males. Fowler's position is used for patients of all ages, and we intend to analyze the effects of trunk angles in Fowler's position among elderly persons, patients who are confined to bed or frail and those with cardiovascular disease. Considering that Fowler's position is clinically applied, we also plan to compare hemodynamics between Fowler's and supine positions. We will also investigate combinations of various angles of upper trunk inclination to determine the optimal position for patients in the clinical environment.
4.4 Clinical implications
We studied the effects of slight differences in trunk posture during Fowler's position on hemodynamics and left ventricular function. Although trunk posture is defined without dividing the upper and lower segments in Fowler's position during clinical practice, we found that some physiological values significantly differed between the upright upper and upright whole trunk positions. That is, our findings suggested that trunk posture should be subdivided into upper and lower segments. Ventricular filling, SV and Q were easier to maintain without increases in HR via vagal withdrawal when only the upper trunk was upright (UT60°) compared with the whole trunk (WT60°) while in Fowler's position. Hence, an upright upper trunk in Fowler's position might help to maintain circulatory volume and allow patients to raise their head and neck area to participate in routine activities and conversation. In addition, controlling upper trunk posture is important when evaluating the effects of clinical or experimental therapies using hemodynamic measurements.
We found that slight differences in trunk posture while in Fowler's position affect hemodynamics. An upright upper trunk results in higher stroke volume and preload and a lower heart rate compared with an upright whole trunk in Fowler's position.
We thank Sumiko Yamamoto for the critical and valuable advice. This study was supported by JSPS KAKENHI Grant Number 22792159 . The experiments complied with the current laws of the country in which they were performed.
None of the authors have any commercial or financial involvements in connection with this study that represent or appear to represent any conflicts of interest.
Stroke volume equation for impedance cardiography.