Effects Of Head-down Tilt Research Articles

Effects of head-down tilt on optic nerve sheath diameter in healthy subjects.

Intracranial pressure increases in head-down tilt (HDT) body posture. This study evaluated the effect of HDT on the optic nerve sheath diameter (ONSD) in normal subjects. Twenty six healthy adults (age 28 [4.7] years) participated in seated and 6° HDT visits. For each visit, subjects presented at 11:00 h for baseline seated scans and then maintained a seated or 6° HDT posture from 12:00 to 15:00 h. Three horizontal axial and three vertical axial scans were obtained at 11:00, 12:00 and 15:00 h with a 10 MHz ultrasonography probe on the same eye, randomly chosen per subject. At each time point, horizontal and vertical ONSD (mm) were quantified by averaging three measures taken 3 mm behind the globe. In the seated visit, ONSDs were similar across time (p > 0.05), with an overall mean (standard deviation) of 4.71 (0.48) horizontally and 5.08 (0.44) vertically. ONSD was larger vertically than horizontally at each time point (p < 0.001). In the HDT visit, ONSD was significantly enlarged from baseline at 12:00 and 15:00 h (p < 0.001 horizontal and p < 0.05 vertical). Mean (standard error) horizontal ONSD change from baseline was 0.37 (0.07) HDT versus 0.10 (0.05) seated at 12:00 h (p = 0.002) and 0.41 (0.09) HDT versus 0.12 (0.06) seated at 15:00 h (p = 0.002); mean vertical ONSD change was 0.14 (0.07) HDT versus -0.07 (0.04) seated at 12:00 h (p = 0.02) and 0.19 (0.06) HDT versus -0.03 (0.04) seated at 15:00 h (p = 0.01). ONSD change in HDT was similar between 12:00 and 15:00 h (p ≥ 0.30). Changes at 12:00 h correlated with those at 15:00 h for horizontal (r = 0.78, p < 0.001) and vertical ONSD (r = 0.73, p < 0.001). The ONSD increased when body posture transitioned from seated to HDT position without any further change at the end of the 3 h in HDT.

Alteration in facial skin blood flow during acute exposure to -10 and -30° head-down tilt in young human volunteers.

What is the central question of this study? Facial skin blood flow (SBF) might increase during head-down tilt (HDT). However, the effect of HDT on facial SBF remains controversial. In addition, the changes in facial SBF in the cheek (cheek SBF) during a steeper angle of HDT (>-12° HDT) have not been investigated. What is the main finding and its importance? This study showed that cheek SBF decreased during -30° HDT, alongside increased vascular resistance. Furthermore, vascular impedance was suggested to be elevated, accompanied by an increased hydrostatic pressure gradient caused by HDT. Constriction of the facial skin vascular bed and congestion of venous return owing to the steep angle of HDT can decrease facial SBF. Head-down tilt (HDT) has been used to simulate microgravity in ground-based studies and clinical procedures including the Trendelenburg position or in certain surgical operations. Facial skin blood flow (SBF) might be altered by HDT, but the effect of a steeper angle of HDT (>-12° HDT) on facial SBF remains unclear. We examined alterations in facial SBF in the cheek (cheek SBF) using two different angles (-10 and -30°) of HDT and lying horizontal (0°) in a supine position for 10min, to test the hypothesis that cheek SBF would increase with a steeper angle of HDT. Cheek SBF was measured continuously by laser Doppler flowmetry. Cheek skin vascular resistance and the pulsatility index of cheek SBF were calculated to assess the circulatory effects on the facial skin vascular bed in the cheek. Cheek SBF decreased significantly during -30° HDT. In addition, the resistance in cheek SBF increased significantly during -30° HDT. The pulsatility index of cheek SBF increased during both -10 and -30° HDT. Contrary to our hypothesis, cheek SBF decreased during -30° HDT along with increased skin vascular resistance. Vascular impedance, estimated by the pulsatility index in the cheek SBF, was elevated during both -10 and -30° HDT, and elevated vascular impedance would be related to increased hydrostatic pressure induced by HDT. Skin vascular constriction and venous return congestion would be induced by -30° HDT, leading to deceased cheek SBF. The present study suggested that facial SBF in the cheek decreased during acute exposure to a steep angle of HDT (∼-30° HDT).

Ocular Outcomes in Healthy Subjects Undergoing a Short-Term Head-Down Tilt Test

PURPOSE: This study aimed to examine the effect of head-down tilt (HDT) on vascular autoregulation in different age groups and determine its effects on intraocular pressure (IOP) and central corneal thickness (CCT).METHODS: Included were 43 eyes of 23 men. The optic nerve head and parafoveal vascular densities were measured by optical coherence tomography angiography before and after 20 min 10 HDT. Also, the study comprised an examination of the IOP and CCT in a subset of 8 participants (14 eyes) in the sitting position and during 15 min of 10 HDT.RESULTS: Grid-based inside disc all-vessel density (GBID) was statistically significantly lower after the HDT test in subjects under 30 yr (1.26). Whole image and peripapillary capillary vessel density (WICVD, PCVD), and whole image and peripapillary all-vessel density (WIAVD, PAVD) were significantly higher after the HDT test in subjects ages 30-39 yr (1.34, 2.16, 1.05, 1.72, respectively). Inside disc capillary, all-vessel density (IDCVD, IDAVD) and GBID were significantly higher after HDT in subjects over 40 yr (2.48, 2.15, 1.52, respectively). In a subset of eight participants, IOP was significantly higher (3.7 mmHg) and CCT was unchanged after 15 min of HDT.CONCLUSION: Our study showed that simulated microgravity induced optic nerve head vessel density at the inside disc area, especially in persons over 40 years. In addition, IOP was increased by HDT, although no change in CCT was observed.Özelbaykal B, Öğretmenoğlu G, Tunçez I.H. Ocular outcomes in healthy subjects undergoing a short-term head-down tilt test. Aerosp Med Hum Perform. 2021; 92(8):619-626.

Effects of Head-down Tilt on Nerve Conduction in Rhesus Monkeys.

Background:Few studies have focused on peripheral nerve conduction during exposure to microgravity. The −6° head-down tilt (HDT) comprises an experimental model used to simulate the space flight environment. This study investigated nerve conduction characteristics of rhesus monkeys before and after prolonged exposure to HDT.Methods:Six rhesus monkeys (3–4 years old) were tilted backward 6° from the horizontal. Nerve conduction studies (NCSs) were performed on the median, ulnar, tibial, and fibular motor nerves. Analysis of variance with a randomized block design was conducted to compare the differences in the NCS before and 7, 21, and 42 days after the −6° HDT.Results:The proximal amplitude of the CMAP of the median nerve was significantly decreased at 21 and 42 days of HDT compared with the amplitude before HDT (4.38 ± 2.83 vs. 8.40 ± 2.66 mV, F = 4.85, P = 0.013 and 3.30 ± 2.70 vs. 8.40 ± 2.66 mV, F = 5.93, P = 0.004, respectively). The distal amplitude of the CMAP of the median nerve was significantly decreased at 7, 21, and 42 days of HDT compared with the amplitude before HDT (7.28 ± 1.27 vs. 10.25 ± 3.40 mV, F = 4.03, P = 0.039; 5.05 ± 2.01 vs. 10.25 ± 3.40 mV, F = 6.25, P = 0.04; and 3.95 ± 2.79 vs. 10.25 ± 3.40 mV, F = 7.35, P = 0.01; respectively). The proximal amplitude of the CMAP of the tibial nerve was significantly decreased at 42 days of HDT compared with the amplitude before HDT (6.14 ± 1.94 vs. 11.87 ± 3.19 mV, F = 5.02, P = 0.039).Conclusions:This study demonstrates that the compound muscle action potential amplitudes of nerves are decreased under simulated microgravity in rhesus monkeys. Moreover, rhesus monkeys exposed to HDT might be served as an experimental model for the study of NCS under microgravity.

Effect of Head down Tilt on Cardiovascular Autonomic responses in males and females

Aim: The present study was done to evaluate the effect of Head Down Tilt (HDT) on Blood Pressure (BP) in males and females. The parameters which represent the cardiovascular autonomic function involved the Heart Rate (HR), Systolic Blood Pressure (SBP), Diastolic Blood Pressure (DBP), Mean Arterial Pressure (MAP) and Pulse Pressure (PP). Materials and Methods: 100 healthy males and females were selected. There was a limit in the age group from 18-45 years. The effect of HDT on cardiovascular autonomic responses was conducted in them. Parameters like HR and BP were recorded at supine and 300, 600, 800 HDT. In each position the HR was determined by recording the ECG for 5 min in lead II. Niviquire software was used. BP was measured within 20 seconds of the change in tilt position. Observation and Results: On HDT the HR, DBP, MAP, decreased. In contrast the SBP increased on HDT. These results were same in both males and females. Conclusion: This study will help to know the cardiovascular autonomic differences in males and females. By conducting the HDT test it is easy for the medical students and the clinicians to understand and identify the cardiovascular reflex responses in healthy or diseased individuals. Keywords: Autonomic nervous system, Posture, Tilt Table, Head down tilt, Tilt table testing

PTH-317 The effect of trendelenburg positioning during laparoscopic colorectal surgery on intra-ocular pressure (IOP)

Introduction Laparoscopic colorectal surgery is the preferred approach for colorectal resections. It often requires extreme positioning for many hours, including extreme head-down tilts. The incidence of perioperative visual loss after colorectal surgery is quoted as 1.24 per 10,000 in USA 1 and is thought to be due to raised IOP leading to reduced optic nerve head perfusion. 1,2 Aim This study assesses the effect of head-down tilt on IOP during laparoscopic colorectal surgery. Method Patients undergoing laparoscopic colorectal resections are included in this study. It is hypothesised that right-sided resections generally spend less time in the head-down position compared to left-sided and sub-total resections. Therefore, Group 1 included those undergoing right-sided resections and Group 2 included left-sided and sub-total resections. Baseline eye examination and IOP measurements using a Tonopen® XL applanation tonometer were carried out every hour during surgery and each time when the operating table was tilted. Results There were 30 patients (16 males, 14 females) in the study with an average age of 63 years (Range 25–78); with Group 1’s mean age 63.5 years (SD 14.35) and Group 2 had a mean age of 62.3 years (SD 17.68), (p > 0.05). Group 1 had 14 patients and Group 2 had 16 patients. The mean length of surgery (LOS) for Group 1 was 2 hrs. 27 mins (SD 0.73) and for Group 2 mean LOS was 4 hrs. 7 mins (SD 1.63) with P = 0.001. The baseline IOP was similar in both groups with the mean baseline IOP in Group 1 being 16.69mmHg, and in Groups 2 it was 16.87 mmHg (p > 0.05). The median IOP rise from baseline during surgery was 10.5 mmHg (IQR = 4.5) in Group 1 and 18 mmHg (IQR = 4.4) in Group 2 (p ° (IQR = 7.2) and Group 2 was 18.3° (IQR = 8.4), (p Conclusion A rise in IOP occurs during laparoscopic colorectal surgery and it appears to be more pronounced in those with a greater degree of head down tilt for a prolonged time. This may have important implications for those patients undergoing prolonged surgery and for those with a history of glaucoma. Disclosure of interest None Declared. References Molloy BL. Implications for postoperative visual loss: steep Trendelenburg position and effects on intraocular pressure. AANA J. 2011;79(2):115–121 Awad H, Santilli S, Ohr M, Roth A, Yan W, Fernandez S, Roth S, Patel V. The effects of steep trendelenburg positioning on intraocular pressure during robotic radical prostatectomy. Anesth Analg. 2009;109(2):473–8

Effects of a head-down tilt on cerebral blood flow in mice during bilateral common carotid artery occlusion

Objectives:Data on the optimal head position for patients with acute ischemic stroke are unavailable. We evaluated the effects of mild head-down tilt (HDT) on cerebral blood flow (CBF) in mice during bilateral common carotid artery occlusion (BCCAO).Materials and Methods:We used mice with BCCAO (35 minutes) and divided these into 2 groups ( n=16): BCCAO at 0°-HDT and 5°-HDT. CBF was measured for both hemispheres with a non-invasive laser Doppler blood perfusion imager. Changes in CBF during BCCAO were examined in both groups.Results:A significantly greater increase in CBF in both hemispheres was observed in 5°-HDT mice than in 0°-HDT mice (126.1% (8.715)% vs. 102.1% (4.718)%; P=0.0294).Conclusion:HDT enhanced the increase in CBF in both hemispheres in the mouse BCCAO model. The potential mechanism underlying CBF increase enhanced by HDT during BCCAO warrants further investigation.

Effect of head-down tilt on respiratory responses and human inspiratory muscle activity

The effect of head-down tilt on respiration and diaphragmal and parasternal muscles activity was investigated in 11 healthy subjects. The short-time (30 min) head-down tilt posture (-30 degrees relatively horizont) increased the inspiratory time (P < 0.05), decreased breathing frequency (P < 0.05), inspiratory and expiratory flow rate (P < 0.05) and increased the airway resistance (P < 0.05) compared with values in vertical posture. There were no significant changes in tidal volume and minute ventilation. Constant values of tidal volume and minute ventilation during head-down tilt were provided by increasing in EMG activity of parasternal muscles more then twice. It was established that the contribution of chest wall inspiratory muscles increased while the diaphragm's contribution decreased during head-down spontaneous breathing. Maximal inspiratory effort (Muller's maneuver) during head-down tilt evoked the opposite EMG-activity pattern: the contribution of inspiratory thoracic muscles was decreased and diaphragm's EMG-activity was increased compared with vertical posture. These results suggest that coordinate modulations in inspiratory muscles activity allows to preserve the functional possibility of human inspiratory muscles during short-time head-down tilt.

Diastolic function: the influence of pneumoperitoneum and Trendelenburg positioning during laparoscopic hysterectomy

Several reports concerning the haemodynamic changes during gynaecologic laparoscopy have been published so far, and the effects of head-down tilt and pneumoperitoneum have not been clearly separated. However, its main effect seems to be an increase in systemic vascular resistance. We investigated how the augmented afterload can affect diastolic function. : Our study involved 20 healthy women, classified as having ASA status I: 10 undergoing laparoscopic hysterectomy and 10 undergoing conventional open hysterectomy. Measurements were made in awake patients and after induction of anaesthesia and then repeated after carbon dioxide insufflation and head-down positioning and at the end of surgery. Diastolic function was primarily studied by transthoracic echocardiography. We observed that pneumoperitoneum caused a significant reduction in stroke volume, cardiac output and left ventricular end-diastolic volume; the diastolic filling times showed a progressive reduction in the E-velocity (the velocity of early mitral inflow, corresponding to the ventricular passive filling phase, measured by pulsed-wave Doppler), a prolonged deceleration time and an augmented isovolumetric relaxation time. After head-down tilting, stroke volume, cardiac output and left ventricular end-diastolic volume increased in both laparoscopic hysterectomy and conventional open hysterectomy groups. We have found that pneumoperitoneum has important effects on left ventricular volumes, causing a drop in left ventricular end-diastolic volume; it also affects diastolic function with a delay in deceleration time and isovolumetric relaxation time without any effects on intracavitary pressures.

Effects of head-down tilt on the intracranial pressure in conscious rabbits

Head-down tilt (HDT) causes a fluid shift towards the upper body, which increases intracranial pressure (ICP). In the present study, the time course of ICP changes during prolonged exposure to HDT was investigated in conscious rabbits through a catheter chronically implanted into the subarachnoid space. The production of cerebrospinal fluid (CSF) after exposure to 7-days HDT was also examined by a ventriculo-cisternal perfusion method. The ICP increased from 4.3±0.4 (mean±S.E.M.) mmHg to 8.0±0.8 mmHg immediately after the onset of 45° HDT, reached a peak value of 15.8±1.9 mmHg at 11 h, and then decreased to 10.4±1.1 mmHg at 24 h. During 7-days HDT, it also increased from 4.8±0.9 mmHg to 9.2±1.6 mmHg immediately after the onset of 45° HDT, reached a peak value of 12.8±2.5 mmHg at 12 h of HDT, and then decreased gradually towards the pre-HDT baseline value for 7 days. The rate of CSF production was 10.1±0.6 μl/min in rabbits exposed to 7-days HDT, and 9.7±0.5 μl/min in control rabbits. These results suggest that the rabbits begin to adapt to HDT within a few days and that the production of CSF is preserved after exposure to 7-days HDT. The time course of ICP changes during HDT in conscious rabbits seems to be considerably different from that in anesthetized rabbits.

Passive leg elevation and head-down tilt: effects and duration of changes.

I am intrigued by the recent further evaluation of passive leg elevation (PLE) in the perioperative period for patients undergoing cardiac surgery [1]. Following prior involvement with somewhat similar evaluations [2], I am left wondering what conclusions may have been reached if the experimental protocol had been expanded to include a penultimate assessment with the legs still elevated (i.e. between time point 3 and time point 4), and also to examine the effects of head-down tilt (HDT) as used during central venous cannulation. On the one hand, the autonomic [2] and haemodynamic effects [3-5] of postural manipulation have been shown to be both minimal and short lived. I surmise that the adverse effects observed may have been self-correcting during the course of PLE, rather than only after resumption of the supine position (i.e. before time point 4). This perhaps thereby minimises the clinical importance of these effects. On the other hand, the patterns of changes seen with HDT are very similar to those induced by PLE [2,3]. It follows that any caution advised regarding PLE for patients known to have reduced right ventricular ejection fraction should be extrapolated to the use of HDT for central venous catheter placement. The data provided could justifiably be added to a list of reasons for avoiding the indiscriminate use of PLE as a therapeutic manoeuvre in hypotensive conditions. As for other applications, it is difficult to think of an immediate alternative to PLE for the preparation of the sterile field required for a coronary artery bypass vein graft requiring use of harvested saphenous veins (or to HDT in central venous catheter placement). The advised caution deserves due consideration for both PLE and HDT.

Effects of Head-Down Tilt on Cerebral Blood Flow and Somatosensory-Evoked Potentials in Rabbits.

Changes in cerebral blood flow (CBF) and somatosensory-evoked potentials (SEPs) were studied in rabbits exposed to head-down tilt (HDT) at 45 degrees and 75 degrees. The animals were anesthetized with alpha chloralose and the lungs were artificially ventilated. CBF was continuously measured by laser Doppler flowmetry (LDF), and SEPs were recorded as responses of the cortex to median nerve stimulation. In the 45 degrees HDT rabbits, CBF did not change significantly in the parietal cortex during 1 h of HDT. In contrast, in the 75 degrees HDT rabbits, CBF did not change significantly within 5 min after the onset of HDT, but decreased gradually to 79% of the pre-HDT baseline value at the end of 1 h of HDT. The latency and amplitude of SEPs did not change significantly throughout the experiment in any group. These results suggest that CBF and SEPs do not change significantly during 1 h of 45 degrees HDT and that 75 degrees HDT disturbs the regulation of the cerebral circulation but does not affect cortical somatosensory response, at least for 1 h.

Contractile responses of the basilar artery isolated from rabbits exposed to 8-day head-down tilt.

In the present study, the effects of head-down tilt (HDT) on contractile responses of basilar and popliteal arteries were investigated in vitro. The arteries were isolated from rabbits exposed to 8 d of 45 degrees HDT. Isometric tension and intracellular calcium were measured in an organ bath perfused with physiological salt solution. In the HDT rabbits, contractile response to norepinephrine (NE) was attenuated in the basilar artery, but not in the popliteal artery compared with control rabbits. HDT did not change the responses to either KCl or 5-hydroxytryptamine in both arteries. In the response of basilar artery to NE, the difference in both [Ca(2+)](i) transient and Ca sensitivity between control and HDT rabbits were statistically not significant. The response of the basilar artery to phenylephrine, a selective alpha(1)-adrenergic agonist, was also attenuated by HDT. Treatment with propranolol, a beta-adrenergic blockade, did not affect the response to NE in the basilar arteries isolated from control rabbits, but it significantly enhanced the response in the basilar arteries from HDT rabbits. These results suggest that exposure to 8-d HDT decreases a contractile response of the basilar artery to NE in rabbits. The reduction of NE-induced contraction is probably attributable to both decreased Ca transient and decreased Ca sensitivity, and decreased alpha(1)-adrenoceptor activity and increased beta-adrenoceptor activity seem to be involved in the mechanism.

Dynamic change of ERPs related to selective attention to signals from left and right visual field during head-down tilt

To study further the effect of head-down tilt(HDT) on slow positive potential in the event-related potentials(ERPs), the temporal and spatial features of visual ERPs changes during 2 hour HDT(−10 °) were compared with that during HUT(+20°) in 15 normal subjects. The stimuli were consisted of two color LED flashes appeared randomly in left or right visual field(LVF or RVF) with same probability. The subjects were asked to make switch response to target signals(T) differentially: switching to left for T in LVF and to right for T in RVF, ignoring non-target signals(N). Five sets of tests were made during HUT and HDT. ERPs were obtained from 9 locations on scalp. The mean value of the ERPs in the period from 0.32–0.55 s was taken as the amplitude of slow positive potential(P400). The main results were as follows. 1)The mean amplitude of P400 decreased during HDT which was more significant at the 2nd, 3rd and 5th set of tests; 2)spatially, the reduction of mean P400 amplitude during HDT was more significant for signals from RVF and was more significant at posterior and central brain regions than that on frontal locations. As that the positive potential probably reflects the active inhibition activity in the brain during attention process, these data provide further evidence showing that the higher brain function was affected by the simulated weightlessness and that this effect was not only transient but also with interesting spatial characteristics.

The effect of head-down tilt on brain potentials related to visual attention

To study the possible effect of simulated weightlessness on brain function state, the brain event-related potentials (ERPs) in a simple visual selective response task were compared between HDT and HUT in 9 normal subjects. The results were: The Target(T) and non-Target(NT) flash signals both induced significant slow positive potentials which were supposed to related to the attention state; the amplitude of the positive potentials in frontal regions decreased significantly especially for NT-ERPs during HDT comprared with that during HUT. The data reported provide new evidence indicating that more attention should be paid on the brain function study during space flight.

Effects of head-down tilt on atrial natriuretic peptide and the renin system in pregnancy.

We studied the effects of head-down tilt to 10 degrees for 30 minutes on plasma atrial natriuretic peptide and the renin-aldosterone system in 8 preeclamptic pregnant women, 8 healthy pregnant women, and 11 nonpregnant women of fertile age. Mean arterial blood pressure did not change in the pregnant groups but increased significantly in the nonpregnant control subjects. Heart rate decreased significantly in preeclamptic women but remained unchanged in both control groups. Baseline atrial natriuretic peptide concentration was significantly higher in both preeclamptic (66 +/- 4 pmol/L) and pregnant (54 +/- 6 pmol/L) control subjects compared with nonpregnant subjects (40 +/- 2 pmol/L), but the difference between the pregnant groups was not significant. Head-down tilting induced a significant increase in atrial natriuretic peptide only in healthy pregnant women. Baseline plasma renin activity and aldosterone concentrations were significantly higher in pregnant control subjects compared with both the preeclamptic and nonpregnant groups. The differences between the preeclamptic and nonpregnant control groups were nonsignificant. After head-down tilting, plasma renin activity decreased significantly only in nonpregnant control subjects, whereas aldosterone decreased significantly in preeclamptic and nonpregnant control subjects. In preeclampsia, atrial natriuretic peptide release followed blood pressure and not changes in cardiac output. When all 27 women were studied, a correlation between atrial natriuretic peptide and mean arterial pressure was found in the left lateral supine position. The results suggest that pregnant women developing preeclampsia lose their usual hemodynamic control and show reactions resembling the nonpregnant state when subjected to head-down tilt.

Effect of Head-Down Tilt on Cardiovascular Control in Healthy Subjects: A Spectral Analytic Approach

1. The purpose of this investigation was to determine non-invasively the changes in autonomic cardiovascular control observed in normal subjects submitted to acute cardiopulmonary blood volume expansion by 100 degrees head-down tilt. The effect of head-down tilt on finger blood pressure and heart rate fluctuations was studied by means of power spectral analysis in 12 healthy men. 2. Amplitude spectra of heart rate and blood pressure rhythmicity were estimated at the low-frequency (60-140 mHz, 10-s rhythm) and high-frequency (area under the curve at mean respiration rate +/- 50 mHz) component. Transfer gain and phase were calculated between systolic blood pressure and heart rate. Forearm vascular resistance was estimated to validate the head-down procedure. 3. Forearm vascular resistance decreased significantly from 19.82 (16.34-26.46) mmHg ml-1 min 100 ml to 18.05 (13.69-22.88) mmHg ml-1 min 100 ml (P < 0.01) during head-down tilt (values are medians and 25 and 75 percentiles). The overall variability (total area under the curve of the spectrum from 20 to 500 mHz) of blood pressure and heart rate time series was consistently reduced with head-down tilt. 4. The spectral pattern of systolic blood pressure showed a diminution of the absolute and relative low-frequency component during head-down tilt: absolute log-transformed values, 2.86 (2.80-2.94) mmHg/Hz1/2 versus 2.77 (2.72-2.82) mmHg/Hz1/2 (P < 0.05); relative values, 35% (32-37%) versus 32% (29-32%) (P < 0.05). In heart rate spectra only the absolute low-frequency component decreased.(ABSTRACT TRUNCATED AT 250 WORDS)

The Effect of Head-down Tilt on Arterial Blood Pressure After Spinal Anesthesia

We examined the usefulness of 10 degrees head-down tilt for hypotension after spinal block. Two different investigations were performed, one employing head-down tilt after arterial blood pressure had decreased (n = 40), and the other using a prophylactic tilt (n = 50). When the head-down tilt was applied to treat hypotension after spinal block (n = 40), arterial blood pressure increased only in patients whose systemic blood pressure decreased more than 30% from the control (severe hypotension group, n = 11). However, even in the severe hypotension group, systolic blood pressure did not increase in two patients after 10 degrees head-down tilt. When the head-down tilt was performed immediately after spinal block (n = 24), the changes in systolic blood pressure were the same as in the horizontal group (n = 26). The cephalad spread of analgesia at 20 min after spinal block was higher, however, in the head-down tilt group (T3.8 +/- 1.6) than the horizontal group (T5.2 +/- 1.9). From these results we conclude that head-down tilt for hypotension after spinal block increases arterial blood pressure only for severe hypotension, and that prophylactic head-down tilt has no effect in maintaining blood pressure.

The effect of head-down tilt on internal jugular vein diameter and cerebral function

The effect of head-down tilt on internal jugular vein diameter and cerebral function

Effect of head-down tilt on basal plasma norepinephrine and renin activity in humans.

The effects of loading cardiopulmonary baroreceptors on basal norepinephrine and renin activity were studied in six normal subjects. Loading of cardiopulmonary baroreceptors was accomplished by a 60-min 30 degrees head-down tilt with small supplemental saline infusions. Central venous pressure was measured continuously by intrathoracic catheter; arterial pressure was measured indirectly by cuff. During the tilt, central venous pressure increased from 5.1 +/- 1.3 to 8.9 +/- 1.7 mmHg (P less than 0.001), whereas arterial pressure was unchanged. Plasma norepinephrine (185 +/- 85 pg/ml) and plasma renin activity (3.9 +/- 5.7 ng . ml-1 . h-1) did not change. Moderate sustained loading of cardiopulmonary baroreceptors is therefore without effect on unstressed plasma norepinephrine and renin activity in normal humans, suggesting that the tonic inhibitory effects of these receptors on these neurohumoral control systems are not readily increased in the basal state.