Novel method for intraoperative assessment of cerebral autoregulation by paced breathing

Background

Cerebral autoregulation (CA) is the mechanism that maintains constancy of cerebral blood flow (CBF) despite variations in blood pressure (BP). Patients with attenuated CA have been shown to have an increased incidence of peri-operative stroke. Studies of CA in anaesthetized subjects are rare, because a simple and non-invasive method to quantify the integrity of CA is not available. In this study, we set out to improve non-invasive quantification of CA during surgery. For this purpose, we introduce a novel method to amplify spontaneous BP fluctuations during surgery by imposing mechanical positive pressure ventilation at three different frequencies and quantify CA from the resulting BP oscillations.

Methods

Fourteen patients undergoing sevoflurane anaesthesia were included in the study. Continuous non-invasive BP and transcranial Doppler-derived CBF velocity (CBF V ) were obtained before surgery during 3 min of paced breathing at 6, 10, and 15 bpm and during surgery from mechanical positive pressure ventilation at identical frequencies. Data were analysed using frequency domain analysis to obtain CBF V -to-BP phase lead as a continuous measure of CA efficacy. Group averages were calculated. Values are means ( sd ), and P <0.05 was used to indicate statistical significance.

Results

Preoperative vs intraoperative CBF V -to-BP phase lead was 43 (9) vs 45 (8)°, 25 (8) vs 24 (10)°, and 4 (6) vs -2 (12)° during 6, 10, and 15 bpm, respectively (all P =NS).

Conclusions

During surgery, cerebral autoregulation indices were similar to values determined before surgery. This indicates that CA can be quantified reliably and non-invasively using this novel method and confirms earlier evidence that CA is unaffected by sevoflurane anaesthesia.

Clinical trial registration

NCT03071432.

Sevoflurane based anaesthesia does not affect already impaired cerebral autoregulation in patients with type 2 diabetes mellitus

BACKGROUND

The baroreflex regulates arterial blood pressure (BP). During periods when blood pressure changes, cerebral blood flow (CBF) is kept constant by cerebral autoregulation (CA). In patients with diabetes mellitus (DM), low baroreflex sensitivity (BRS) is associated with impaired CA. As sevoflurane-based anaesthesia obliterates BRS, we hypothesised that this could aggravate the already impaired CA in patients with DM resulting in a 'double-hit' on cerebral perfusion leading to increased fluctuations in blood pressure and cerebral perfusion.

METHODS

On the day before surgery, we measured CBF velocity (CBFV), heart rate, and BP to determine BRS and CA efficacy (CBFV_mean-to-BP_mean-phase lead) in 25 patients with DM and in 14 controls. During the operation, BRS and CA efficacy were determined during sevoflurane-based anaesthesia. Patients with DM were divided into a group with high BRS (DM_BRS↑) and a group with low BRS (DM_BRS↓). Values presented are median (inter-quartile range).

RESULTS

Preoperative vs intraoperative BRS was 6.2 (4.5-8.5) vs 1.9 (1.1-2.5, P<0.001) ms mm Hg^-1 for controls, 5.8 (4.9-7.6) vs 2.7 (1.5-3.9, P<0.001) ms mm Hg^-1 for patients with DM_BRS↑, and 1.9 (1.5-2.8) vs 1.1 (0.6-2.5, P=0.31) ms mm Hg^-1 for patients with DM_BRS↓. Preoperative vs intraoperative CA efficacy was 43° (38-46) vs 43° (38-51, P=0.30), 44° (36-49) vs 41° (32-49, P=0.52), and 34° (28-40) vs 30° (27-38, P=0.64) for controls, DM_BRS↑, and DM_BRS↓ patients, respectively.

CONCLUSIONS

In diabetic patients with low preoperative BRS, preoperative CA efficacy was also impaired. In controls and diabetic patients, CA was unaffected by sevoflurane-based anaesthesia. We therefore conclude that sevoflurane-based anaesthesia does not contribute to a 'double-hit' phenomenon on cerebral perfusion.

CLINICAL TRIAL REGISTRATION

NCT 03071432.

Cerebral autoregulatory performance and the cerebrovascular response to head-of-bed positioning in acute ischaemic stroke

Background and purpose

Cerebrovascular responses to head‐of‐bed positioning in patients with acute ischaemic stroke are heterogeneous, questioning the applicability of general recommendations on head positioning. Cerebral autoregulation is impaired to various extents after acute stroke, although it is unknown whether this affects cerebral perfusion during posture change. We aimed to elucidate whether the cerebrovascular response to head position manipulation depends on autoregulatory performance in patients with ischaemic stroke.

Methods

The responses of bilateral transcranial Doppler ultrasound‐determined cerebral blood flow velocity (CBFV) and local cerebral blood volume (CBV), assessed by near‐infrared spectroscopy of total hemoglobin tissue concentration ([total Hb]), to head‐of‐bed lowering from 30° to 0° were determined in 39 patients with acute ischaemic stroke and 17 reference subjects from two centers. Cerebrovascular autoregulatory performance was expressed as the phase difference of the arterial pressure‐to‐CBFV transfer function.

Results

Following head‐of‐bed lowering, CBV increased in the reference subjects only ([total Hb]: + 2.1 ± 2.0 vs. + 0.4 ± 2.6 μM; P < 0.05), whereas CBFV did not change in either group. CBV increased upon head‐of‐bed lowering in the hemispheres of patients with autoregulatory performance <50th percentile compared with a decrease in the hemispheres of patients with better autoregulatory performance ([total Hb]: +1.0 ± 1.3 vs. −0.5 ± 1.0 μM; P < 0.05). The CBV response was inversely related to autoregulatory performance (r = −0.68; P < 0.001) in the patients, whereas no such relation was observed for CBFV.

Conclusion

This study is the first to provide evidence that cerebral autoregulatory performance in patients with acute ischaemic stroke affects the cerebrovascular response to changes in the position of the head.

Middle cerebral artery diameter changes during rhythmic handgrip exercise in humans

Transcranial Doppler (TCD) sonography is a frequently employed technique for quantifying cerebral blood flow by assuming a constant arterial diameter. Given that exercise increases arterial pressure by sympathetic activation, we hypothesized that exercise might induce a change in the diameter of large cerebral arteries. Middle cerebral artery (MCA) cross-sectional area was assessed in response to handgrip exercise by direct magnetic resonance imaging (MRI) observations. Twenty healthy subjects (11 female) performed three 5 min bouts of rhythmic handgrip exercise at 60% maximum voluntary contraction, alternated with 5 min of rest. High-resolution 7 T MRI scans were acquired perpendicular to the MCA. Two blinded observers manually determined the MCA cross-sectional area. Sufficient image quality was obtained in 101 MCA-scans of 19 subjects (age-range 20-59 years). Mixed effects modelling showed that the MCA cross-sectional area decreased by 2.1 ± 0.8% (p = 0.01) during handgrip, while the heart rate increased by 11 ± 2% (p < 0.001) at constant end-tidal CO₂ (p = 0.10). In conclusion, the present study showed a 2% decrease in MCA cross-sectional area during rhythmic handgrip exercise. This further strengthens the current concept of sympathetic control of large cerebral arteries, showing in vivo vasoconstriction during exercise-induced sympathetic activation. Moreover, care must be taken when interpreting TCD exercise studies as diameter constancy cannot be assumed.

Hyperventilation, cerebral perfusion, and syncope

This review summarizes evidence in humans for an association between hyperventilation (HV)-induced hypocapnia and a reduction in cerebral perfusion leading to syncope defined as transient loss of consciousness (TLOC). The cerebral vasculature is sensitive to changes in both the arterial carbon dioxide (PaCO2) and oxygen (PaO2) partial pressures so that hypercapnia/hypoxia increases and hypocapnia/hyperoxia reduces global cerebral blood flow. Cerebral hypoperfusion and TLOC have been associated with hypocapnia related to HV. Notwithstanding pronounced cerebrovascular effects of PaCO2 the contribution of a low PaCO2 to the early postural reduction in middle cerebral artery blood velocity is transient. HV together with postural stress does not reduce cerebral perfusion to such an extent that TLOC develops. However when HV is combined with cardiovascular stressors like cold immersion or reduced cardiac output brain perfusion becomes jeopardized. Whether, in patients with cardiovascular disease and/or defect, cerebral blood flow cerebral control HV-induced hypocapnia elicits cerebral hypoperfusion, leading to TLOC, remains to be established.

Orthostatic leg blood volume changes assessed by near-infrared spectroscopy

Standing up shifts blood to dependent parts of the body, and blood vessels in the leg become filled. The orthostatic blood volume accumulation in the small vessels is relatively unknown, although these may contribute significantly. We hypothesized that in healthy humans exposed to the upright posture, volume accumulation in small blood vessels contributes significantly to the total fluid volume accumulated in the legs. Considering that near-infrared spectroscopy (NIRS) tracks postural blood volume changes within the small blood vessels of the lower leg, we evaluated the NIRS-determined changes in oxygenated (Δ[O(2)Hb]), deoxygenated (Δ[HHb]) and total haemoglobin tissue concentration (Δ[tHb]) and in total leg volume by strain-gauge plethysmography during 70 deg head-up tilt (HUT; n = 7). In a second experiment, spatial and temporal reproducibility were evaluated with three NIRS probes applied on two separate days (n = 8). In response to HUT, an initially fast increase in [O(2)Hb] was followed by a gradual decline, while [HHb] increased continuously. The increase in [tHb] during HUT was closely related to the increase in total leg volume (r(2) = 0.95 ± 0.03). After tilt back, [O(2)Hb] declined below and [HHb] remained above baseline, whereas all NIRS signals gradually returned to baseline. Spatial heterogeneity was observed, and for two probes [tHb] was highly correlated between days (r(2) = 0.92 ± 0.09 and 0.91 ± 0.12), but less for the third probe (r(2) = 0.44 ± 0.36). The results suggest a non-linear accumulation of blood volume in the small vessels of the leg, with an initial fast phase followed by a more gradual increase at least partly contributing to the relocation of fluid during orthostatic stress.

The postural reduction in middle cerebral artery blood velocity is not explained by PaCO2

In the normocapnic range, middle cerebral artery mean velocity (MCA Vmean) changes approximately 3.5% per mmHg carbon-dioxide tension in arterial blood (PaCO2) and a decrease in PaCO2 will reduce the cerebral blood flow by vasoconstriction (the CO2 reactivity of the brain). When standing up MCA Vmean and the end-tidal carbon-dioxide tension (PETCO2) decrease, suggesting that PaCO2 contributes to the reduction in MCA Vmean. In a fixed body position, PETCO2 tracks changes in the PaCO2 but when assuming the upright position, cardiac output (Q) decreases and its distribution over the lung changes, while ventilation (VE) increases suggesting that PETCO2 decreases more than PaCO2. This study evaluated whether the postural reduction in PaCO2 accounts for the postural decline in MCA Vmean). From the supine to the upright position, VE, Q, PETCO2, PaCO2, MCA Vmean, and the near-infrared spectrophotometry determined cerebral tissue oxygenation (CO2Hb) were followed in seven subjects. When standing up, MCA Vmean (from 65.3+/-3.8 to 54.6+/-3.3 cm s(-1) ; mean +/- SEM; P<0.05) and cO2Hb (-7.2+/-2.2 micromol l(-1) ; P<0.05) decreased. At the same time, the VE/Q ratio increased 49+/-14% (P<0.05) with the postural reduction in PETCO2 overestimating the decline in PaCO2 (-4.8+/-0.9 mmHg vs. -3.0+/-1.1 mmHg; P<0.05). When assuming the upright position, the postural decrease in MCA Vmean seems to be explained by the reduction in PETCO2 but the small decrease in PaCO2 makes it unlikely that the postural decrease in MCA Vmean can be accounted for by the cerebral CO2 reactivity alone.

Stroke volume of the heart and thoracic fluid content during head-up and head-down tilt in humans

BACKGROUND

The stroke volume (SV) of the heart depends on the diastolic volume but, for the intact organism, central pressures are applied widely to express the filling of the heart.

METHODS

This study evaluates the interdependence of SV and thoracic electrical admittance of thoracic fluid content (TA) vs. the central venous (CVP), mean pulmonary artery (MPAP) and pulmonary artery wedge (PAWP) pressures during head-up (HUT) and head-down (HDT) tilt in nine healthy humans.

RESULTS

From the supine position to 20 degrees HDT, SV [112 +/- 18 ml; mean +/- standard deviation (SD)], TA (30.8 +/- 7.1 mS) and CVP (3.6 +/- 0.9 mmHg) did not change significantly, whereas MPAP (from 13.9 +/- 2.7 to 16.1 +/- 2.5 mmHg) and PAWP (from 8.8 +/- 3.4 to 11.3 +/- 2.5 mmHg; P < 0.05) increased. Conversely, during 70 degrees HUT, SV (to 65 +/- 24 ml) decreased, together with CVP (to 0.9 +/- 1.4 mmHg; P < 0.001), MPAP (to 9.3 +/- 3.8 mmHg; P < 0.01), PAWP (to 0.7 +/- 3.3 mmHg; P < 0.001) and TA (to 26.7 +/- 6.8 mS; P < 0.01). However, from 20 to 50 min of HUT, SV decreased further (to 48 +/- 21 ml; P < 0.001), whereas the central pressures did not change significantly.

CONCLUSIONS

During both HUT and HDT, SV of the heart changed with the thoracic fluid content rather than with the central vascular pressures. These findings confirm that the function of the heart relates to its volume rather than to its so-called filling pressures.

Orthostatic blood pressure control before and after spaceflight, determined by time-domain baroreflex method

Reduction in plasma volume is a major contributor to orthostatic tachycardia and hypotension after spaceflight. We set out to determine time- and frequency-domain baroreflex (BRS) function during preflight baseline and venous occlusion and postflight orthostatic stress, testing the hypothesis that a reduction in central blood volume could mimic the postflight orthostatic response. In five cosmonauts, we measured finger arterial pressure noninvasively in supine and upright positions. Preflight measurements were repeated using venous occlusion thigh cuffs to impede venous return and “trap” an increased blood volume in the lower extremities; postflight sessions were between 1 and 3 days after return from 10- to 11-day spaceflight. BRS was determined by spectral analysis and by PRVXBRS, a time-domain BRS computation method. Although all completed the stand tests, two of five cosmonauts had drastically reduced pulse pressures and an increase in heart rate of ∼30 beats/min or more during standing after spaceflight. Averaged for all five subjects in standing position, high-frequency interbeat interval spectral power or transfer gain did not decrease postflight. Low-frequency gain decreased from 8.1 (SD 4.0) preflight baseline to 6.8 (SD 3.4) postflight ( P = 0.033); preflight with thigh cuffs inflated, low-frequency gain was 9.4 (SD 4.3) ms/mmHg. There was a shift in time-domain-determined pulse interval-to-pressure lag, Tau, toward higher values ( P < 0.001). None of the postflight results were mimicked during preflight venous occlusion. In conclusion, two of five cosmonauts showed abnormal orthostatic response 1 and 2 days after spaceflight. Overall, there were indications of increased sympathetic response to standing, even though we can expect (partial) restoration of plasma volume to have taken place. Preflight venous occlusion did not mimic the postflight orthostatic response.

Human cerebral venous outflow pathway depends on posture and central venous pressure

Internal jugular veins are the major cerebral venous outflow pathway in supine humans. In upright humans the positioning of these veins above heart level causes them to collapse. An alternative cerebral outflow pathway is the vertebral venous plexus. We set out to determine the effect of posture and central venous pressure (CVP) on the distribution of cerebral outflow over the internal jugular veins and the vertebral plexus, using a mathematical model. Input to the model was a data set of beat-to-beat cerebral blood flow velocity and CVP measurements in 10 healthy subjects, during baseline rest and a Valsalva manoeuvre in the supine and standing position. The model, consisting of 2 jugular veins, each a chain of 10 units containing nonlinear resistances and capacitors, and a vertebral plexus containing a resistance, showed blood flow mainly through the internal jugular veins in the supine position, but mainly through the vertebral plexus in the upright position. A Valsalva manoeuvre while standing completely re-opened the jugular veins. Results of ultrasound imaging of the right internal jugular vein cross-sectional area at the level of the laryngeal prominence in six healthy subjects, before and during a Valsalva manoeuvre in both body positions, correlate highly with model simulation of the jugular cross-sectional area (R(2) = 0.97). The results suggest that the cerebral venous flow distribution depends on posture and CVP: in supine humans the internal jugular veins are the primary pathway. The internal jugular veins are collapsed in the standing position and blood is shunted to an alternative venous pathway, but a marked increase in CVP while standing completely re-opens the jugular veins.

Comparison of the time courses and potencies of the vasodilator effects of nifedipine and felodipine in the human forearm

In a previous study we investigated the differential time courses of the vasodilator effect of various calcium antagonists (CA) in small isolated rat mesenteric arteries (van der Lee et al., Fundam Clin Pharmacol, 1998: 12: 607-12). We concluded that the differences observed could be due to differences in lipophilicity between the CA studied. A measure for lipophilicity is the logarithm of the membrane-partition coefficient (log P). The log P values of nifedipine and felodipine are 2.50 and 4.46, respectively. It was the aim of the present study to compare the time courses of nifedipine and felodipine effects by means of forearm venous occlusion plethysmography in healthy subjects. Healthy male non-smoking volunteers (age 31 +/- 7 years, n = 14) were studied. Informed consent was obtained prior to each experiment from all subjects. The study commenced with the vehicle of either CA (NaCl 0.9% or a PEG400-solution for nifedipine and felodipine, respectively). In four subsequent runs, increasing concentrations of CA were studied for 20 min each, at an infusion rate of 0.3 ml/min. During experiments both hands were excluded from the circulation using small wrist cuffs, inflated to at least 40 mmHg over systolic blood pressure. Mean arterial pressure remained stable in all subjects (88 +/- 3 and 83 +/- 3 mmHg for nifedipine and felodipine, respectively), thus a systemic effect of the CA was not likely. Log IC50 values were -7.46 +/- 0.17 and -8.47 +/- 0.14 for nifedipine and felodipine, respectively (p < 0.01). Averaged KD values were 4.3 +/- 0.6 and 4.6 +/- 0.6 for nifedipine and felodipine, respectively (n.s.). In this model, felodipine appears to be a more potent vasodilator than nifedipine. The 100-fold difference in lipophilicity between the two CA tested is apparently not sufficient to cause major differences in K(D) values in the plethysmography experimental set-up.

Mechanisms underlying the impairment in orthostatic tolerance after nocturnal recumbency in patients with autonomic failure

In the present study, we have assessed in patients with neurogenic orthostatic hypotension the haemodynamics underlying the reduced tolerance to standing after prolonged recumbency at night. In 10 patients with neurogenic orthostatic hypotension (age 33-68 years), of which seven were being treated with fludrocortisone and/or sleeping in the 12 degrees head-up tilt position, 24 h continuous non-invasive finger blood pressure was recorded by a Portapres device. Beat-to-beat blood pressure, heart rate, stroke volume, cardiac output and total peripheral vascular resistance obtained by pulse contour analysis were assessed during 5 min of standing in the evening (at 22.30 hours) and in the morning (at 06.30 hours). On average, the inverse of the normal 24 h blood pressure profile was found, with a large diversity in blood pressure profiles among patients. Supine blood pressure values were similar, but standing blood pressure values were lower in the morning than in the evening (P<0.01). This resulted from larger falls in stroke volume and cardiac output upon standing in the morning compared with the evening, while total peripheral resistance did not change. There was no relationship between the decrease in body weight during the night (mean 0.9 kg; range 0.2-1.6 kg) and the evening-morning difference in standing blood pressure. We conclude that, in patients with neurogenic orthostatic hypotension, the impaired tolerance to standing in the morning is due to larger falls in stroke volume and cardiac output. Not only nocturnal polyuria, but also a redistribution of body fluid, are likely mechanisms underlying the pronounced decreases in stroke volume and cardiac output after prolonged recumbency at night.

Muscle tensing during standing: effects on cerebral tissue oxygenation and cerebral artery blood velocity

BACKGROUND AND PURPOSE

When standing up causes dizziness, tensing of the leg muscles may alleviate the symptoms. We tested the hypothesis that leg tensing improves orthostatic tolerance via enhanced cerebral perfusion and oxygenation.

METHODS

In 10 healthy young adults, the effects of leg tensing on transcranial Doppler-determined middle cerebral artery (MCA) mean blood velocity (V(mean)) and the near-infrared spectroscopy-determined frontal oxygenation (O(2)Hb) were assessed together with central circulatory variables and an arterial pressure low-frequency (LF) (0.07 to 0.15 Hz) domain evaluation of sympathetic activity.

RESULTS

Standing up reduced central venous pressure by (mean+/-SEM) 4.3+/-2.6 mm Hg, stroke volume by 49+/-7 mL, cardiac output by 1.9+/-0.4 L/min, and mean arterial pressure at MCA level by 9+/-4 mm Hg, whereas it increased heart rate by 30+/-4 beats per minute (P<0.05). MCA V(mean) declined from 67+/-4 to 56+/-3 cm/s, O(2)Hb decreased by 7+/-2.8%, and LF spectral power increased (P<0.05). Leg tensing increased central venous pressure by 1.4+/-2.7 mm Hg and cardiac output by 1.8+/-0.4 L/min with no significant effect on blood pressure, whereas heart rate decreased by 11+/-3 beats per minute (P<0.05). MCA V(mean) increased to 63+/-3 cm/s and O(2)Hb increased by 2.1+/-2.6%, whereas LF power declined (P<0.05). Within 2 minutes after leg tensing, these effects had disappeared.

CONCLUSIONS

During standing, tensing of the leg muscles attenuates a reduction in cerebral perfusion and oxygenation as it stabilizes central circulatory variables and reduces sympathetic activity.

Cerebrovascular and cardiovascular responses associated with orthostatic intolerance and tachycardia

Idiopathic orthostatic intolerance syndrome is characterized by postural symptoms of cerebral hypoperfusion without arterial hypotension. Abnormal baroreceptor responses with deranged cerebral autoregulation leading to cerebral vasoconstriction have been proposed as a causative mechanism. The authors report the cerebrovascular and cardiovascular responses in a patient who recovered from orthostatic intolerance and tachycardia. Changes in the orthostatic responses of mean arterial pressure (MAP), heart rate (HR), cardiac output (CO), and transcranial Doppler middle cerebral artery (MCA) mean blood flow velocity (Vmean) were assessed at admission and again 6 months after recovery. Normal cardiovascular responses to forced breathing and to standing indicated intact overall baroreflex integrity with normal baroreflex sensitivity (10.2 msec.mm Hg(-1)). After the patient stood for 8 minutes, presyncopal symptoms developed, with unchanged MAP but increased HR (+41 beats/min) and reduced stroke volume (SV) (-69%), CO (-50%), and MCA Vmean (-46%; 57 to 31 cm. s(-1)). After a reconditioning program and recovery, the patient was reexamined. The supine MCA Vmean was larger (79 cm. s(-1)), as were MAP (76 versus 70 mm Hg) and CO (+15%). The orthostatic HR increase was smaller (+5 beats/min), as was the reduction in SV (-44%) and CO (-30%), with an increase in MAP to 93 mm Hg. The orthostatic reduction in MCA Vmean was smaller (-13 versus -26 cm.s(-1)) and standing cerebrovascular resistance decreased (1.41 versus 2.39 mm Hg.cm. s(-1)). In this patient who had intact baroreflex control and no postural decrease in blood pressure, the reduction in MCA Vmean, concomitant with a large decrease in CO, seemed reversible. The result suggests that a symptomatic reduction in cerebrovascular conductance during standing is to be interpreted as being an adaptive response to a critically limited systemic blood flow, rather than to derangement of cerebral autoregulation.

Orthostatic tolerance, cerebral oxygenation, and blood velocity in humans with sympathetic failure

BACKGROUND AND PURPOSE

Patients with orthostatic hypotension due to sympathetic failure become symptomatic when standing, although their capability to maintain cerebral blood flow is reported to be preserved. We tested the hypothesis that in patients with sympathetic failure, orthostatic symptoms reflect reduced cerebral perfusion with insufficient oxygen supply.

METHODS

This study addressed the relationship between orthostatic tolerance, mean cerebral artery blood velocity (V(mean), determined by transcranial Doppler ultrasonography), oxygenation (oxyhemoglobin [O(2)Hb], determined by near-infrared spectroscopy), and mean arterial pressure at brain level (MAP(MCA), determined by finger arterial pressure monitoring [Finapres]) in 9 patients (aged 37 to 70 years; 4 women) and their age- and sex-matched controls during 5 minutes of standing.

RESULTS

Supine MAP(MCA) (108+/-14 versus 86+/-14 mm Hg) and V(mean) (84+/-21 versus 62+/-13 cm. s(-1)) were higher in the patients. After 5 minutes of standing, MAP(MCA) was lower in the patients (31+/-14 versus 72+/-14 mm Hg), as was V(mean) (51+/-8 versus 59+/-9 cm. s(-1)), with a larger reduction in O(2)Hb (-11. 6+/-4 versus -6.7+/-4.5 micromol. L(-1)). Four patients terminated standing after 1 to 3.5 minutes. In these symptomatic patients, the orthostatic fall in V(mean) was greater (45+/-6 versus 64+/-10 cm. s(-1)), and the orthostatic decrease in O(2)Hb (-12.0+/-3.3 versus -7.6+/-3.9 micromol. L(-1)) tended to be larger. The reduction in MAP(MCA) was larger after 10 seconds of standing, and MAP(MCA) was lower after 1 minute (25+/-8 versus 40+/-6 mm Hg).

CONCLUSIONS

In patients with sympathetic failure, the orthostatic reduction in cerebral blood velocity and oxygenation is larger. Patients who become symptomatic within 5 minutes of standing are characterized by a pronounced orthostatic fall in blood pressure, cerebral blood velocity, and oxygenation manifest within the first 10 seconds of standing.

Middle cerebral artery blood velocity during a valsalva maneuver in the standing position

Occasionally, lifting of a heavy weight leads to dizziness and even to fainting, suggesting that, especially in the standing position, expiratory straining compromises cerebral perfusion. In 10 subjects, the middle cerebral artery mean blood velocity (V(mean)) was evaluated during a Valsalva maneuver (mouth pressure 40 mmHg for 15 s) both in the supine and in the standing position. During standing, cardiac output decreased by 16 +/- 4 (SE) % (P < 0.05), and at the level of the brain mean arterial pressure (MAP) decreased from 89 +/- 2 to 78 +/- 3 mmHg (P < 0.05), as did V(mean) from 73 +/- 4 to 62 +/- 5 cm/s (P < 0.05). In both postures, the Valsalva maneuver increased central venous pressure by approximately 40 mmHg with a nadir in MAP and cardiac output that was most pronounced during standing (MAP: 65 +/- 6 vs. 87 +/- 3 mmHg; cardiac output: 37 +/- 3 vs. 57 +/- 4% of the resting value; P < 0.05). Also, V(mean) was lowest during the standing Valsalva maneuver (39 +/- 5 vs. 47 +/- 4 cm/s; P < 0.05). In healthy individuals, orthostasis induces an approximately 15% reduction in middle cerebral artery V(mean) that is exaggerated by a Valsalva maneuver performed with 40-mmHg mouth pressure to approximately 50% of supine rest.

Fludrocortisone and sleeping in the head-up position limit the postural decrease in cardiac output in autonomic failure

Treatment with head-up tilt sleeping and low-dose fludrocortisone effectively minimizes orthostatic symptoms and increases orthostatic blood pressure in patients with neurogenic orthostatic hypotension. The aim of the present study was to examine whether the improvement in orthostatic blood pressure during combined treatment with low-dose fludrocortisone and nocturnal head-up tilt in patients with neurogenic orthostatic hypotension can be attributed to expansion of plasma volume or to increased total peripheral resistance. The effects of a 3-week treatment with fludrocortisone and nocturnal head-up tilting on the postural changes in arterial pressure, heart rate, and cardiac output (pulse contour) were evaluated in eight consecutive patients with orthostatic hypotension. The period during which the patients were able to remain in the standing position without orthostatic complaints increased minimally from 3 to 10 minutes. The decrease in arterial pressure after 1 minute of standing--(means with standard deviations in parentheses) systolic, 49 (20) mm Hg; diastolic, 18 (11) mm Hg--before treatment was produced by a greater than normal decrease in cardiac output: 37% (10%) in patients with neurogenic orthostatic hypotension versus -14% (8%) in control subjects. Treatment increased upright arterial pressure from 83 (19) mm Hg systolic and 55 (13) mm Hg diastolic to 114 (22) mm Hg systolic and 60 (16) mm Hg diastolic by limiting the decrease in cardiac output. Body weight increased but hematocrit did not change. Leg pressure-volume relationship decreased in the two patients studied. The responses of plasma renin activity and aldosterone to orthostatic stress prior to treatment were subnormal and became even lower after treatment. The improvement in upright blood pressure in orthostatic hypotension during treatment with fludrocortisone and nocturnal head-up sleeping is the result of a reduction in the orthostatic decrease in cardiac output. Preliminary data suggest that the expanded body fluid volume is allocated to the perivascular space rather than to the intravascular space.

Estimation of beat-to-beat changes in stroke volume from arterial pressure: a comparison of two pressure wave analysis techniques during head-up tilt testing in young, healthy men

OBJECTIVE

The aim of this study was to compare beat-to-beat changes in stroke volume (SV) estimated by two different pressure wave analysis techniques during orthostatic stress testing: pulse contour analysis and Modelflow, i.e., simulation of a three-element model of aortic input impedance.

METHODS

A reduction in SV was introduced in eight healthy young men (mean age, 25; range, 19-32 y) by a 30-minute head-up tilt maneuver. Intrabrachial and noninvasive finger pressure were monitored simultaneously. Beat-to-beat changes in SV were estimated from intrabrachial pressure by pulse contour analysis and Modelflow. In addition, the relative differences in Modelflow SV obtained from intrabrachial pressure and noninvasive finger pressure were assessed.

RESULTS

Beat-to-beat changes in Modelflow SV from intrabrachial pressure were comparable with pulse contour measures. The relative difference between the two methods amounted to 0.1+/-1% (mean +/- SEM) and was not dependent on the duration of tilt. The difference between Modelflow applied to intrabrachial pressure and finger pressure amounted to -2.7+/-1.3% (p = 0.04). This difference was not dependent on the duration of tilt or level of arterial pressure.

CONCLUSIONS

Based on different mathematical models of the human arterial system, pulse contour and Modelflow compute similar changes in SV from intrabrachial pressure during orthostatic stress testing in young healthy men. The magnitude of the difference in SV derived from intrabrachial and finger pressure may vary among subjects; Modelflow SV from noninvasive finger pressure tracks fast and brisk changes in SV derived from intrabrachial pressure.

Continuous cardiac output in septic shock by simulating a model of the aortic input impedance: a comparison with bolus injection thermodilution

BACKGROUND

To compare continuous cardiac output obtained by simulation of an aortic input impedance model to bolus injection thermodilution (TDCO) in critically ill patients with septic shock.

METHODS

In an open study, mechanically ventilated patients with septic shock were monitored for 1 (32 patients), 2 (15 patients), or 3 (5 patients) days. The hemodynamic state was altered by varying the dosages of dopamine, norepinephrine, or dobutamine. TDCO was estimated 189 times as the series average of four automated phase-controlled injections of iced 5% glucose, spread equally over the ventilatory cycle. Continuous model-simulated cardiac output (MCO) was computed from radial or femoral artery pressure. On each day, the first TDCO value was used to calibrate the model.

RESULTS

TDCO ranged from 4.1 to 18.2 l/min. The bias (mean difference between MCO and TDCO) on the first day before calibration was -1.92 +/- 2.3 l/min (mean +/- SD; n = 32; 95% limits of agreement, -6.5 to 2.6 l/min). The bias increased at higher levels of cardiac output (P < 0.05). In 15 patients studied on two consecutive days, the precalibration ratio TDCO:MCO on day 1 was 1.39 +/- 0.28 (mean +/- SD) and did not change on day 2 (1.39 +/- 0.34). After calibration, the bias was -0.1 +/- 0.8 l/min with 82% of the comparisons (n = 112) < 1 l/min and 58% (n = 79) < 0.5 l/min, and independent of the level of cardiac output.

CONCLUSIONS

In mechanically ventilated patients with septic shock, changes in bolus TDCO are reflected by calibrated MCO over a range of cardiac output values. A single calibration of the model appears sufficient to monitor continuous cardiac output over a 2-day period with a bias of -0.1 +/- 0.8 l/min.

Pathophysiological mechanisms underlying vasovagal syncope in young subjects

The occurrence of vasovagal fainting is common in young subjects, but the origin of the precipitating hemodynamic mechanisms involved remain a subject of considerable speculation. Vasovagal fainting is not a sudden onset phenomenon, early failure of vascular resistance responses occurs in faint-prone young subjects. The variability of hemodynamic responses during the actual faint is large, but the main mechanism operative during, is withdrawal of sympathetic outflow to blood vessels in skeletal muscle with impairment of ability to maintain vasomotor tone.

Neural circulatory control in vasovagal syncope

The orthostatic volume displacement associated with the upright position necessitates effective neural cardiovascular modulation. Neural control of cardiac chronotropy and inotropy, and vasomotor tone aims at maintaining venous return, thus opposing gravitational pooling of blood in the lower part of the body. The present concept of the vasovagal response or "common faint" implicates the development of inappropriate cardiac slowing due to sudden augmentation of efferent vagal activity, and arteriolar dilatation by sudden reduction or cessation of sympathetic activity. The venous pooling associated with lasting orthostatic stress results in development of central hypovolemia. At a certain point during the ongoing reflex adaptation to the hypovolemia in progress, a depressor reflex is set in train. The depressor reflex input along this second "peripheral" afferent pathway is postulated to originate from various sites in the cardiovascular system but remains uncertain. The common faint in humans is of both vaso- and vagal origin; the pure vagal response is less common than its vasodepressor variant. There is strong evidence for an early loss of vasomotor tone in the majority of fainting subjects. Blocking the vagus nerve or cardiac pacing is not of much help in preventing vasovagal syncope; though atropine or pacing may prevent bradycardia in vasovagal fainting, they have never been proven to prevent hypotension. Baroreflex modulation of autonomic outflow remains present during the presyncopal stages until it becomes offset by an opposing depressor reflex with relative bradycardia and relaxation of arterial resistance vessels. The nature of the vasodilatation associated with the vasovagal response has still not been settled.

Finger arterial versus intrabrachial pressure and continuous cardiac output during head-up tilt testing in healthy subjects

1. The aims of this study were to determine the clinical feasibility of continuous, non-invasive Finapres recordings as a replacement for intrabrachial pressure during a 30 min head-up tilt, and the reliability of continuous cardiac output computation by pulse contour analysis from the finger arterial versus the brachial waveform. 2. In eight healthy subjects a 30 min 70 degrees passive head-up tilt was performed. Finger arterial (FINAP) and intrabrachial (IAP) pressures were measured simultaneously. Beat-to-beat changes in stroke volume were computed using a pulse contour algorithm. 3. Accuracy (the group-averaged FINAP-IAP difference) and precision (the SD of the difference) of Finapres measurements were 4 and 9 mmHg for systolic blood pressure, -5 and 9 mmHg for mean blood pressure and -5 and 9mmHg for diastolic blood pressure. 4. The time course of the FINAP-IAP differences during head-up tilt showed a linear trend (P < 0.001 for all pressure levels). Averaged for the group, the difference increased 7 mmHg for mean blood pressure. The difference in stroke volume computed from FINAP and IAP was 0.3 +/- 5% (mean +/- SD), and independent of the duration of the tilt (P > 0.05). This difference did not change at low blood pressure levels (0.5 +/- 6%). 5. The qualitative performance of the Finapres allows it to be used in the clinical setting as a monitor of sudden changes in blood pressure induced by a 30 min head-up tilt. Relative changes in stroke volume, as obtained by pulse contour analysis of the finger arterial waveform, closely follow intrabrachial values during long-duration head-up tilt and associated arterial hypotension.

Circulatory response evoked by a 3 s bout of dynamic leg exercise in humans

1. The mechanisms underlying the pronounced transient fall in arterial blood pressure evoked by a 3 s bout of bicycle exercise were investigated in twenty healthy young adults and four patients with hypoadrenergic orthostatic hypotension. 2. In healthy subjects a 3 s bout of upright cycling induced a 28 +/- 3 mmHg fall in mean arterial pressure at 12 s. The fall in mean arterial pressure was preceded by a 12 +/- 2 mmHg rise in right atrial pressure at 3 s and accompanied by a 54 +/- 7% increase in left ventricle stroke volume at 6 s. Systemic vascular resistance dropped 48 +/- 2% at 7 s after the start of the manoeuvre to remain at that level for approximately 5 s. The total response lasted about 20 s. During sustained upright cycling the initial fall in mean arterial pressure was also present, but less pronounced (17 +/- 2 vs. 26 +/- 3 mmHg). A 3 s bout of supine cycling in four patients with hypoadrenergic orthostatic hypotension also elicited a pronounced fall in mean arterial pressure (22 +/- 4 mmHg) and in systemic vascular resistance (38 +/- 4%). 3. A bout of exercise with a large muscle mass induces two main effects. First, it mechanically increases filling of the heart due to activation of the muscle pump, resulting in an increase in cardiac output. Second, it induces a drop in systemic vascular resistance. The increase in cardiac output is not sufficient to compensate fully for the pronounced fall in systemic vascular resistance and the result is a transient fall in arterial pressure at the onset of whole-body exercise. The rise in right atrial pressure evoked by 3 s cycling is abrupt and large, but the almost immediate onset and rapid fall of the systemic vascular resistance is too fast for sympathetically mediated reflex effects due to stimulation of the cardiopulmonary afferents. An important factor involved in the drop in systemic vascular resistance appears to be local, non-autonomically mediated vasodilatation in exercising muscles, since it also occurs in patients with autonomic failure.

Effects of leg muscle pumping and tensing on orthostatic arterial pressure: a study in normal subjects and patients with autonomic failure

1. The effects of leg muscle pumping (tiptoeing) and tensing (leg-crossing) on orthostatic blood pressure were investigated in six healthy adult subjects (aged 28-34 years) and in seven patients with severe hypoadrenergic orthostatic hypotension (aged 20-65 years). 2. Finger arterial pressure was monitored. Relative changes in left ventricular stroke volume were computed by a pulse contour method. 3. Tiptoeing increased mean arterial pressure (7 +/- 5 mmHg) in the healthy subjects, but not in the patients, whereas cardiac output increased in both groups, although by more in the healthy adults than in the patients (35 +/- 10% versus 20 +/- 11%, P < 0.05). Systemic vascular resistance decreased substantially in both groups while tiptoeing. Leg-crossing did not affect arterial pressure in the healthy subjects, although stroke volume had increased. In contrast, in the patients an increase in cardiac output (16 +/- 12%) and mean blood pressure (13 +/- 13 mmHg) was observed. 4. Tiptoeing and leg-crossing have different effects on orthostatic blood pressure in healthy adult subjects and in patients with autonomic failure. In normal humans, tiptoeing increases arterial pressure, whereas leg-crossing has little effect. In the patients, in contrast, tiptoeing has little effect, whereas leg-crossing increases arterial pressure considerably. Patients with autonomic failure should be instructed to apply leg-crossing to combat orthostatic dizziness.

Investigation and treatment of autonomic circulatory failure

Investigation and treatment of patients suffering from autonomic circulatory failure should be based on the application of knowledge of basic physiology to the problem of a particular patient. For an overall evaluation of cardiovascular autonomic dysfunction, an analysis of the heart rate and blood pressure responses to standing usually is sufficient. Instruction in physical maneuvers such as squatting and leg crossing should be part of the treatment programs to reduce disabilities from orthostatic hypotension in patients with circulatory autonomic failure.

Differences in circulatory control in normal subjects who faint and who do not faint during orthostatic stress

We have determined if there are differences in normal subjects who fainted and those who did not faint during prolonged standing. We studied the short-term orthostatic responses in relation to heart rate, blood pressure measured by Finapres, left ventricular stroke volume analysed by pulse contour method, cardiac output and systemic vascular resistance, and also postural blood pressure and heart rate variability as assessed by spectral analysis. Thirteen healthy males without a history of syncope were studied. Three fainted after 10-13 min standing; the ten non-fainters remained upright for 20 min. The initial (first 30 s) postural circulatory adjustment was comparable for blood pressure but the rebound bradycardia was smaller in the fainters (heart rate at 22 s amounted to +13 +/- 10 beats/min above control vs. +1 +/- 5 beats/min in the non-fainters). Upright heart rate at 2 min standing was higher in the fainters (+31 +/- 2 beats/min vs. +20 +/- 5 beats/min), and blood pressure at 7 min standing was lower (-2/+5/+8 +/- 5/5/5 mmHg vs. +11/+13/+16 +/- 10/6/5 mmHg). The responses of stroke volume and cardiac output were comparable but systemic vascular resistance gradually decreased in the fainters from 5 min standing to the onset of fainting (+4 +/- 13% vs. +33 +/- 19% at 7 min standing). In fainters, the variability in upright blood pressure around 0.1 Hz was larger (8.8 mmHg2/Hz for diastolic blood pressure vs. 5.7 +/- 1.5 mmHg2/Hz in non-fainters).(ABSTRACT TRUNCATED AT 250 WORDS)

[Immunotherapy using the anti-endotoxin antibody HA-1A (Centoxin) in patients with sepsis syndrome; fair results following protocol selection of patients]

OBJECTIVE

Evaluation of HA-1A treatment in patients with the sepsis syndrome.

DESIGN

Descriptive.

SETTING

Department of intensive care, Academic Medical Centre, Amsterdam.

PATIENTS AND METHODS

Intensive-care patients with the sepsis syndrome and shock or organ failure with a presumptive diagnosis of Gram-negative infection were eligible for treatment with HA-1A. We analysed and compared the results with those of the double-blind, randomized HA-1A study by Ziegler et al.

RESULTS

Between May 1991 and March 1992, 27 patients were treated with HA-1A. The mortality rate was 59% (16/27). Among the 11 patients with a Gram-negative bacteraemia mortality was 7/11, much higher than in the Ziegler study (30%). In comparison with the HA-1A study we selected sicker patients: the mean APACHE II score was higher, 93% of our patients were in shock and 85% had organ failure. More patients presented with an intra-abdominal sepsis and mortality in this group was very high (11/14). In patients with a Gram-negative bacteraemia the delay between the onset of the sepsis syndrome and the administration of HA-1A was longer (median 22 h versus 14.3 h in the Ziegler study, mean 30 versus 20 h).

CONCLUSION

HA-1A does not appear to be beneficial in critically ill patients with a longstanding sepsis syndrome, especially not if an intra-abdominal sepsis is apparent. Therefore, we decided not to use H-1A until additional data become available. Additional objective inclusion criteria are needed to improve the identification of the patient group that may benefit from treatment with HA-1A.

Physical manoeuvres that reduce postural hypotension in autonomic failure

A young female with autonomic failure is described. She successfully reduced the symptoms of orthostatic hypotension by application of physical manoeuvres like leg-crossing, bending forward and placing a foot on a chair. The beneficial effects of these manoeuvres can be explained by a small (10-15 mmHg) increase in mean arterial pressure to a level just sufficient to maintain adequate cerebral blood flow. The underlying common mechanism appears to be an increase of thoracic blood volume by translocation of blood from the vascular beds below the diaphragm to the chest. Instruction in these physical manoeuvres should be part of the management programme to reduce the disabilities arising from postural hypotension in patients with autonomic failure.

Physical manoeuvres for combating orthostatic dizziness in autonomic failure

Some patients with orthostatic hypotension combat orthostatic dizziness by leg-crossing and squatting. Changes in blood pressure with these manoeuvres were studied in 7 patients with hypoadrenergic orthostatic hypotension and in 6 healthy subjects. Without leg-crossing, 5 of the patients reported dizziness within 10 min of standing up. Crossing of the legs allowed all to stand for 10 min or more, and there was an associated increase in mean blood pressure of 13 (SD 6) mm Hg compared with 1 (4) in healthy controls; the corresponding figures for squatting were 44 (18) and 8 (6) mm Hg. Patients with orthostatic intolerance should be told about these blood-pressure-raising manoeuvres.

Assessment of cardiovascular reflexes is of limited value in predicting maximal +Gz-tolerance

The importance of +Gz-induced loss of consciousness as a major cause of inflight incapacitation emphasizes the need for predicting +Gz-tolerance and investigating its possible determinants. The cardiovascular changes from +Gz-stress are initially counteracted reflexly by the cardiovascular autonomic system. The integrity of neural cardiovascular reflex control can be assessed by analysing the blood pressure (BP) and heart rate (HR) responses to different maneuvers, such as the Valsalva maneuver, standing and forced respiratory sinus arrhythmia. The aim of the present study was to investigate a possible relation between the cardiovascular responses to these tests and +Gz-tolerance. In 10 healthy subjects continuous Finapres BP and HR responses to the tests have been determined and correlated with their G-levels of peripheral light loss (PLL) during centrifuge-runs (0.1 G/s). Only mean BP recovery during Valsalva maneuver correlated marginally significantly with PLL (r = 0.63, p = 0.049). Cardiovascular findings were within normal range revealing no cardiovascular autonomic dysfunction. These results indicate that intact neural cardiovascular control seems to be a condition for tolerating +Gz-stress without determining maximal +Gz-tolerance. We conclude that assessment of cardiovascular reflexes may only confirm baroreflex integrity. However, they have limited value in predicting +Gz-tolerance.

The vasovagal response

The vasovagal response is the development of inappropriate cardiac slowing and arteriolar dilatation. Vasovagal responses reflect autonomic neural changes: bradycardia results from sudden augmentation of efferent vagal activity, and hypotension results from sudden reduction or cessation of sympathetic activity and relaxation of arterial resistance vessels. Two different neural pathways are thought to be involved, one originating in the hypothalamus, the other in the heart. Direct hypothalamic activation of the medullary cardiovascular centres triggered by emotional stress or pain causes a vasovagal response (central type). The combination of a reduced central blood volume secondary to venous pooling or blood loss, and an increased inotropic state of the heart, may stimulate ventricular mechanoreceptors and provoke vasodilatation and bradycardia (peripheral type). Cardiovascular afferents originating from stretch receptors in various parts of the vascular tree sometimes induce opposite reflexes when compared with those from ventricular afferents. The depressor reflex involved in the peripheral type of vasovagal response originates in the heart itself and overrides normal baroreflex circulatory control; an antagonism between the control of volume and pressure on the filling side of the heart and the control system of arterial pressure becomes apparent. Vasovagal responses are not necessarily abnormal; the neural pathways involved in the vasovagal response are probably present in all healthy subjects who individually mainly differ in susceptibility.

Circulatory autonomic failure 50 years after acute poliomyelitis

A 59-year old woman who presented with postural dizziness 50 years after an acute episode of poliomyelitis is described. There were no new neurological signs and no evidence of motor neuron disease. She had postural hypotension with an abnormal Valsalva. Investigations led to a diagnosis of hypo-adrenergic orthostatic hypotension, with a predominantly preganglionic sympathetic lesion and intact vagal baroreflex pathways. Although pure autonomic failure and multiple system atrophy are possible causes of circulatory autonomic failure, no other new neurological or autonomic features have developed during a 2 year follow-up. We propose that hypo-adrenergic orthostatic hypotension may be a late complication of poliomyelitis. Deterioration in ambulatory ability in a patient with previous poliomyelitis should additionally include assessment of cardiovascular autonomic function.

Spectrum of orthostatic disorders: classification based on an analysis of the short-term circulatory response upon standing

1. In 31 consecutively referred patients (20 females, 11 males) with overt or suspected orthostatic disorders, the changes in blood pressure and heart rate that occur in the first 2 min of standing were analysed. 2. Blood pressure was measured continuously by Finapres. The blood pressure and heart rate responses after 1-2 min of standing (early steady-state response) were used to classify the patients as follows: group I (n = 17, age 42 +/- 17 years), normal early steady-state blood pressure and heart rate responses; group II (n = 5, age 40 +/- 14 years), combination of normal early steady-state blood pressure and postural tachycardia; group III (n = 9, age 51 +/- 14 years), hypotensive orthostatic response with (4/9) or without (5/9) postural tachycardia. We examined whether additional information could be obtained by beat-to-beat analysis of the initial circulatory response (first 30 s). It was quantified by identifying the blood pressure trough and overshoot and the maximum heart rate and relative bradycardia. 3. The initial drop in systolic and diastolic blood pressures did not differ between the three groups. A recovery of blood pressure with a systolic and/or diastolic blood pressure overshoot was present in all group I and II patients, but was absent in all except two patients in group III. The initial maximum heart rate increase did not differ between the three groups. The relative bradycardia was less in groups II and III than in group I. 4. We conclude that analysis of the beat-to-beat blood pressure changes in the first 30 s after the onset of standing provides almost all the information that is necessary to determine abnormalities in orthostatic circulatory control.

Contrasting effects of acute and chronic volume expansion on orthostatic blood pressure control in a patient with autonomic circulatory failure

We studied the effects of acute and chronic volume expansion on orthostatic blood pressure control in a 22-year-old female with hypoadrenergic orthostatic hypotension. Acute volume expansion on two occasions had unexpected effects: a decrease respectively no change in orthostatic tolerance and no change in upright blood pressure immediately after volume expansion followed by a marked improvement 8 h thereafter. The time course of changes in haematocrit and serum protein indicated an initial extravasation of plasma followed by a subsequent larger fluid shift back to the intravascular space. These effects had vanished after 3 days. Chronic volume expansion by head-up tilt at night and fludrocortisone resulted in a marked improvement in orthostatic blood pressure control at a comparable increment in body weight and sodium balance for the next 7 years until now. The circadian circulatory variation with orthostatic blood pressure lowest in the morning remained present after chronic volume expansion. We conclude in this patient that the effects of acute volume expansion on orthostatic blood pressure in autonomic failure are complex and not predictive for the beneficial effects of chronic volume expansion.

Circulatory responses to stand up: discrimination between the effects of respiration, orthostasis and exercise

The initial circulatory responses to an active change in posture (stand up from supine) were compared with the responses induced by a passive change in posture (head-up tilt) and a burst of muscular exercise on a bicycle ergometer (upright cycling) in order to differentiate between exercise- and orthostasis-induced effects. In eight subjects heart rate responses and in four subjects intra-arterial pressure transients were measured. In addition the effects of respiration on heart rate responses to the three manoeuvres were assessed. Both stand up and cycling induced almost superimposable and pronounced heart rate responses lasting for about 30 s. This contrasts with the more gradual increases following head-up tilt. Changing the respiratory phase during the performance of the manoeuvres exerted its effect on heart rate responses in the first 5 s only. Like stand up, cycling induced a transient blood pressure fall lasting for 30 s on average. As both manoeuvres were performed during inspiration the transients observed are not caused by involuntary Valsalva straining. In conclusion, the maximum and duration of the heart rate responses induced by stand up, cycling and head-up tilt are not influenced by respiratory activity. The initial fall in blood pressure following stand up is probably the result of the muscular effort of the manoeuvre and not due to the effects of orthostasis or Valsalva straining.

Cardiovascular instability and baroreflex activity in a patient with tetanus

In a patient with tetanus we tested the hypothesis that the hyperadrenergic cardiovascular instability might be due to impairment of the baroreceptor reflex by the tetanus toxin. Baroreflex sensitivity assessed with the phenylephrine method was found to be normal. Changes in arterial pressure correlated inversely with relative changes in plasma volume but not with plasma catecholamine levels. There were both extreme hypo- and hyper-adrenergic episodes. We conclude that sympathetic overactivity in tetanus temporarily overrules a functionally intact baroreflex leading to severe blood pressure instability with episodes of hypertension.

Singing-induced hypotension: a complication of a high spinal cord lesion

Cardiovascular autonomic control was studied in a patient with an incomplete high spinal cord lesion who presented with the symptoms of severe dizziness during debating and singing but not during orthostasis. The marked falls in blood pressure upon singing and orthostasis (45 degrees passive head-up tilt) were comparable in magnitude but different in time course. The fall in blood pressure upon graded Valsalva manoeuvres, however, was comparable to singing in magnitude and time course; similarly, 20 and 30 mmHg strain evoked complaints of dizziness. These differential circulatory responses upon orthostasis versus singing and Valsalva in tetraplegic patients have not been described before. We suggest that rapidly developing hypotension such as that induced by a moderate Valsalva strain represents the instantaneous mechanical effects of a raised intrathoracic pressure with lack of abdominal compression on the cardiovascular system when baroreflex vasomotor modulation is disrupted.