Cardiovascular dynamics during orthostasis and the influence of intravascular instrumentation

doi:10.1016/0002-9149(66)90354-7

The American Journal of Cardiology

Volume 17, Issue 2, February 1966, Pages 211-218

https://doi.org/10.1016/0002-9149(66)90354-7 Get rights and content

Abstract

1.
1. Orthostatic tolerance of normal subjects is markedly diminished by the use of intravascular instrumentation such as venous catheters and intraarterial needles.
2.
2. Psychic stimulation due to discomfort or unconscious fear and anxiety associated with such instrumentation may interfere with compensatory reflexes activated by the upright posture and mediated through the central nervous system.
3.
3. Individual reactions to orthostasis vary greatly. Within three minutes following upright tilting, late fainters who were then asymptomatic showed the same decrease in cardiac index and stroke volume as nonfainters. With the onset of symptoms of presyncope, the cardiac index further decreased to 39 per cent of baseline, but nonfainters manifested only a 19 per cent decrease after 20 min. of orthostasis.
4.
4. Recumbent systolic pressure was found to be significantly lower in early fainters than in nonfainters. After three minutes of orthostasis, asymptomatic late fainters had a significantly lower systolic and narrower pulse pressure than nonfainters at the same time. This finding suggests an incomplete compensatory response to the orthostatic stress.
5.
5. Effective cerebral arterial pressure was maintained in nonfainters, but markedly decreased in fainters prior to syncope.
6.
6. The 19 per cent decrease in cardiac index found after 20 min. of orthostasis is associated with a 36 per cent increase in peripheral resistance and a 7 per cent increase in mean arterial pressure. A similar decrease in cardiac index in the recumbent position was associated with only a 14 per cent increase in peripheral resistance and a 5 per cent decrease in mean arterial pressure. This discrepancy in response to central cardiac changes is probably related to the relative effects of the upright posture on cerebral arterial pressure or peripheral vascular receptors.

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This maximized the effects of the normal compensatory baroreflex response, caused by a more pronounced, acute reduction in preload from baseline. The changes in vital parameters observed in our study during tilting were consistent with the described normal baroreceptor reflex that follows postural changes in healthy individuals, and it is mainly the result of an increase in sympathetic activity, reflected by an increase in heart rate and arterial vasoconstriction in order to maintain constant blood pressure.32 We found that acute reductions in preload were also associated with changes in a wide range of echocardiographic parameters.
Left atrial (LA) longitudinal strain is a novel parameter used for the evaluation of LA function, with demonstrated prognostic value in several cardiac diseases. However, the extent of load dependency of LA strain is not well known. The aim of this study was to evaluate the impact of acute changes in preload on LA strain, side by side with LA volume, in normal subjects.
Twenty-five healthy volunteers (13 men; mean age, 31 ± 2 years) were prospectively enrolled, who underwent two-dimensional and three-dimensional echocardiographic imaging during acute stepwise reductions in preload using a tilt maneuver: baseline at 0°, followed by 40° and 80°. Left ventricular and LA size and function parameters were measured using standard methodology, and LA strain-time curves were obtained using speckle-tracking software (TomTec), resulting in reservoir, conduit, and contractile strain components. All parameters were compared among the three loading conditions using one-way analysis of variance for repeated measurements.
Although there were no significant changes in blood pressure, heart rate increased significantly with tilt. As expected, LA volumes, left ventricular volumes, and left ventricular ejection fraction, as well as E wave, A wave, and e′ significantly decreased with progressive inclination. In parallel, LA reservoir, conduit, and contractile strain values decreased with reduction in preload (reservoir: 42.9 ± 3.9% to 27.5 ± 3.8%, P < .001; conduit: 29.3 ± 2.7% to 20.2 ± 5.0%, P < .001; contractile: 13.6 ± 2.9% to 7.3 ± 3.5%, P < .001). Paired post hoc analysis showed that all LA strain values were significantly different among all three tilt phases. Of note, percentage change in LA reservoir strain was significantly smaller than that in LA maximum volume.
In normal subjects, LA strain is preload dependent but to a lesser degree than LA volume. This difference underscores the relative advantage of LA strain over maximum volume, when LA assessment is used as part of the diagnostic paradigm.
Cardiac output and vasodilation in the vasovagal response: An analysis of the classic papers
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The simple faint is secondary to hypotension and bradycardia resulting in transient loss of consciousness. According to Ohm’s law applied to the circulation, BP = SVR × CO, hypotension can result from a decrease in systemic vascular resistance (SVR), cardiac output (CO), or both. It is important to understand that when blood pressure (BP) is falling, SVR and CO do not change reciprocally as they do in the steady state. In 1932, Lewis, assuming that decreased SVR alone accounted for hypotension, defined “the vasovagal response” along pathophysiologic lines to denote the association of vasodilation with vagal-induced bradycardia in simple faint. Studies performed by Barcroft and Sharpey-Schafer between 1940 and 1950 used volume-based plethysmography to demonstrate major forearm vasodilation during extreme hypotension and concluded that the main mechanism for hypotension was vasodilation. Plethysmographic measurements were intermittent and not frequent enough to capture rapid changes in blood flow during progressive hypotension. However, later investigations by Weissler, Murray, and Stevens performed between 1950 and 1970 used invasive beat-to-beat BP measurements and more frequent measurements of CO using the Fick principle. They demonstrated that CO significantly fell before syncope, and little vasodilation occurred until very late in the vasovagal reaction Thus, since the 1970s, decreasing cardiac output rather than vasodilation has been regarded as the principal mechanism for the hypotension of vasovagal syncope.
The Role of the Syncope Management Unit
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The time and/or degree of stress required to provoke this response provide the measure of OT. Any technique used to determine OT should enable distinction between patients with orthostatic intolerance (of various causes) and asymptomatic control subjects; be highly reproducible, enabling evaluation of therapeutic interventions; and avoid invasive procedures, which are known to impair OT.55 Head-upright tilt testing (HUTT) was first described for the diagnosis of syncope in the late 1980s.56
The role of the syncope management unit
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For example, in nitro-glycerine induced vasovagal syncope studies [8] the effects of this drug on cardiac output make it difficult to ascertain what would be the hemodynamic responses in healthy or non-medicated subjects. Furthermore, intravenous cannulation is avoided, which has been shown to spuriously affect cardiovascular responses and orthostatic tolerance [9]. For details on the combined HUT and graded LBNP protocol, please refer to [10].
At orthostatic vasovagal syncope there appears to be a sudden decline of sympathetic activity. As mental challenge activates the sympathetic system, we hypothesized that doing mental arithmetic in volunteers driven to the end point of their cardiovascular stability may delay the onset of orthostatic syncope. We investigated this in healthy male subjects. Each subject underwent a head up tilt (HUT)+ graded lower body negative pressure (LBNP) up to presyncope session (control) to determine the orthostatic tolerance time, OTT (Time from HUT commencement to development of presyncopal symptoms/signs). Once the tolerance time was known, a randomized crossover protocol was used: either 1) Repeat HUT + LBNP to ensure reproducibility of repeated run or 2) HUT + LBNP run but with added mental challenge (2 min before the expected presyncope time). Test protocols were separated by 2 weeks. Our studies on five male test subjects indicate that mental challenge improves orthostatic tolerance significantly. Additional mental loading could be a useful countermeasure to alleviate the orthostatic responses of persons, particularly in those with histories of dizziness on standing up, or to alleviate hypotension that frequently occurs during hemodialysis or on return to earth from the spaceflight environment of microgravity.

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Clinical studyCardiovascular dynamics during orthostasis and the influence of intravascular instrumentation

Abstract

Am. J. Cardiol.

Am. J. Med.

Am. J. Cardiol.

The effects of four weeks of absolute bed rest on circulatory function of man

Aerospace Med.

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Aerospace Med.

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Aerospace Med.

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J. Aviation Med.

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J.A.M.A.

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Clinical study
Cardiovascular dynamics during orthostasis and the influence of intravascular instrumentation