Many times , an Innocuous question poses a real challenge, to our life time understanding of circulatory physiology.
This question is very important , if we want to understand the true mechanism of postural hypotension, a commonest autonomic disorder in elderly (& also the newly recognised entity orthostatic hypertension)
Mechanism
Orthostatic hypotension is defined as a sustained drop of at least 20 mm Hg in systolic blood pressure (SBP) or 10 mm Hg in diastolic blood pressure (DBP) within 3-5 minutes of going from a supine to a standing position. In patients with associated supine hypertension, the criteria for orthostatic hypotension allows for a drop of at least 30 mm Hg in SBP or 15 mm Hg in DBP.
Can you discuss the answer to the question please ?
The correct answer is likely to be this. In normal adult, the systolic BP falls and diastolic BP is either static or raises by few mm . DBP is never expected to fall on standing, as the fall in systolic BP is invariable, that trigger a vasoconstriction which will increase the PVR and make sure the DBP doesn’t fall. In the process mean BP is kept near normal and regulated.
However, hemodynamic response to standing has no fixed rules. That’s why we keep definitions, like acceptable fall in systolic and diastolic BP . It depends on , age , central and peripheral neurological nervous system, preconditioning of vascular tone.(Not only arteriolar, even the much neglected venous tone).
For the seekers of evidence, there is one study which was specifically done to find orthostatic BP response .(Smith et al Ref 3) .It highlights the point, the immediate vascular response ie (<30sec) is totally different from prolonged standing, Implying, the response time of even an intact autonomic nervous system can be quiet variable.
Orthostatic hypertension :
It is relatively new entity, and claimed to be equally a problem A proposed consensus definition of “an exaggerated orthostatic pressor response” in subjects in whom systolic blood pressure increases ≥20 mmHg when going from the supine to standing posture. (Jordan J, Consensus statement on the definition of orthostatic hypertension endorsed by the American Autonomic Society and the Japanese Society of Hypertension. Clin Auton Res. 2023 Feb;33(1):69-73. ) Surprisingly, diastolic BP is not included in the defining criteria. There must be valid reason for that I guess.
Secret link between both these entities
Obviously it will add on to the complexity, if we say there are common roots in pathogenesis between the two entities. It is indeed true. Both comes under the basket of dysregulated autonomic nervous system. Please recall , we have a condition called supine hypertension and standing hypotension , POTS syndrome etc that should stimulate us to search for those hidden secrets.
As the name itself suggest, autonomic nervous system has its own control(or no control ) with a complex and poorly understood brainstem and cortical network. This operates through physical wiring directly as cranial nerves or piggy packing along the spinal cord tracts , vascular tree, and somatic nerves. The biochemical orchestra of this system is played by a delicate balance of adrenergic vs cholinergic forces.
Next question
What happens to BP during exercise ?
This again has more dynamic and interesting changes especially in the diastolic BP .Read Guyton’s physiology or the good old Rushmer’s hemodynamics monograph, linked in this site elsewhere.
Reference
Dr.S.Venkatesan MD 