Baroreceptor Physiology
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Global Cerebral Ischemia (Presyncope, Syncope)
Global Cerebral Ischemia (Presyncope, Syncope)
Presentations
Presyncope
- Ill-defined light-headedness
- Warmth, pallor, diaphoresis
- Vision loss (progressing from peripheral to medial)
- Auditory illusion of sounds being more distant
Syncope
- Loss of consciousness with brief loss of muscle tone
Etiologies
Vasovagal syncope (most common cause)
- Sudden increase in vagal (parasympathetic) activity
- Results in hypotension, bradycardia, and decreased vascular tone.
- Common causes include prolonged standing, emotional stress, pain (eg, from venipuncture), micturition.
Orthostatic hypotension
- Common causes: autonomic dysfunction, anti-hypertensive agents, or beta-blockers
- Manifests with: persistent/progressive generalized weakness and fatigue, visual blurring, leg buckling, "coat hanger" headache (due to trapezius ischemia).
- Provoked by: physical exertion, prolonged standing (poor venous return to the heart), a meal (due to shunting of blood to the intestines), increased body temperature.
Cardiogenic arrythmias
Common causes
- Ventricular Tachycardia, Heart block (Mobitz II or III), Symptomatic bradycardia, Supraventricular tachycardia
Nonarrhythmic cardiovascular causes
- Coronary ischemia, Severe aortic stenosis or prosthetic valve dysfunction, Hypertrophic cardiomyopathy, Pulmonary embolism, Aortic dissection, Heart failure and cardiac tamponade
Anemia
- Acute GI bleed, Chronic anemia
Recommendations for vasovagal syncope
Non-pharmaceutical
- Avoidance of triggers (prolonged standing, warm environments, emotional triggers).
- Supine position and physical counter-pressure maneuvers (leg crossing, limb and/or abdominal contraction, squatting).
- Salt: 6 to 9 grams/day
- Fluids: 2 to 3 liters/day
Pharmaceutical
- Midodrine
- Fludrocortisone
- Beta-blockers (in patients > 42 yo)
Recommendations for neurogenic orthostatic hypotension
Non-pharmaceutical
- Acute free water ingestion: >240 ml
- sympathetically-driven vasopressor effect
- avoid added salt or glucose
- Physical counter-pressure maneuvers
- Isometric contraction: leg crossing, lower body tensing, forceful handgrip
- Thigh-high compression stockings (preferably also include abdominal compression).
Pharmaceutical
- Midodrine
- peripheral alpha-agonist, so may cause urinary retention, supine hypertension, piloerection, scalp tingling.
- Droxidopa
- may cause supine hypertension, headache, dizziness, nausea
- Fludrocortisone
- may cause supine hypertension.
- avoid doses higher than 0.3mg/day as they may cause adrenal suppression and immunosuppression.
- Pyridostigmine
- cholinergic side effects: salivation, abdominal cramping, urinary incontinence , etc…
- Octreotide
- helpful to reduce postprandial hypotension (reduces splanchnic blood flow)
Complications of prolonged global cerebral ischemia
- Watershed or border zone infarction.
- Global hypoxic-ischemic injury.
- Outcomes range from mild cognitive impairment to deep coma.
- Cooling (32°C to 36°C [89.6°F to 96.8°F] core body temperature) for the first 24 to 48 hours after cardiac arrest may be beneficial.
Core Body Temperature Cooling for out-of-hospital cardiac arrest (OHCA)
- When patients who suffer from out-of-hospital cardiac arrest (OHCA) [those in whom the original rhythm is pulseless ventricular tachycardia or ventricular fibrillation] are subjected to therapeutic hypothermia, there is improved survival and neurological outcome.
- Originally, it was believed the core body temperature needed to be cooled to 32 to 34 °C for 12 to 24 hours.
- Current thinking is that temperature control to 36°C (96.8ºF) is comparable to cooler temperatures (33°C [91.4ºF]), which important since colder core body temperatures were associated with adverse effects and required heavy sedation and paralysis to control shivering.
Key References
- Kelly, Adam. Cerebrovascular Disorders. American Academy of Neurology Institute. 2016.
- Nielsen N, Wetterslev J, Cronberg T, et al. Targeted temperature management at 33°C versus 36°C after cardiac arrest. N Engl J Med 2013;369(23):2197–2206. doi:10.1056/NEJMoa1310519.
- Shen W-K, Sheldon RS, Benditt DG, Cohen MI, Forman DE, Goldberger ZD, Grubb BP, Hamdan MH, Krahn AD, Link MS, Olshansky B, Raj SR, Sandhu RK, Sorajja D, Sun BC, Yancy CW. 2017 ACC/AHA/HRS guideline for the evaluation and management of patients with syncope: a report of the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines and the Heart Rhythm Society. Circulation. 2017;136:e60–e122. DOI: 10.1161/CIR.0000000000000499.