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Altitude and atrial fibrillation.



To the Editor: A 56-year-old man described a 9-year history of intermittent palpitations. This was confirmed to be recurrent paroxysmal paroxysmal (per´ksiz´ml),
adj recurring in paroxysms.
 atrial fibrillation. The episodes would typically last for hours and were well tolerated, with heart rates up to the 120s. He described the episodes as not being related to any specific activity, food, or mental state. A healthy lifestyle was noted that included regular exercise. The patient's medical history, family history, review of systems, and physical examination were unremarkable except that his father had coronary heart disease coronary heart disease: see coronary artery disease.
coronary heart disease
 or ischemic heart disease

Progressive reduction of blood supply to the heart muscle due to narrowing or blocking of a coronary artery (see atherosclerosis).
 and congestive heart failure congestive heart failure, inability of the heart to expel sufficient blood to keep pace with the metabolic demands of the body. In the healthy individual the heart can tolerate large increases of workload for a considerable length of time.  and died at 89 years of age. Pertinent laboratory data includes a normal free T4, thyroid-stimulating hormone, and chemistry profile. M-mode and two-dimensional echocardiography revealed all valves, left ventricle, and ejection fraction to be normal. The left atrium was at the upper limits of normal, at 3.56 cm (normal, 1.9 to 4.0 cm). A stress echocardiogram ech·o·car·di·o·gram
n.
A visual record produced by echocardiography.


Echocardiogram
A non-invasive ultrasound test that shows an image of the inside of the heart.
 was normal with the exception of an episode of rapid atrial fibrillation during the later part of the exercise period, which resolved several minutes into recovery phase.

The patient lived at an altitude of 1,008 m (3,300 feet) until 2 1/2 years ago, at which time he moved to an altitude of 2,750 m (9,000 feet). The recurrent episodes of palpitations disappeared after the move. No episodes were noted until a 7-week stay at an altitude of 763 m (2,500 feet), which occurred 10 months after the move to the higher altitude. The palpitations then returned and were confirmed to be atrial fibrillation. After the 7-week stay, the patient returned to live at 2,750 m and the palpitations again went away completely. Ten months later, after no evidence of recurrent atrial fibrillation, the patient again returned to an altitude of 763 m. The recurrent episodes of palpitations returned and were again confirmed to be atrial fibrillation. After a 3-month stay, he returned to the 2,750-m elevation and the recurrent palpitations resolved completely. The initial testing was done at an altitude of 1,008 m.

The fact that the recurrent atrial fibrillation resolved specifically when the resident altitude increased and reappeared when resident altitude returned to a lower level, on several occasions without any other variables, suggests an association with resident altitude. Lone atrial fibrillation lone atrial fibrillation Cardiology A Fib in a Pt < age 60, in absence an underlying pathology–eg, DM, CHD, HTN. See Atrial fibrillation.  describes atrial fibrillation in the absence of demonstrable underlying cardiac disease or history of hypertension. (1) This patient had no obvious preexisting pre·ex·ist or pre-ex·ist  
v. pre·ex·ist·ed, pre·ex·ist·ing, pre·ex·ists

v.tr.
To exist before (something); precede: Dinosaurs preexisted humans.

v.intr.
 or coexisting medical conditions that could be associated with atrial fibrillation, which placed him into a category of lone atrial fibrillation. To approach this situation, it seems appropriate to examine the effect of altitude on the cardiovascular system.

Pulmonary hypertension is usual on ascent to higher altitude in the normal cardiovascular system. The cause of pulmonary hypertension is hypoxic pulmonary vasoconstriction Hypoxic pulmonary vasoconstriction is a physiological phenomenon in which pulmonary arteries constrict in the presence of hypoxia (low oxygen levels) without hypercapnia (high carbon dioxide levels), redirecting blood flow to alveoli with higher oxygen tension. . Studies involving inhabitants of high altitude locations have revealed an increased prevalence of right ventricular hypertrophy right ventricular hypertrophy Cardiology An ↑ in myocardial mass which may be due to interventricular septal defects or ↑ blood flow–eg, hyperthyroidism  and pulmonary hypertension. The following conclusion was made concerning the study of natives at high altitude in the Himalayan region. Echocardiographic examination showed normal left ventricular function in all, whereas 27% of the natives at an altitude of 4,500 to 5,000 m had evidence of pulmonary hypertension and rise of normalized right ventricular pre-ejection period. (2)

A study made the following observations at high altitude. Stroke volume falls and pre-ejection period to left ventricular ejection time left ventricular ejection time
n.
Abbr. LVET The time measured from onset to notch of the carotid pulse.
 ratio rises at 3,100 m because of diminished venous return despite preservation of left ventricular systolic Systolic
The phase of blood circulation in which the heart's pumping chambers (ventricles) are actively pumping blood. The ventricles are squeezing (contracting) forcefully, and the pressure against the walls of the arteries is at its highest.
 performance. (3)

Also noted is a decrease in plasma volume reflected by a rise in hematocrit, which in part explains the finding of a decreased venous return.

Considering the above facts, it is possible to propose the following mechanism for the above-described case. Hypoxic pulmonary artery vasoconstriction vasoconstriction /vaso·con·stric·tion/ (-kon-strik´shun) decrease in the caliber of blood vessels.vasoconstric´tive

va·so·con·stric·tion
n.
 secondary to increased resident altitude would contribute to a decrease in venous return, along with altitude-associated decreased plasma volume. This would reduce left atrial size by decreasing atrial wall tension, thereby eliminating a trigger for atrial fibrillation.

The above deduction is proposed from the facts listed. More studies would be needed to further examine mechanisms for this association.

References

1. Falk RH. Atrial fibrillation. N Engl J Med 2001;344:1067-1076.

2. Sharma S. Clinical, biochemical, electrocardiographic electrocardiographic

emanating from or pertaining to electrocardiography.


electrocardiographic monitoring
maintenance of a more or less continuous surveillance of a patient's cardiac status by means of electrocardiography.
 and noninvasive hemodynamic he·mo·dy·nam·ics  
n. (used with a sing. verb)
The study of the forces involved in the circulation of blood.



he
 assessment of cardiovascular status in natives at high to extreme altitudes (3000m-5500m) of the Himalayan region. Indian Heart J 1990;42:375-379.

3. Alexander JK, Grover RF. Mechanism of reduced cardiac stroke volume at high altitude. Clin Cardiol 1983;6:301-303.

V. Taylor Smith, MD

Lake City, CO

Retired, Department of Internal Medicine

Covenant Medical Group

Lubbock, TX
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Author:Smith, V. Taylor
Publication:Southern Medical Journal
Article Type:Letter to the Editor
Date:Jan 1, 2005
Words:761
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