Revealing Arrhythmias in the Himalayas

Revealing Arrhythmias in the Himalayas

My colleagues and I had an opportunity to study the effects of high altitude on arrhythmias when Dr David Woods participated in a British Army expedition to the Himalayas.

The evaluation of patients with unexplained palpitations or syncope, as we know, can be difficult. Traditionally external Holter monitoring is often used to diagnose these events. 24hr holter monitoring is most commonly used and yet provides a very small diagnostic yield. This can be improved with the use of prolonged monitoring but is still prone to the problem of electrodes detaching from their skin placement. Prolonged monitoring or the use of external patient activated recorders, even if the electrodes are changed regularly, may still produce difficulties if the event occurs during dressing or showing etc.

Dr David Woods interrogating a reveal with Mount Everest in the background

Dr David Woods interrogating a reveal with Mount Everest in the background

Medtronic Reveal Implantable Loop Recorder - ILR

Medtronic Reveal Implantable Loop Recorder - ILR

To date there has been little study of the effects of altitude on the electrophysiological properties of the heart; empirical evidence suggests that at altitude humans are more prone to arrhythmias. A number of cases of sudden cardiac death (SCD) have been documented during high altitude expeditions and in the majority of cases this is attributed to high altitude pulmonary oedema (HAPE), but it is unknown whether the initial or final event was an arrhythmia.

Holter monitoring in extreme temperatures is fraught with technical difficulties and constraints. These include failure of the surface electrodes to maintain adequate contact, mechanical failure of the holter monitor tape-driving mechanism, and restraints of layers of clothing and temperature. An alternative way of recording arrhythmias at altitude is thus needed.

The Reveal™ Implantable Loop Recorder (ILR) (shown above) is a subcutaneous ECG monitor developed by Medtronic Ltd to aid in the diagnosis of patients with unexplained infrequent syncope. The device stores segments obtained during symptomatic events in its memory, and storage of these events occur through a manually triggered external patient activator. These events occur with an automatic storage facility in the later model.

The quality of the ECG recorded by the Reveal is demonstrated by the two examples on the following page. Two patients, whilst under the care of Southampton General Hospital and not part of the expedition, experienced several events of unexplained syncope for over 12 months, and serial external monitoring failed to uncover the cause. This is further highlighted by the examples on the following page.

Fig. 1 demonstrates a 25 second pause, which occurred 5 months post Reveal implant; the patient subsequently received a pacemaker.

Fig. 2 demonstrates an episode of Torsades de Pointes whereby the patient experienced an attack of syncope 9 months post Reveal implant; this patient received an ICD.

Figure 1

Figure 1

Figure 2

Figure 2

Studies concerned with the effects of altitude on cardiac arrhythmias is limited, these have concentrated on either repeated 12 lead surface electrocardiograms or intermittent holter monitoring. It is apparent that altitude is associated with an increased incidence of arrhythmia; however, the majority of these arrhythmias have been relatively benign in nature. It is apparent that the baseline heart rate, (both) when asleep and awake, is increased with altitude, and this is proportional to the degree of elevation. Both atrial and ventricular ectopic rhythms may be observed in healthy subjects at high altitude, and again the degree of elevation plays a role. Ectopy occurs almost always during exertion and tends to diminish with acclimatization (2.7).

In one study at altitude it was noted that a profound sinus arrhythmia occurred during sleep, and it was felt that this represented a mechanism similar to that found in patients with sleep apnoea. It is important to note that the majority of these studies have monitored the ECG during rest rather than exercise or strenuous activity. It would seem logical that most arrhythmias might occur during the latter as a result of the increased circulating catecholamines and relative hypoxia. It is clear to see why the data is scarce during strenuous activity as palpitations at high altitude have been experienced, but seldom recorded. The explorer D’Orbigny, on the crest of the Peruvian Cordilleras 1826-1834, is quoted as saying “At the least movement, I felt violent palpitations”[1]. Two other studies using single Holter monitoring up to altitudes of up to 5,895 meters showed sinus arrhythmia, occasional ectopy, and ventricular ectopics. A simulated accent to 8,448m by 8 individuals, who spent 40 days in a hypobaric chamber, showed an increase in heart rate and ventricular ectopics, but no significant arrhythmias.

Our study was to use an ILR Reveal recorder implanted in the subcutaneous tissue of 9 healthy volunteers to examine the effect of altitude on the heart during an expedition to 6325m in the Himalayas. At altitudes above 5000m all subjects reported palpitations during exercise, abnormal heart rhythms were identified in all, and significant ischaemia in one.

The partial pressure of oxygen at Everest base camp (5300m) is half that at sea level, a fact manipulated by the Levy test of the 1940s, which simulated the hypoxic environment of 5500m as a diagnostic tool for coronary artery disease [2]. Hypoxia, sympathetic activation, and alkalosis of altitude predispose to cardiac ischaemia and arrhythmia.
Indeed, sudden cardiac death (SCD) accounts for 30% of all deaths during mountain sports at altitude; 642 SCDs over 8 years in Austria alone[3].

Despite these factors a paucity of data exist due to the logistical difficulties of monitoring the ECG at altitude. Early pioneers recorded interference from the static electricity generated by flapping nylon tents in the high winds on Makalu.

Current conviction that maximal exercise at high altitude is not accompanied by significant ischaemia or arrhythmia is primarily based on data from a simulated ascent of Everest i.e. resting ECGs on 8 subjects at 5 altitudes and exercise ECGs at two altitudes[2]. This accepted wisdom seems incongruous with the 10% of fatalities while trekking in Nepal that are due to heart attacks [4], or the 642 SCDs over an 8 year period in the Austrian Alps[3].

We therefore sought to establish the previously undetermined nature of palpitations at high altitude and the incidence of ischaemia and arrhythmia using an implanted ECG recorder during ascent to high altitude. Nine healthy male volunteers, aged 29.9 ± 5.2 years, had a normal cardiovascular examination, a normal 12-lead ECG, a normal 2-D cardiac echo, and achieved at least stage 5 of the Bruce protocol without any abnormality pre-expedition.

Implantable loop recorders (ILR, Reveal, Model 9525 Medtronic, USA) were inserted subcutaneously in the left pectoral region under local anaesthesia 6 weeks prior to departure. Subjects flew to Kathmandu (1250m) and then Lukla (2800m) before immediately commencing an identical ascent and descent profile. All subjects reached intermediate camp at 5600m (day 15), six members attained 5700m, four members attained 6070m and two members attained 6325m. No subjects used prophylactic medication against acute mountain sickness. The ILR was activated remotely using an electromagnetic induction unit that ‘freezes’ the loop of ECG recording, which is then stored in the memory ‘bins’ of the unit. Data was downloaded to diskette daily from the ILR using a pacemaker programmer (Model 9790, Medtronic) driven by an altitude modified petroleum generator.

The ILR was programmed alternately to record 3 activations (6 minutes pre, 1 minute post) or 1 single activation (40 minutes pre, 2 minutes post).

Devices were activated during episodes of palpitations, and during exercise, rest and sleep. Arterial oxygen saturation was also assessed concurrently during device activations. 263 ECG recordings were made during the expedition, 29.2 ± 2.6 per subject. Analysis of R-R intervals at increasing altitudes demonstrated a progressive increase in mean heart rate both during exercise, at rest, and also mean maximum achieved heart rate during exercise (98.6 ± 24.2, 106.7 ± 27.8, 144.1 ± 13.7, and 152 ± 23.8, mean±sd, at 2-2999m, 3-3999m, 4- 4999m, 5-5999m respectively).

All subjects experienced palpitations during exercise above 5000m with 2 symptomatic episodes at rest (5,600m and 6300m).

Figure 3: Atrial Flutter post exercise at 4500m, oxygen saturation 76%

Figure 3: Atrial Flutter post exercise at 4500m, oxygen saturation 76%

Figure 4: Atrial flutter 1:1 conduction on exercise at 4,600m, oxygen saturation 87%

Figure 4: Atrial flutter 1:1 conduction on exercise at 4,600m, oxygen saturation 87%

Figure 5: Sinus arrhythmia/non conducted p -wave during sleep at 4,300m, oxygen saturation 84%

Figure 5: Sinus arrhythmia/non conducted p -wave during sleep at 4,300m, oxygen saturation 84%

Figure 6: Repolarisation Abnormality. Significant ST segment depression seen at 6300m (59% oxygen saturation).

Figure 6: Repolarisation Abnormality. Significant ST segment depression seen at 6300m (59% oxygen saturation).

Analyses of all symptomatic recordings were found to correlate with sinus tachycardia. In one subject an episode of asymptomatic atrial flutter with 2:1 conduction was observed for 8.5 minutes immediately after a period of severe exertion at 4500m (SaO2 76%).

Sleep recordings revealed sinus arrhythmia in all individuals with non-conducted ectopic p waves in one subject. Another subject demonstrated sinus arrhythmia during exercise (not previously reported at altitude). In one individual, during exercise at 6300m, there was clear evidence of a change in repolarisation associated with dramatic ST segment depression (SaO2 59%). It is possible that these repolarisation changes represent cardiac ischaemia.

In two tracings derived from asymptomatic individuals during exercise, it was not possible to clearly identify the underlying cardiac rhythm. In both cases analysis of the apparent R-R intervals identified a frequency of 300 beats per minute consistent with a diagnosis of atrial flutter with 1:1 conduction. It is possible that these tracings represent artifact but extensive provocation during both rest and exercise post-expedition failed to reproduce such a pattern.

This expedition demonstrated that the implantable loop recorder is effective for recording ECGs even under extreme conditions and is more favourable than conventional holter monitoring for recording the possible arrhythmic effects of high altitude. The ILR demonstrated abnormal heart rhythms in all subjects and significant repolarisation changes in one. Above 5,000m all subjects reported palpitations during exercise that were associated with sinus tachycardia.

Current evidence regarding the risk of ischaemia and arrhythmia at high altitude is based on a paucity of data and is incongruous with the rate of SCD. Further evaluation is required, particularly in the elderly who account for 15% of the 100 million visitors to high altitude annually. These figures are of real concern due to the fact that up to 60% of elderly individuals in Western societies have significant coronary lesions at autopsy [5].

Acknowledgements:

  • Dr Paul Roberts ,Consultant Cardiologist and Electrophysiologist, Southampton General Hospital
  • Dr David Woods, Consultant in Endocrinology and Diabetes, Northumbria and Newcastle.
  • The members of the expedition from the Army Training Regiment, Lichfield, UK.

References:

  1. Bert P. La Pression Barométrique, recherches de physiologie expérimental. Masson, Paris (1878). English translation (1943) by Hitchcock MA and Hitchcock FA, College Book Co., Columbus, Ohio, p37.
  2. Malconian M, Rock P, Hultgren H, et al. The electrocardiogram at rest and exercise during a simulated ascent of Mt. Everest (Operation Everest II). Am J Cardiol. 1990; 65: 1475-1480.
  3. Burtscher M, Mittleman MA. Time-dependent SCD risk during mountain sports changes with age. Circulation. 1995; 92: 3151-3152.
  4. Shlim DR, Gallie J. The causes of death among trekkers in Nepal. Int J Sports Med. 1992; 13: S74-6.
  5. Levine BD, Zuckerman, JH, deFilippi CR. Effect of High-Altitude Exposure in the Elderly. Circulation. 1997; 96: 1224-1232.

 

Author:

Stuart Allen
Principal Cardiac Physiologist
Manchester Heart Centre
Manchester, UK

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