A Study of Circadian Rhythm and Meteorological Factors Influencing Acute Myocardial Infarction

Keywords : Acute myocardial infarction (AMI) - AMI and meteorological parameters -

AMI and surface pressure - AMI and solar activity

Acute myocardial infraction (AMI) is one of the leading causes of death in adults. The last two decades have witnessed rapid strides in the management of acute myocardial infarction. Consequently there is a significant reduction in the mortality from AMI and most of the death from infarction now occur before medical care is available. This realization has led to a renewed interest in primary prevention and the probable triggering factors in the causation of AMI. Several major studies have demonstrated a circadian (diurnal) variation in AMI incidence with a peak in the morning hours (Master 1960; Muller et al. 1985; Gilpin et al. 1990; Ridker et al. 1990; Hansen et al. 1992; Willich 1990; Levine et al. 1992; Mulcahy et al. 1988; Lucente et al. 1988).

Prominent and reproducible temporal patterns occurring regularly over twenty four hour periods are referred to as circadian rhythms. Many physiological processes exhibit circadian rhythm. A few prominent examples include sleep and wakefulness, blood pressure, growth hormone release etc. (Ganong 1989). This normal circadian rhythm is influenced by both internal control and external factors. The observed diurnal variation in acute coronary syndromes had been attributed to a complex interplay between internal and external factors (Rocco 1990).

Recognition of a circadian rhythm in the onset of acute cardiovascular events suggests the need for enhanced pharmacological protection during the vulnerable periods and provides clues to the mechanisms, the investigation of which may ultimately lead to improved methods of prevention (Muller et al. 1989).

Man is not immune to his meteorological environment. The possible relationship between sudden changes in weather and the development of diseased states has always generated curiosity among physicians and meteorological scientists. There has, however been a paucity of data concerning the association between changes in weather and the development of AMI.

The present study is an attempt to define the circadian variation in symptom onset of AMI with reference to possible modifying factors. The study also examines the correlation between a few meteorological factors, solar activity and the incidence of AMI. All cases of AMI admitted to a leading Institute of Cardiology in Pune, India during the 2-year period June 1992 to May 1994 were used in the study.

The meteorological conditions in the region is classified into the following four major seasons : (1) Winter (DJF : December, January, February), (2) Premonsoon (MAM: March, April, May), (3) Monsoon (JJA : June, July, August), (4) Postmonsoon (SON: September, October, November).

The surface pressure variations and associated weather activity in the region is influenced to a large extent by the seasonal migration of the Intertropical Convergence Zone (ITCZ) which follows the sun from the southern hemisphere in winter to the northern hemisphere in summer. Figure 1 gives the location of ITCZ during January (winter) and July (summer). A brief description of ITCZ is given in the following. From geometrical considerations, the latitude of 23.5° N is called the Tropic of Cancer and the latitude of 23.5° S is called the Tropic of Capricorn. The region between these two sides of the geometrical equator is called the tropical region. However, the weather systems of the two hemispheres are not geometrically fixed like the geometrical latitudes. These move with the sun. During the northern (southern) summer season, the weather systems of both hemispheres shift towards the north (south). The center of the weather systems of the two hemispheres is to the north of the geometrical equator during northern summer and to the south of the geometrical equator during the southern summer. This center of the weather systems is called the meteorological equator or the Inter Tropical Convergence Zone (ITCZ) (Asnani 1993). ITCZ is a region of low pressure associated with cloudy conditions and rainfall. The northward movement of ITCZ into the Indian region from May to July is associated with summer monsoon onset and rainfall. The southward migration of ITCZ from July to November heralds the gradual withdrawal of the summer monsoon over India. The average meteorological conditions during the four seasons are given below.

Figure 2: Climatological normals (1931-1960) for meteorological parameters

The meteorological data used for the study, namely, maximum and minimum temperatures, relative humidity and surface pressure were obtained from India Meteorological Department.

Data relating to cosmic ray index (Climax neutron monitor pressure), geomagnetic aa index, grouped solar flare counts, smoothed sunspot number and 2800 Mhz (10.7 cm) solar flux data were obtained from the publication SOLAR-GEOPHYSICAL DATA, NOAA National Geophysical Data Center, Boulder, Colorado.

The complete data set (monthly totals/averages) used for the study is given in Table 1.

Figure 4 shows the frequency of occurrence of AMI for the above defined four quarters of the day. The maximum number of infarcts (38.7%) occurred between 6 a.m. and 12 noon and this was found to be statistically significant.

Figure 4: Circadian rhythm of AMI - four quarters of the day

The hourly frequency (annual) of occurrence of AMI shown in Figure 5a revealed the occurrence of a smaller peak in the late evening in addition to the morning peak. This double oscillation in the circadian rhythm of AMI is seen in the seasonal averages also (Figure 5b). The circadian rhythms during the four seasons are significantly correlated to the annual mean circadian rhythm (Table 2). The frequency of occurrence of AMI exhibits a broad maximum with peak at 10 a.m. and a smaller narrow peak at about 10 p.m.

Figure 6: Seasonal variation of AMI

The MILIS study and the study by Fogel and Righthand (1964) failed to show any seasonal variation in incidence.

The most important factor responsible for modifying the onset of myocardial infarction may be the sudden changes in weather (Al Yusuf 1986). The sudden shifts in weather were thought to overtax vasomotor mechanism producing an acute myocardial infarction.

The present study failed to show any statistically significant circ-seasonal rhythm in AMI incidence. Poona has comfortable weather conditions for most parts of the year irrespective of the season and this absence of marked seasonal variation may be responsible for this.

In order to investigate the association ,if any, between AMI and solar-geophysical parameters, graphs were drawn between monthly total AMI incidence and month-to-month variation of mean (a) surface pressure (Figure 7a) (b) cosmic ray index (Figure 7b) and (c) solar flare counts(Figure 7c). Figure 7d shows the month-to-month variation of AMI cases and total solar flare counts. It is seen that AMI incidence follows closely (positive correlation) the month-to-month variation of mean surface pressure, cosmic ray index and solar flare counts(Figures 7a to 7c).On the other hand ,AMI incidence shows a negative association(correlation) with total solar flare counts(Figure 7d).Statistical correlation coefficients were computed and shown in Table 3 for monthly total AMI incidence and month-to-month variation of mean surface pressure, cosmic ray index and solar flare counts(Figure 7a to 7c).Table 3 also shows the correlation coefficient between monthly total AMI incidence and total solar flare counts.

Figure 7a

Figure 7b

Figure 7c

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Figure 7d

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Figures 8a and 8b

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Incidentally, it is now established that surface pressure fluctuations are closely associated with cosmic rays and solar events such as solar flares, sunspots etc. (Herman and Goldberg 1985).

The close association between month-to-month variability in mean surface pressure and AMI incidence was also reported in a similar study in Bombay , India (Rodrigues and Pinto 1982) where the surface pressure changes and the associated AMI incidence were attributed to the movement of ITCZ (Inter-Tropical Convergence Zone). Teng and Heyer (1955) give reference to observed correlation between number of deaths from cardiovascular diseases and barometric pressure. Kveton (1991) reported that the association between the incidence of AMI and weather fronts were due to the variations in surface pressure.

Though a statistically significant association between AMI and surface pressure variation was demonstrated in the present study, it might not amount to causal association. Larger prospective studies over longer periods are mandatory before causality can be attached.

2. There was no statistically significant seasonal or monthly variation in AMI. Significant correlation was found between the monthly incidence of AMI and month-to-month variation in surface pressure ,cosmic ray index and solar flare counts. The pattern of incidence was seen to be modified by full moon and new moon. No significant correlation was seen with the other meteorological parameters like temperature and relative humidity.