Cold weather increases the risk of heart attacks and strokes in people with high blood pressure, with people over 60 being at greatest risk.
In Delhi, hospital admissions for heart attacks rose 15 to 30 per cent in December 2006 as compared to June.
In Mumbai, said Dr Harish Mehta, consulting cardiologist at PD Hinduja Hospital, "There is a rise in the number of heart attacks during winter."
Pollution combined with the cold was one of the main reasons. "It constricts blood vessels and the aged become more prone to heart attacks," said Dr Mehta.
Dr Naresh Trehan, director, Escorts Heart Institute, added: "In the winter, the body requires more blood to maintain body temperature. People with marginal heart disease can suddenly develop symptoms or have an attack."
Dr OP Yadava, CEO and chief cardiac surgeon at the National Heart Institute in Delhi, explained that for every degree that the temperature drops, the upper blood pressure (systolic) increases by 1.3 mm of mercury. The lower blood pressure (diastolic) increases by 0.6 mm.
The cold also activates the sympathetic system — responsible for the 'fight or fright syndrome' — that increases the secretion of adrenaline and nor-adrenaline, further heightening blood pressure and the heart rate, and raise the body's oxygen demand.
The solution? "You can protect yourself by walking in the sun and vaccinating yourself against viral infection," said Dr Ashok Seth of Max Heart and Vascular Institute.