While enjoying summer weather, remember that warm months pose the greatest risk of dehydration. This can be particularly hazardous for patients with a history of heart disease, because dehydration can rapidly stress an already overburdened cardiovascular system.

Here's how it works.

As the temperature rises, perspiration evaporates to cool your body. You may not even notice how much fluid you are losing, but depending on your level of activity, fluid loss through perspiration can rise to one quart of fluid per hour. In high heat, perceptions can be compromised so quickly that thirst receptors don't have time to signal the need to replace lost fluids.

High humidity also makes a difference. If the atmosphere is saturated, sweat has nowhere to go and remains on the skin, so it does not carry heat away from the body. If exposed to prolonged intense heat, the part of your brain that regulates body temperature malfunctions, and the body can't cool itself properly.

People with a cardiac history are particularly vulnerable in this situation. Why? Because fluid lost as sweat comes from the blood stream, decreasing the volume of blood and throwing off the fluid/electrolyte balance, so the heart rate increases to compensate for inadequate blood flow. If the heart rate rises above certain levels, people with heart disease may be at special risk for dangerous arrhythmias.

To avoid these complications, make sure you drink at least six to eight ounces of fluid after every 15 minutes of activity, as well as two full glasses two hours before physical activity. Take frequent breaks, and when the heat is intense - stay indoors!