Cardiac Disease & Summer Considerations
Julia Shih, VMD, DACVIM (Cardiology)
Veterinary Specialty & Emergency Center
Cardiac disease in veterinary patients is not confined to any season; however, special considerations are necessary to maintain the comfort and stability of our cardiac patients in the hot summer months. Heat and humidity affects dogs and cats more than people due to differences in how they are able to cool themselves. Unlike humans who shed large amounts of heat through sweat, dogs and cats do not have sweat glands all over their body. Instead they release heat primarily by panting and by sweating through their foot pads.
Both panting and sweating cools by promoting evaporative heat loss across a wet surface, be it the tongue and oropharynx or the footpads. Through these means, animals move large amounts of heat out of the body. The degree of cooling is dependent on the water vapor pressure gradient between the wet skin and the surrounding surface, thus less evaporative cooling occurs in high humidity. Cooling also occurs through convection where heat delivered to the skin is lost due to the temperature gradient between the animal and the environment. In normal animals, the heat that is naturally produced by metabolism and other bodily functions is easily released; however, as the environmental air temperature rises, animals have more difficulty shedding this heat to the environment. The internal body temperature will begin to rise in animals that cannot effectively expel heat.
Cardiac patients are predisposed to heat related conditions such as heat stroke because they have impaired heat dissipation compared to dogs without cardiac disease.
There are several potential factors which may play a role in this impaired heat loss:
- Limited Panting
Panting is the primary means for cooling; however, panting must be paused for dogs to breathe regularly. Cardiac patients who may require deeper or more rapid respirations to obtain adequate oxygenation may be unable to pant as effectively and thus are not able to dissipate heat as well.
- Decreased Blood Volume
Cardiac patients are at a greater risk of dehydration in hot weather due to the increased evaporative losses and concurrent medical therapy from diuretics and angiotensin converting enzyme inhibitors which prevent fluid retention. The evaporative losses and the fluid loss from medications results in a drop in blood volume from the central veins. This requires the heart to increase heart rate and contractility to sustain adequate cardiac output to maintain appropriate circulation and blood pressure. In extreme temperatures, the increased work that is imposed on the heart can tax patients with compromised cardiac function.
- Increased Respiratory Work
Animals with cardiac disease experience increased work of the intercostal musculature to maintain oxygenation in warmer temperatures. This respiratory work generates more heat in tachypneic or dyspneic patients and can promote heat stroke.
- Impaired Peripheral Vasodilation
In humans with stable heart failure, tolerance to heat stress is impaired due to a marked decrease in cutaneous vasodilatory capacity. Normal vasodilatory responses are important because they maintain normal skin blood flow which is necessary to promote heat loss via convection. It has been suggested that vascular remodeling may limit the ability for the peripheral vasculature to vasodilate during hyperthermia and may be a potential mechanism for heat intolerance in veterinary patients with congestive heart failure. In addition, cardiac patients are prone to dehydration which limits blood flow to the skin and results in progressive increases in internal temperature.
Cavalier King Charles Spaniels are prone to chronic valvular disease which may progress to congestive heart failure. When the temperature is hot, limiting outdoor activity to the early morning or late evening when the temperature is cooler may help limit the potential for heat retention and heat related complications
In cardiac patients, heat syncope or weakness can develop when these patients are exposed to excessive heat. Syncope is the transient loss of consciousness resulting from a reduction in cerebral blood flow. Heat syncope occurs due to marked vasodilation and pooling of blood which limits venous return to the heart and results in a drop in cardiac output. Concurrently, the cutaneous vasodilation may increase vagal tone, triggering bradycardia and worsening cardiac output. This results in hypotension and subsequent syncope. Cardiac patients are more prone to dehydration and are thus at a greater risk of heat syncope.
Because of these abnormalities, cardiac patients may be more prone to decompensation in hotter temperatures. It is important to instruct owners to limit activity in cardiac patients because they cannot dissipate heat well and even normal activity can be harmful. These patients should avoid exercise in the heat and instead should be walked in the early morning or late evening when the temperatures are cooler. Access to water must be provided at all times. Cardiac patients should be kept in cool areas within the house and air conditioning is one of the best ways to achieve this.
As clinicians we can also limit heat stress in our patients by avoiding muzzles in our cardiac patients, if possible, as panting is necessary to expel excess heat and maintain a normal internal temperature. With recognition of the physiologic limitations that are present in our cardiac patients, we can take actions to maintain their comfort and stability in the hot summer months.
Julia Shih is a Board-Certified Cardiologist at the Veterinary Specialty & Emergency Center. The Veterinary Specialty & Emergency Center operates state-of-the-art emergency and specialty veterinary hospitals that are open 24/7/365 in both Levittown PA and Philadelphia PA. For more information about our world-class emergency and specialty care, please visit VSEC on the web at www.VSECVET.com.
Cui J, Arbab-Zadeh A, Prasad A, Durand S et al. Effects of heat stress on thermoregulatory responses in congestive heart failure patients. Circulation 2005;112(15):2286-2292.
Green DJ, Maiorana AJ, Siong JH, Burke V et al. Impaired skin blood flow response to environmental heating in chronic heart failure. Eur Heart J 2006;27(3):338-343.