Sparky's Heart Medicine

My dog Sparky has been taking this medication for a year, but this is the first time I've checked to see what it actually does. It makes for interesting reading:

BRAND NAME: ENACARD OR VASOTEC

BACKGROUND
Enalapril is an angiotensin converting enzyme inhibitor, also called an ACE inhibitor, or simply an ACEI. In order to understand what this means, it is important to have some understanding of the body’s renin-angiotensin system, an important hormonal mechanism used in times of blood pressure drop.

The kidney is a uniquely well perfused organ, receiving approximately 25% of the blood pumped by the heart directly. Given this fact, it is not surprising that the kidney would possess special areas for the sensing blood pressure changes. In the event of a drop in blood pressure, as might occur with a significant bleed or in heart failure, the kidney’s sensors perceive this drop and release a special hormone called renin.

The healthy liver normally produces a substance called angiotensinogen that innocuously floats around in the blood in case of a blood pressure emergency. Should angiotensinogen meet up with renin, an activation reaction occurs leading to the production of angiotensin I. As angiotensin I in the blood circulates eventually into the lungs where an enzyme called angiotensin converting enzyme converts angiotensin I into angiotensin II.

Angiotensin II acts as the superhero in this time of need. It is probably the most powerful constrictor of blood vessels known. This helps re-route circulation so as to preserve blood flow to the most important organs: the brain, heart and kidney.

As you might guess, an angiotensin converting enzyme inhibitor curtails the conversion of angiotensin I to angiotensin II. Why would we want to reduce levels of a life-saving protein like angiotensin II?

One must remember that the above system evolved to protect us from blood loss due to injury as might occur in an attack from an enemy or predator. It was not designed to protect us from a more chronic blood pressure drop such as progressive heart failure. The re-routing of circulation produced by angiotensin II leads to more blood returning to the heart and more blood for the heart to pump forward. The failing heart cannot handle this extra work and will fail more rapidly.

Enalapril effectively acts as a dilator of blood vessels. This effect opens up circulation peripherally. (If one thinks of the circulation as a roadway system, this is analogous to achieving less highway congestion by opening more side streets.) Blood pressure drops and the heart has less work.