cardiac physiology

My heart skip skips a beat


The pause is to allow the atria to fully empty into the ventricle.

Heartbeat on an ECG trace

P Interval (Ventricular Diastole)

  • Atria and ventricles are relaxed
  • blood is flowing into the atria from the veins. 
  • Atrial pressure increases above that of the ventricle, AV valves open allowing blood to flow into the ventricle

P Wave (Atrial Systole) P-Q

Signal transduction from SA to AV nodes. 

  • SA node fires 
  • Atria contract causing atrial systole 
  • which forces all blood into the ventricles
  • emptying the atria.

Q Interval (End of Ventricular Diastole)

Depolarisation of interventricular (IV) septum 

  • AV valves remain open - all remaining blood squeezed into the ventricles. 
  • impulse from the SA node reaches the AV node 
  • which spreads the signal throughout the walls of the ventricles via bundles of His and Purkinje fibres
  • R peak is the end of ventricular diastole and the start of systole.

R Interval (Ventricular Systole)

Ventricular contraction

  • All blood is now within the ventricles
  • so pressure is higher than in the atria - AV valves close
  • ventricles start to contract although pressure is not yet high enough to open the SL (semilunar) valves

ST Segment (Ventricular Systole)

Ventricular contraction

  • Pressure increases until it equals Aortic pressure,
  • SL valves open
  • blood is ejected into the Aorta (and pulmonary artery) as ventricles contract
  • At this time the atria are in diastole and filling with blood returning from the veins.
  • plateau in ventricular arterial pressure

T Wave (Ventricular Diastole)

T= moment of Ventricular repolarisation immediately before ventricular relaxation

  • Ventricles relax
  • ventricular pressure is once again less than the aortic pressure 
  • so SL valves close
Are We feeding Snakes to Death?

In the world of herpetoculture there are as many opinions on how to properly keep species as there are species themselves. Some of it is backed by science and facts while much of it is baseless and is just done that way because that’s how it always has been. One of the major spheres of influence comes from breeders of reptiles. The goal of reptile breeders is to produce reptiles for sale, which is their job. Therefore many breeders keep lots of animals in rack systems to conserve space and feed their animals frequently to encourage rapid growth and sexual maturation. Whether or not rack systems are wrong is the subject of a different post. What I would like to concentrate on is how we sometimes over feed reptiles, especially snakes, and what the side effects are.

It is well known that after snakes eat their metabolisms are kicked into high gear to help digest the meal. Studies in Burmese pythons show that their hearts actually grow larger in size after a meal to accommodate the increase need for oxygenation of the tissues and to supply more blood to the GI tract. I never really paid much attention to this as a clinician until I had a client who breeds several species of python tell me that his snakes just aren’t living as long as they did when he first got into the business. After spending several days pouring over all of his data (he keeps meticulous records on each snake) and running diagnostics on a few snakes and necropsies on those that had died, I only really found one main issue- he was feeding larger prey items more often starting about a decade ago. I looked into the literature and spoke with some other herp vets and found that animals that are fed more often age faster than those that eat less.

Who cares right? The difference seems only to be a few years in longevity. For breeders whose main goal is producing young to sell, it probably is insignificant. But for people that own snakes just for their own enjoyment or those in zoological institutions those extra years may really matter, especially for endangered species. I see a lot more obesity in reptiles than I used to due to over feeding so not only are we making reptiles fat which causes health problems, we are shortening their lives just be speeding up their metabolisms.

Below are a few of the articles if you would like to read them and form your own conclusion.

Eating increases oxidative damage in a reptile.
J Exp Biol. July 2016;219(Pt 13):1969-73.
Michael W Butler 1, Thomas J Lutz 2, H Bobby Fokidis 3, Zachary R Stahlschmidt 4

Effects of meal size on postprandial responses in juvenile Burmese pythons (Python molurus)
Am J Physiol. March 1997;272(3 Pt 2):R902-12.
S M Secor 1, J Diamond

The effects of fasting duration on the metabolic response to feeding in Python molurus: an evaluation of the energetic costs associated with gastrointestinal growth and upregulation
Physiol Biochem Zool. 2002 Jul-Aug;75(4):360-8.
Johannes Overgaard 1, Johnnie B Andersen, Tobias Wang

Physiology: postprandial cardiac hypertrophy in pythons
Nature. March 2005;434(7029):37-8.
Johnnie B Andersen 1, Bryan C Rourke, Vincent J Caiozzo, Albert F Bennett, James W Hicks

Respiratory consequences of feeding in the snake Python molorus
Comp Biochem Physiol A Mol Integr Physiol. November 1999;124(3):359-65.
J Overgaard 1, M Busk, J W Hicks, F B Jensen, T Wang