MadSci Network: Anatomy
Query:

Re: Does blood pH affect heart rate, and, if so, how?

Date: Tue Apr 11 14:33:50 2006
Posted By: Amit Agarwal, Grad student, Neuroscience/Neurogenetics,
Area of science: Anatomy
ID: 1141092705.An
Message:

Dear Casey, 
It’s indeed an interesting question, which still keeps many
scientific minds busy. Many researchers working in the field of cardiac
biology are trying to understand detailed electrophysiological and
neurochemical mechanisms which can affect heart rate and its efficiency.
There are several studies available on rodents which outline factors
responsible for the maintenance of normal heart rate. Your question also
targets one of the aspects of such studies. Several studies have shown that
acidosis can lead to reduced heart rate. So before going ahead let’s
understand what is acidosis? 
When breathing is restricted, the body cannot eliminate carbon dioxide as
it should, and the amount of carbon dioxide in the blood increases. Carbon
dioxide (CO2) and water (H2O) exist in equilibrium with carbonic acid
(H2CO3) in a reaction catalyzed by an enzyme called carbonic anhydrase.

CO2 + H2O --> H2CO3 

A molecule of carbonic acid dissociates on its own into a molecule
bicarbonate (HCO3-) and an (acidic) hydrogen ion. (H+) 

H2CO3 <> HCO3- and H+ 

Thus the overall pattern is: 

H2O + CO2 <> H2CO3 <> HCO3- + H+ 

Therefore, if breathing is restricted, CO2 builds up and the reaction
shifts to the right in an attempt to balance things out, ultimately making
the blood more acidic and thus decreasing its pH. This is called
respiratory acidosis. As you can see, either a build-up in the blood of
carbon dioxide or a decrease in the blood of oxygen will cause the pH of
the blood to fall. If both occur at the same time, as they do in cases of
suffocation, the pH of the blood will plummet to life-threatening levels
within a very few minutes. The pH of normal human blood is in the 7.35 to
7.45 range (slightly alkaline). A pH falling to 6.9 (or raising to 7.8) is
"incompatible with life." 
Respiratory acidosis produces a marked decrease in heart rate and an
increase in P—R
Interval with little apparent effect on the duration of the QRS complex
(Abbera et.al.,2000). Acidosis alters the electrical activity of cardiac
muscle, having noticeable effects on most of the membrane currents that
have been studied and hence on the configuration of the action potential.

For details you can refer following reference:

The effect of acidosis on the ECG of the rat heart
A. Aberra, K. Komukai, F. C. Howarth and C. H. Orchard.
Experimental Physiology 2001;86;27-31

Hopefully this reference will help to answer some of your technical and
more advanced questions. Since I am not specialist of this subject I
couldn’t give you more detailed explanation.
 





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