UNDERSTANDING HOW TO APPLY THE HENDERSON-HASSELBALCH EQUATION

by / Wednesday, 09 February 2011 / Published in Biochemistry

NOTE:  This is a series of notes taken from a popular review course that gives you a real-life look at how you can properly apply this equation to many medical situations.

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Henderson-Hasselback Eq’n:    pH = pk – log(B/A)

** must understand its application, but won’t be given on the USMLE exams.

when 50% is dissociated, pH = pk = maximum buffering capacity.

ex. protein with pk of 5, when do we have max buffer capacity?? when pH = 5

*** this is because the antilog cancels out.

***** best buffer capacity when pk is same as pH

Best buffer intracell = protein    Best buffer extracell = bicarb (HCO3-)

Take pK number first, then make comparison to that # of the pH (ex. pk = 3 and pH = 5, this means +2 = 99% dissociation).

Example question… pH = 5 (weak acid), change to 6… this is +1 change, giving a 90% dissociation.  This happens when we drink acidic drinks (same principle for that alkalotic water that is supposed to prevent cancers).

Another ex:  pK = 4 (weak acid), pH of 2 (is -2, ie 2 below) leads to 1% dissociation.

These numbers %’s represent a charged form for acids and neutral form for bases.

***Neutral crosses membranes more

REVIEW of concept:

1.  Application #1

– aspirin pk = 4, how to absorb more –> keep in neutral form, thus stay below the pK, at least 2 points below.

* if you dont want to absorb an acid, keep charged and thus 2 points above pK

– base: to absorb more, keep in neutral form, keep 2 points above pH

Ex. pt on an acid, he is toxic on this acid now… Thus he must have mixed it with another acid (same principle for bases).

REMEMBER – acid dissociates becomes charged, base dissociates becomes neutral

******IF A DRUG IS TO BE TAKEN ON AN EMPTY STOMACH, IT IS AN ACID.

******IF A DRUG IS TO BE TAKEN WITH FOOD, IT IS A BASE (food prevents too much acid and thus base

Stomach 1-2  Duodenum 3-5  early jejunum 5-7  late jejunum 7-9  (these are pH’s)

2. Mortality drops after taking an aspirin post-MI, b/c is quickly absorbed in stomach

3. Cinchonism (ringing of the ears) – due to aspirin toxicity, b/c it was probably mixed with another acid.  **Should not take aspirin with orange juice (too much acids can cause aspirin toxicity).  Don’t drink it with any acidic juices.

THEREFORE:  If you want to absorb more acid, mix it with an acid.  If you want to absorb more base, take it with more base.   AND VICE VERSA.

Q: Someone ingests toxic acids, what to do?? Give him base –> milk, alka seltzer, baking soda

Someone ingests base, what to do? Give him acids –> juice

CAUSTIC INGESTIONS – strong acids or strong bases mixing is caustic… Reason: reaction is violent and releases lots of heat (especially mixing a strong acid into a strong base – remember this with: never PEE into the water).

The worst type of ingestion is thus a strong base.  Instead of giving acid, scope these people because we want to see what happened when this strong base mixed with the strong acid of the stomach.  Worried about esophageal perforation and bleeding (acutely), chronically worried about scarring/stricture formation.

***strong bases are in drain cleaners (liquid plumr, drano, hair relaxers).

***what to do when we can’t give another substance to decrease absorption?  Flush with lots of water because we want to dilute it and thus prevent its movement into the cells (prevents concentration gradient). This will minimize the damage, and the cells will eventually regenerate.

When they say “take on an empty stomach“, you are taking an acid –> mixing acid drug with acid in stomach = better absorption.

When they say “take with food“, we raise stomach pH and the drug given was a base (neutralizes base and gets absorbed faster).

Tetracycline/quinolone (acids) –> if pt isn’t getting better on these meds, the best assumption is that he’s taking it with food (basic), and thus the medication is not absorbed and he doesnt get any better.

Giving base taken with no food –> remains charged in stomach,

ENTERIC COATED ASPIRINS (wrapped with a basic coating – prevents stomach pain)

– the base that wraps it is NH3+, this remains charged in stomach acid and is not absorbed here

– once reaching the jejunum, it is more basic and the NH3+ charge goes away, allowing the basic coating to absorb here, and the aspirin is then able to bind to the wall of the intestines and get absorbed there!

ALCOHOL – is an acid, thus increases drug absorbtion, this is why we shouldnt mix drugs and alcohol.

Question: someone ingests something we dont know what it is… what do we do???

GIVE them activated charcoal –> WHY?

WHY? –> is a base, it will save 90% of all patients who took a drug (b/c most drugs are acids)… This pt will die if we do nothing, thus can take this chance and give the activated charcoal.

Aspirin causing GI problems? Give them a little bit of food, which increases pH and will help them absorb less.

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One Response to “UNDERSTANDING HOW TO APPLY THE HENDERSON-HASSELBALCH EQUATION”

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