47 - understanding ABGs part 4

KYJ 47 - blood gas series part 4.

We measure blood gases for two reasons. First to determine pH of blood and secondly to look at gas exchange of oxygen.  Low oxygen values are called Hypoxaemia.

Normal values are
pH = 7.35 - 7.45
PaO2 = 80-100mmHg
PaCO2 = 35-45mmHg
HCO3 = 22-26 mEq

If pH drops below normal (acidosis) the cause is too much acidic CO2, or not enough HCO3.

High pH (alkalosis) is caused by too much HCO3 or not enough CO2.

Now that we have established the diagnosis of acidosis or alkalosis being purely a numbers game. Let's look
At qualifying types of each extreme

Acidosis can be called Respiratory acidosis or Metabolic Acidosis.

Respiratory acidosis means the cause is high CO2 (>45).  Remember CO2 makes blood acidic. You excrete CO2 by breathing.  So if CO2 is too high, it is a problem with ventilating it off, hence it's called a Respiratory Acidosis.

Metabolic Acidosis is caused by an underproduction of bicarb (HCO3) or an accumulation of acids in the blood eg Lactic acid, ketones, and chloride.  In these patients the pH is always low. But instead of seeing a high respiratory acid value (CO2) there is an abnormally low HCO3 value.
.....

Alkalosis
Alkalosis is the same as Acidosis but a mirror image.

Respiratory Alkalosis is a high pH with an abnormally low CO2. The patient has blown off (hyperventilated) all their CO2 (acid) leaving them alkalotic.  I like to call this the "Justin Beiber Effect".   Tweenies seeing a live Justin Beiber concert get hysterical and hyperventilate - and pass out wasting a $150 concert ticket!!

Metabolic Alkalosis is a high pH with abnormally high HCO3.  In this alkalosis the patient has retained HCO3 ( an alkali) or excreted too many metabolic acids.  A classic example is hyperemesis (vomiting) seen in early pregnancy with morning sickness.  Middle ear vertigo, sea sickness, diarrhoeal illness and diuretic use, all cause the retention of bicarb, and loss of acids (eg stomach acid).

...
So let's look at some values.(normal in brackets)

pH 7.02. (7.35-7.45)
PaCO2 55. (35-45)
HCO3 24. (22-26)

Note that pH is too low (Acidosis), and the CO2 is high (Respiratory)

This is Respiratory Acidosis- this bloke needs to breathe more to vent off his CO2.  Suspect this in sleepy shallow breathers waking up from anaesthetic.

Another:
pH 7.22 (7.35-7.45)
PaCO2 36 (35-45)
HCO3. 18 (22-26)

pH is too low ( acidotic again) but CO2 is normal.  Bicarb is too low.  There is not enough metabolic alkali (bicarb) in the blood so it becomes acidic.  This is a case of metabolic acidosis, and frequently caused by diabetics with extremely high BGLs.  They start melting fat, which produces large amounts of ketones. These ketone by products of fat catabolism cause the diabetic to develop this lethal diabetic ketoacidosis a type of metabolic acidosis .

What you may have noticed in these examples, is that I have not mentioned Oxygen values.  The reason is, that oxygen levels are are not used in the analysis of acid/base balance when interpreting ABGs.  In fact PaO2 is the last value that we are interested in. It is merely an indicator of blood oxygenation and never pH.

Look at these two next examples and have a crack at telling me what they are.

A).
pH 7.54
paCO2 =24
HCO3 = 24

And
B).
pH 7.61
PaCO2 = 42
HCO3 = 28

I'll answer these in the next post, and we will start our exploration of what Compensation means.