The Bodies Acid Base Balance (Aka PH Level): Basic Concepts

A recent thread here In the ATS Medical Consp. Forum titledYour Ph balance and diseases, cancer and aging raised the question of PH and how it effects the body. Below you will find the text of a basic Arterial Blood Gas primer. In addition to my role as a transport specialist I am an instructor with the hospitals department of education. I use this both as an education poster and as part of a combined lecture regarding labratory valuse.

Arterial Blood Gases measure among other things the bodies Ph.
(Ill admit its a bit dorkey, but its presented to the new graduate nurses in color with lots of diagrams and pictures and I have found it perks them up after beeing lulled to death by endless power point lectures)




ABG ABC's

Kirk: Captain’s Log Star Date 97687.9 We have just entered the Acidosis and Hypoxia Zone.......
Scotty: Captain she’s just not oxygenating She canna stand the strain.
McCoy: I’m just a doctor Jim not a miracle worker.
Kirk: Spock what should I do?
Spock: Logically Captain you have no choice. You must check a STAT Blood Gas.......

Our good friend Mr. Spock is right (For you non Trekers out there, he’s the dude with the pointed ears), we need to check a blood gas. No matter if you don’t trust the reading you are getting off of the transcutaneous monitor, or just taking a “look see” to see if intubation is required, the arterial blood gas is the way to go. One small (easy too if you have an arterial line) can yield volumes of data. Furthermore.....

Spock. Logically you should have got to the point by now. If you don’t I shall be forced to use the Vulcan Death Wedgie on you....
Alright already. Now on with the Blood Gas........

In order for the body to achieve homeostasis, a highly complex balancing act must occur. The body must balance out both acids and base in order to keep the pH between 7.35-7.45. Okay let’s take a look at the individual components.

When you get your blood gas back the first item on the list is the pH. PH is the negative logarithm of hydrogen (H+) ion concentration in the blood. Where do these nasty little buggers come from? They are either created by volatile acids (flux between a liquid and gaseous state) or nonvolatile acids (lactic acids, ketoacids, etc.). There are like a million reasons for a person to be either acidotic or alkalotic.

The Partial Pressure of Carbon Dioxide (PCO2) is the measurement of the pressure (or tension) exerted by CO2 that is dissolved in the blood. This is roughly proportional to the levels of CO2 in the alveoli. The levels of CO2 in the blood and the cerebral spinal fluid have a direct effect on alveolar ventilation. When CO2 levels rise, alveolar ventilation will rise exponentially.

When we talk about the Partial Pressure of Oxygen (PO2), we are referring to the force that oxygen exerts when attempting to diffuse through the pulmonary membrane. Or it reflects the amount of PCO2 passing through the pulmonary alveoli into the blood. The amount of inspired oxygen will directly effect this value.

Sodium Bicarbonate (NaHCO3-) and Base Excess / Deficit go hand in hand. First things first: A positive value indicates a base excess, a negative value indicates a base deficit. This calculated value allows you to treat acid-base imbalances that are non respiratory in nature. It is also referred to as the whole blood base and is the sum of the concentration of the buffer anions (mEq/L) in the blood (This includes the bicarbonate ion (HCO3-) in the plasma and red blood cells, hemoglobin, plasma protein, and phosphates.).

The Alveolar to Arterial Oxygen Gradient (A-aDO2) is a test designed to approximate the concentration of oxygen in the alveoli and the arteries. This test can help spot V / Q mismatch (Ventilation of the alveoli with no perfusion or perfusion of the alveoli with no ventilation, or alveolar collapse). Increased values may indicate mucus plugs, bronchospasm, asthma, and bronchiolitis. Hypoxemia seen with an increase in A-aDO2 difference is seen with an atrial septal defect, pneumothorax, atelectasis, emboli, and edema.

Compensation (No, not the monetary type) is the body’s response to changes in the overall pH. If one part of this complex equation (either respiratory or metabolic) moves out of the normal ranges, the other will also change in an attempt to offset the other. The chart below demonstrates this. For example if the body is acidotic because of an increased level of Carbon Dioxide, the body’s response is to raise the level of bicarbonate and vice versa. It is important that you remember to treat the cause and not treat the symptoms.

please note the chart would not TX over

ABG Normal Values*
pH (Arterial)7.35-7.45
PCO2 35-45 mmHg
PO2 80-105 mmHg
Standard Base Excess
Newborns: -8 to -2
Infants: -6 to -1
Children: -4 to +2
Adults: -2 to +3
*Guideline only, please check with the laboratory for you facilities normal values.

Top 8 Reasons To Check A Blood Gas
8) Assess the adequacy of Oxygenation.
7) Its 3 AM, and you just ran out of catalogs.
6) Assess the adequacy of ventilation
5) You just can’t get enough of the pneumatic tube system.
4) Rapid assessment of the acid-base balance (respiratory components)
3) The other nurse in the room is driving you nuts and it’s the only way to leave.
2) Assessment of the acid-base balance (metabolic components)
.....And the number 1 Reason to check a Blood Gas
The resident just dropped into REM sleep

[edit on 5/20/06 by FredT]

[edit on 5/20/06 by FredT]

[edit on 5/20/06 by FredT]