's Quick and Dirty Guide to Acid Base Balance

Acid Base balance made simple.

pH is a measurement of acidity. It just measures how acidic something is. When referencing acidity, we're talking about excess hydrogen ions (H+), but we're just going to use the general term "acid." Its a scary foreign phrase, but it just means "percentage of acids" Acid isn't always a bad thing like in movies. The pH scale goes from 0-14, so neutral is 7 (water is neutral.)  The body needs a pH of 7.35-7.45 to maintain homeostasis. 0.5 in either direction away is not conducive to life. So, when we find someone whose acid levels are not conducive with life, we counteract the problem and restore homeostasis. You want to keep the body neither too acidic or too alkaline, you want to balance it. This is all Acid-Base balance means.

ACIDIC - TOO MUCH ACID - Lower than 7.35 means that there is too much acid in your system.
NORMAL - Just right! pH between 7.35 and 7.45 is perfect for homeostasis.
ALKALINE - NOT ENOUGH ACID - It just means NOT acidic enough. pH higher than 7.45 is considered Alkaline.

Test yourself
Your patient has a ph of 6.9    Is he acidic or alkalotic?
Your patient has a ph of 7.4    Is he acidic or alkalotic?
Your patient has a ph of 7.7   Is he acidic or alkalotic?
Your patient has a ph of 7.25  Is he acidic or alkalotic?
Your patient has a ph of 7.43  Is he acidic or alkalotic?
Your patient has a ph of 8.0    Is he acidic or alkalotic?


1. acidic 2. normal 3. Alkaline 4. Acidic 5. Normal 6. Alkaline



You take in oxygen by inhaling, your body turns oxygen into carbon dioxide, you exhale and remove the carbon dioxide from your body.
Carbon dioxide is "respiratory acid." When you're not breathing adequately, you are not getting rid of this "respiratory acid" and it builds up in the tissues. The extra CO2 molecules combine with water in your body to form carbonic acid and makes your pH go up. This is bad.

How do we measure it?
We can measure the amount of respiratory acid in the arterial blood using blood gases. They measure the amount of each gas in your blood. We measure the pH, the amount of carbon dioxide (PaCO2) and the amount of oxygen in the blood (PaO2).

PaCO2 is the partial pressure of carbon dioxide. We can measure it to see how much respiratory acid (CO2) there is in the blood. We use arterial blood gas tests to check it. How much respiratory acid (CO2) should there be? The normal value is 35-45 mmHg (mmHg just means millimeters of mercury, its a measurement of pressure.)  The (a) in PaCO2 just stands for arterial. If you measured venous blood gasses, the levels are different and PvCO2 is used. If CO2 is HIGH, it means there is a buildup of respiratory acids because he's not breathing enough CO2 away. If your pH is acidic, and your CO2 is HIGH, its considered respiratory acidosis. If CO2 is LOW, it means there are not enough respiratory acids because he's probably hyperventilating too much CO2 away. When pH is high, and CO2 is low, this is called Respiratory Alkalosis.

Field Note:
On the trucks,  pETCO2 (End Tidal Carbon Dioxide) is sometimes used and is expressed as a percentage. instead of measuring the pressure in mmHg to measure the effectiveness of ventilations and circulation. A pETCO2 of 5-6% roughly equates to a PaCO2 of 35-45mmHg, so titrate your ventilations to achieve 5-6% pETCo2.

EMT Quick Rule for testing

Inadequate breathing  - Respiratory Acidosis - PH will be low, CO2 level will be high, because he's not breathing it off.

Hyperventilating patients - Respiratory Alkalosis - PH will be high, CO2 levels will be low, because they're breathing off too much CO2.

Making it simple:

If your CO2 is LOW, then DECREASE rate and depth of ventilations.

If your CO2 is HIGH, then INCREASE rate and depth of ventilations.


Respiratory Acidosis
NOT Breathing adequately - NOT exhaling enough means you aren't getting rid of CO2 therefore pCO2 (percent of Co2 is HIGH!)
Clinical Presentation: Altered LOC, Tachycardic, Diaphoresis, headache
Cause: Crappy or no respirations (COPD, Overdose, Pneumonia, Smoke inhalation, Pneumothorax, airway obstruction)
 - Increase rate and depth of ventilations - Bag them, so you get rid of some CO2 for them.

Respiratory Alkalosis 
Blowing off too much CO2, as in hyperventilation. When there is not enough CO2 in the blood, the body uses bicarb to compensate for the lack of respiratory acid. (Metabolic compensation)
Clinical Presentation: Numbness or muscle twitch in fingers, toes, seizures
Cause: Shock, DKA Kussmaul Respirations - Deep and FAST breathing! (The body is trying to compensate for its metabolic acidosis by producing a respiratory alkalosis on purpose.) Caused by (anxiety, pain, fever, hypotension, hypoxia, CHF, PE, Sepsis)
Treatment: decrease rate and depth of ventilations. (Calm them down or stop bagging so fast.)


Metabolic just means it has to do with metabolism. Metabolism just means the chemical changes that sustain life. Oxygen combines with glucose to create energy. Carbon dioxide is a waste product of this process. CO2 is then carried from the blood to the alveoli to the lungs for expiration. We blow off this respiratory acid when ventilations are sufficient. Your body produces a buffer known as bicarbonate that is alkaline and it binds up with excess acids and turns it neutral. Bicarbonate is like Tums for your bloodstream.

HCO3 is the amount of bicarbonate in the bloodstream. HCO3 is just the chemical name for bicarbonate. Normal is 22-26/mEq/l (milliequivalents per liter) This measures how much Tums are in your blood binding up the excess acid. If your HCO3 (Tums) level is 10, it means that there is not enough of it, and so the acid in your blood just builds up and the overall pH of your body goes DOWN! This is called Metabolic Acidosis. To fix it, we bring bicarbonate levels up by administering Bicarb via IV! If there's too much, it binds up too much acid and there isn't enough, so now your pH goes alkaline! This is called Metabolic Alkalosis. Fix it by keeping your patient stable long enough for the bicarb to work itself out.


Metabolic Acidosis 
Acid in the blood is used to combine with oxygen and metabolize it. If you do not have enough oxygen rich blood circulating, the acid is not used and builds up.
Clinical Presentation: Tachycardia, Pulmonary edema, Tachypnea, confusion or coma
Cause: Cardiac arrest, renal failure, DKA, diarrhea for a long time, ASA or antifreeze Overdose
Treatment - Increase rate and depth of Ventilations. Consider Bicarbonate - Its like Tums for your blood. It binds up the acid in your body and turns it neutral.

Metabolic Alkalosis
Too much bicarbonate, not enough acid
Clinical Presentation: Seizures, Headache, Dysrhythmias
Cause: Diuretics, cystic fibrosis, chewing tobacco, penicillin, overzealous bicarb administration
Treatment: Keep them alive long enough for their body to use up the excess bicarb.

Handy Dandy Quick Reference


Test Yourself 
If you have blood gases, first consider whether your patient is acidotic or alkalotic by evaluating the pH. Then, determine whether the cause is respiratory or metabolic.

Your patient has a PH of 7.25, a CO2 of 60mmHg and HCO3 22
Your patient has a PH of 7.5, a CO2 of 45mmHg and HCO3 40
Your patient has a PH of 7.4, a CO2 of 40mmHg and HCO3 25
Your patient has a PH of 7.25, a CO2 of 50mmHg and HCO3 22
Your patient has a PH of 7.55, a CO2 of 24mmHg and HCO3 22


Answers: 1. Respiratory acidosis 2. Metabolic alkalosis 3. Normal 4. Respiratory acidosis. 5. Respiratory Alkalosis

The kidneys also have a fail safe switch on them. If your pH is too high when it hits the kidney filter, your kidney senses it and flushes MORE acid out with your urine. Similarly, if your kidneys sense that you don't have enough acid, they kick out less acid into your urine. Unfortunately, the kidneys take awhile to process it, so it takes hours or days for it to compensate.

Field Note:
Your dialysis patients are chronically acidotic because their kidneys are not participating in the compensation system. So, if your dialysis patient crashes, assuring adequate ventilations and bicarb administration is even more vital.

 Advanced Learning Links for Acid Base Balance

End Tidal Carbon Dioxide Monitoring During CPR: A Predictor of Outcome

End Tidal CO2

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