Nurse Files: ABG part 2
pH
Okay, we covered the basics of an arterial blood gas (ABG) in Nurse Files: ABG, Part 1. What is included, how O2 and CO2 get transported, and how CO2 gets turned into H2CO3 then H+ and HCO3-.
Some things to keep in mind:
O2 is oxygen. It binds to hemoglobin in red blood cells (RBC)
CO2 is carbon dioxide. It is the waste gas produced by cellular metabolism. It can bind to hemoglobin too, but most of it is converted into carbonic acid (H2CO3) which then splits into hydrogen (H+) and bicarbonate (HCO3-)
Today we’re covering pH. We don’t have to get into the math (negative logarithms, anyone?) but we do need to know that pH stands for ‘power of hydrogen.’ The pH is a measurement of how much and how active hydrogen atoms are in a solution.
Back to Basics
Something that is ‘acidic’ will easily give up hydrogen atoms. Something ‘basic’ or ‘alkaline’ will easily accept hydrogen atoms. Pure water, with no loose hydrogen atoms (H2O), is a ‘neutral’ substance.
pH is measured on a scale from 0 to 14. 0 is the most acidic, meaning the liquid will give up hydrogen atoms the most enthusiastically. 7 is neutral, with no hydrogen activity, and is 14 the most alkaline. This solution bind hydrogen atoms aggressively.
This pH level is important because it affects how chemical reactions happen. Human blood needs to maintain a pH of 7.35 to 7.45 in order to function appropriately. Too far out of range and proteins will unravel, hemoglobin won’t bind to oxygen, cells will die, and the whole system will crash. This is a tiny window, and the body does an amazing job of holding blood pH where it needs to be.
But you can easily shift your pH into alkaline territory just sitting reading this article. Start breathing deep and fast, or hyperventilating. In a few moments you’ll get light-headed. That’s your body yelling at you to stop, because you are blowing off too much CO2 and shifting your blood pH into alkalosis.
Keep at it and eventually you will pass out and your brain will stop your breathing until things correct themselves. You’ll wake up with a headache and your respiratory compensation system glowering at you.
You can do the opposite. Hold your breath. Hold it and hold it and hold it until your lungs burn with built up CO2. Do it long enough and — you guessed it — you’ll pass out. Your brain will force you to breathe, you’ll wake up with a headache, and your respiratory system will throw up its hands and stomp off in a huff.
Luckily (hopefully) this drama is because your acid-base system is working properly. It has mechanisms in place to keep things in balance. Then we humans come along and muck it up.
Bedside Considerations
So, what does this mean for you as a nurse? What can upset this pH balance?
Fever states and seizures. Increased cellular metabolism will increase the amount of CO2 produced. If the lungs can’t blow it off fast enough, blood pH will drop.
Infection. Especially if lactic acid is being produced. It’s an acid and drops the pH.
GI Diseases. Vomiting and diarrhea will expel potassium ions (K+) and chloride ions (Cl-). This affects how the kidneys resorb HCO3, causing an excess that leads to raised pH.
Restrictive Lung Diseases. Anything that holds CO2 in the body will affect the pH.
It is important to note: O2 doesn’t directly affect pH. O2 has no hydrogen to give or take. Under normal circumstances, hemoglobin preferentially binds with oxygen, which can then bump CO2 off the protein into solution. This is what we want it to do, so yay!
Kidney Diseases. Even acute diseases can shift the pH in either direction.
What we need to know is which disease states cause which change and how to correct it. This gets us into metabolic versus respiratory territory, which we’ll cover in Part 3. For now, when faced with an out-of-whack pH, think about what is happening to the CO2:
Is the body working too hard and producing too much acid? (Seizures? Fevers?)
Is the patient not breathing enough or breathing too much? (Drug overdose, anxiety, COPD?)
Is acid escaping the body somewhere? (Take a look at Check Yourself #2. Any escape of intestinal fluid can drastically shift the pH, especially if there is a fistula or leak.)
Did they ingest something that will override the body’s equilibrium? (Aspirin overdose, Tums overuse, etc.)
Are their kidneys working? (Acute renal failure, chronic renal failure, medication toxicity).
That’s enough for today. You’re doing great! This is a complicated topic that even experienced providers struggle with. We’ll get it nailed down and you’ll be an ABG whiz in no time.
References
Porth’s Pathophysiology: Concepts of Altered Health States (9th Edition). Grossman, S.C., Porth, C.M.
Oxygen–hemoglobin dissociation curve - Wikipedia
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Anna, RN, BSN, CCRN
Necessary disclaimer: I am discussing medications and medical conditions in this article based on my personal experiences as a nurse. Your facility may have different requirements and resources. Use your own nursing judgement to assess and treat your patients according to your governing body and facility guidelines. All information within this article is correct to the best of my knowledge, but should be confirmed through verified evidence-based sources. I am not responsible for any clinical decisions made based on this article.