Pathophysiology behind this ABG??

[tbear1206]

Scenario: An 80 year old female presents to the ED with a 2 day history of persistent vomiting. She is lethargic and weak and has body aches. Her mucous membranes are dry and her capillary refill is >4 seconds. ABG results are pH 7.5, PaC02 25, HC03 17, and Pa02 90%.

Regarding the preceding scenario, I understand the patient is experiencing respiratory alkalosis with metabolic compensation. My theory to the pathophysiology is that the patient is experiencing early hypovolemic shock due to excessive loss of body fluids. Hypovolemic shock results in tachypnea, leading to respiratory alkalosis.

I am just wondering, is the patient compensating or worsening? I am aware that the body also has mechanisms to compensate and regulate the pH. However, hypoperfusion to organs (in late hypovolemic shock) will lead to lactic acidosis (metabolic acidosis). Since the patient's HCO3 leve is 17 (acidodic) is her condition worsening, or is her body compensating?? Or am I just making it more complicated?

[Esme12, ASN, BSN, RN]

Normal values:

PH = 7.35 - 7.45

C02 = 35 - 45

HC03 = 21-26

Respiratory acidosis = low ph and high C02

hypoventilation (eg: COPD, narcs or sedatives, atelectasis)

*Compensated by metabolic alkalosis (increased HC03)

For example:

ph 7.20 C02 60 HC03 24 (uncompensated respiratory acidosis)

ph 7.33 C02 55 HC03 29 (partially compensated respiratory acidosis)

ph 7.37 C02 60 HC03 37 (compensated respiratory acidosis)

Respiratory alkalosis : high ph and low C02

hyperventilation (eg: anxiety, PE, pain, sepsis, brain injury)

*Compensated by metabolic acidosis (decreased HC03)

examples:

ph 7.51 C02 26 HC03 25 (uncompensated respiratory alkalosis)

ph 7.47 C02 32 HC03 20 (partially compensated respiratory alkalosis)

ph 7.43 C02 30 HC03 19 (compensated respiratory alkalosis)

Metabolic acidosis : low ph and low HC03

diabetic ketoacidosis, starvation, severe diarrhea

*Compensated by respiratory alkalosis (decreased C02)

examples:

ph 7.23 C02 36 HC03 14 (uncompensated metabolic acidosis)

ph 7.31 C02 30 HC03 17 (partially compensated metabolic acidosis)

ph 7.38 C02 26 HC03 20 (compensated metabolic acidosis)

Metabloic alkalosis = high ph and high HC03

severe vomiting, potassium deficit, diuretics

*Compensated by respiratory acidosis (increased C02)

example:

ph 7.54 C02 44 HC03 29 (uncompensated metabolic alkalosis)

ph 7.50 C02 49 HC03 32 (partially compensated metabolic alkalosis)

ph 7.44 C02 52 HC02 35 (compensated metabolic alkalosis)

*Remember that compensation corrects the ph.

Now a simple way to remember this......

CO2 = acid, makes things acidic

HCO3 = base, makes things alkalotic

Remember ROME

R-Respiratory

O-Opposite

M-Metabolic

E-Equal

Ok always look at the pH first...

pH

pH>7.45 = alkalosis

Then, if the CO2 is high or low, then it is respiratory...If the HCO3 is high or low then it is metabolic. How you remember that is that the respiratory system is involved with CO2 (blowing air off or slowing RR), and the kidneys (metabolic) are involved with HCO3 (excreting or not excreting).

Here is how you think thru it: pH = 7.25 CO2 = 40 HCO3 = 17

Ok, first, the pH is low so think acidosis. CO2 is WNL. HCO3 is low. Draw arrows if it helps. The abnormal values are both low (think Equal). Metabolic imbalances are equal. So, this must be metabolic acidosis!

Now, for compensation...If you have a metabolic imbalance, the respiratory system is going to try to compensate. Respiratory = CO2. If the CO2 is normal in the ABG, then there is no compensation going on. Compensation in acidosis will decrease the CO2 because you want to get rid of the acid (CO2). In alkalosis, it will increase because you want to add more acid (CO2)

If you have a respiratory imbalance, the kidneys will try to compensate. Kidneys = HCO3. If the HCO3 is normal in the ABG, then there is no compensation going on. Compensation in acidosis will increase HCO3 because you want to hold on to the base to make it more alkalotic. In alkalosis, it will decrease because you want to excrete the base to make it more acidic.

[calivianya, BSN, RN]

I think you're making it a little complicated. Whether the patient is getting better or getting worse depends on what her initial ABG looked like. Or, if that is the initial, to draw a second one. You're not going to be able to tell if a patient is getting better from just one ABG because you have no data points to compare that ABG to.

PaO2 is a very important part of the puzzle, but it doesn't mean much by itself. You have to know how much O2 the patient was on when the gas was drawn to know whether that's a good PaO2 or a bad one. If she is on room air, that PaO2 is awesome. If she is on 100% oxygen by nonrebreather and she's got a PaO2 of 90, she's in some real deep trouble.