Pulmonary Concepts in Critical Care
I. Carbon dioxide is carried in the blood in three forms
1. CO2 is twenty times more soluble in blood than O2
2. The dissolved form for CO2 accounts for less than ten percent of total body CO2
B. When in combination with proteins, and, chiefly, as bicarbonate.
C. Bicarbonate is formed in blood by the following sequence: CO2 + H2O <> H2CO3 <>H+ +HCO3-
1. The Henderson Hasselbach equation is derived from this chemical reaction: pH = pKa + log [HCO3]/[0.03Pco2]
2. The value of pKa is 6.1 and the normal HCO3- concentration in arterial blood is 24 mmol/L.
3. Normal pH is 7.4.
II. How To Interpret an ABG.
A. ABG is usually written as: pH/PaCO2/PaO2/ HCO3-/O2 Saturation
B. Determine if the patient is acidotic or alkalemic: Look at the pH (Normal = 7.35-7.45)
1. If the pH<7.35 then acidemic
2. If the pH is >7.45 then alkalemic
C. There is only one situation in which a normal pH can coexist with an acid-base disorder
1. Chronic respiratory alkalosis.
2. In this situation the kidneys are able to fully compensate for the respiratory alkalosis.
D. If there is an acidosis, is it metabolic or respiratory in origin: Look at the PaCO2 (NL=35 – 45)
E. If the PaCO2 < 35, then the patient has metabolic acidosis
F. Check for Anion Gap: Na - [Cl + HCO3]
1. If normal (12 +/- 2), then Non-anion gap metabolic acidosis
a. Differential Cardiac.
-Expansion with fluid
-Acid loading by ingestion
- RTA types I, IIb, Ivb
- Carbonic Anhydrase Inhibitors or Spironolactone
- Chronic pyelonephritis
- Urethral diversions and GI losses and TPN
b. If abnormal (<8 or >12), then Anion gap metabolic acidosis – Differential.
- Methanol, metformin
- DKA (alcohol, starvation)
- Iron, Isoniazid, Inhalants
- Lactic Acidosis
- Ethylene Glycol
- Salicylates, solvents (toluene)
c. If the PaCO2 > 45, then respiratory acidosis
- if acute, decrease in pH by 0.08 accompanied by increase in PaCO2 by 10 Consider.
- Foreign body obstruction
- if chronic, differential is:
- Restrictive lung disease
2. If alkalosis, determine if metabolic or respiratory in origin: Look at PaCO2 (Normal = 35 – 45)
a. If the PaCO2 is greater than 45 then metabolic alkalosis
- administer NaCl to determine if Cl sensitive or resistant
If Cl sensitive, differential is
post hypercalcemia syndrome
if Cl resistant, differential is
chronic K loss
increase in glucocorticoids (Cushings or adrenal hyperplasia)
b. If PaCO2 is less than 35, then Respiratory Alkalosis
hyperventilation (anxiety, CNS disease, pregnancy)
Congestive Heart Failure