Interpreting Blood Gases



 
ABG (Normal Values)
pH 7.35-7.45
pCO2 35-45
HCO3 22-26
pO2 75-100


BASIC DEFINITIONS

- Acidosis/alkalosis: A process → acidemia/alkalemia
- Acidemia: pH <7.40
- Alkalemia: pH >7.40
- Metabolic disorder: Change in HCO3
- Respiratory disorder: Change in PCO2
- Respiratory or renal compensation: Other system alterations that bring the blood gas toward a normal pH of 7.40


DIAGNOSIS
Use the history and exam for clues to the disorder.

Now look at the blood gas and chemistry.

  1. Look at the pH to determine if there is an acidemia or alkalemia.
  2. Check the PCO2 to determine if there is a respiratory acidosis or alkalosis.
  3. Examine the HCO3 to determine if there is a metabolic acidosis or alkalosis.
  4. Calculate the anion gap [Na+ − (K + Cl)]. If > 12, there is an underlying metabolic gap acidosis.
Normal pH (7.35 - 7.45)
Acidemia
(pH <7.35)
Alkalemia
(pH >7.45)
Respiratory Acidosis
(↓ pH + ↑ PCO2 [>40])
Metabolic Acidosis
(  HCO3 [<20], ↓ PCO2 [<40])
Respiratory alkalosis
(↓ PCO2 [<40])
Metabolic alkalosis
(↑ HCO3)
PaCO2, Serum Cl, Serum K
HYPOventilation:

Inadequate ventilation or increased dead space.

Causes include:
- Head or chest trauma
- Sedatives, and opiates
- Over-sedation, obtundation, or coma
- Neuromuscular disorders
- Pickwickian syndrome (obesity-hypoventilation syndrome)
- COPD

Renal compensation occurs after 48 hrs of steady state.
Anion Gap
(anion gap >12)
Gap
(anion gap <12)
Normal Anion Gap acidosis HYPERventilation:
Acute Asthma
P
anic/Anxiety
Pul Embolus
Pul Edema
Pneumonia
Pregnancy
Progesterone
Cirrhosis
CHF
Hypoxemia (High altitude)
↑ ICP
Toxic salicylates
Sympathomimetics.
Hypovolemia:
 - Vomit
 - Pyloric Stenosis
 - Hyperemesis gravid
 - Diuretics

Hyperaldosteronism
Renin-producing Tumor
Bartter syndrome, Gitelman Syndrom (genetic Testing)
Antacid use
 
concentrations of anions other than K and Cl

 

MUDPILES
- Methanol, Metformin
- Uremia (Pericarditis)
- Diabetic (or alcoholic) keoacidosis
- Propylene
- INH/Iron/Inhalant (ie, CO) poisoning
- Lactic acidosis (sepsis, shock, hypoxia, seizures, cyanide, metformin, Rhabdo, Ischemia)
- Ethylene glycol
- Salicylates, Solvents

 

in unmeasured serum cations (seen in myeloma)

serum albumin

Bromide or iodine poisoning (which are mistaken for chloride in many labs)

 

Diarrhea
Renal tubular acidosis
Ketone wasting
Toluene

 
Cl
K K
- Renal losses:
Renal tubular acidosis or acetazolamide
- GI losses:
Diarrhea or malabsorption
Laxative abuse
- Adrenal insufficiency
- Renal insufficiency
- Posthypocapnia
HARD-ASS:
Hyperalimentation
Addison disease
Renal tubular acidosis
Diarrhea
Acetazolamide
Spironolactone
Saline infusion

Tx:
  - Intubation, mechanical ventilation
  - RTA Type 1,  & 2 ( ↓ K) : Potassium Citrate (Urocit-K) 20-40 mEq PO qd +/- HCTZ 12.5-50mg PO qd.
  - RTA Type 4 ( K) : Lasix 20-80mg po qd  OR bumetanide 0.5-2mg PO qd
  - Refractory Acidosis : Na HCO3 8.4% (1mEq/ml) @ 50 cc/hr.

MOA:
- Likely HypOventilating = Retaining CO2
- Goal = Help pt blow off CO2
 
Tx:

- Ventilatory support
- O2 may be necessary to treat hypoxia, but may worsen hypercapnia in patients with COPD or in heavily sedated patients.

 

NOTE:
Min. Ventilation (VE) = RR x TV

VE = CO2 = pH = Cerabral Flow/ICP
VE = CO2 = pH

Tx:
- May need to be intubated
- Treating the underlying cause is the most important action.
- Sodium bicarbonate treatment is a controversial and potentially dangerous treatment because of the risk of electrolyte disturbances and paradoxical cerebral acidosis. The cerebral acidosis occurs 2° to the inability of HCO3 to quickly cross the blood-brain barrier. Bicarbonate for the treatment of acidosis should only be considered for extremely ill patients with severe acidosis.
- A brief reminder of some special treatments of underlying causes of metabolic acidosis includes:
  - Ethylene glycol and methanol: Fomepizole, Ethanol or 4-methylpyrazole and dialysis
  - Salicylate toxicity: HCO3 to keep serum pH between 7.3 and 7.5 with resultant urine alkalinization; dialysis
  - Iron overdose: Deferoxamine
  - Isoniazid: Pyridoxine (vitamin B6)
 
Acidosis Management: (pH < 7.30)
  If pH 7.15-7.30:

  - Increase RR until pH > 7.30 or PaCO2 < 25  (Maximum set RR = 35).
  If pH < 7.15:
  -
Increase RR to 35.
  - If pH remains < 7.15, VT may be increased in 1 ml/kg steps until pH > 7.15 (Pplat target of 30 may be exceeded).
  - May give NaHCO3 2-5 mEq/kg IV x1
Alkalosis Management: (pH > 7.45)
  - Decrease vent rate if possible
 
Tx:
Treatment focuses on identifying and addressing the underlying cause of
tachypnea.
TX:
Acetazolamide

Three rules assist in identifying a mixed disorder:
1. Neither respiratory nor renal compensation completely normalizes the pH.
2. The PCO2 in patients with a metabolic acidosis is predicted by Winter's formula: PCO2 = 1.5 HCO3 + 8.
       - PCO2 < predicted by Winter's formula = Respiratory Alkalosis.
       - PCO2 > predicted by Winter's formula = Respiratory Acidosis.

3. Δ gap = measured anion gap - normal anion gap. The Δ gap should approximate the decrease in HCO3; if the drop in HCO3 cannot be explained completely by the Δ gap, a non-gap acidosis is also present.