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Blood Gas Analysis 1065.e1

Blood Gas Analysis
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• May obtain venous or mixed sample when
Difficulty level: ♦♦
presence of increased UA.
arterial sample was desired ○ An increased AG value indicates the
Synonym ○ Arterial sample will typically fill syringe • Determine most likely differentials for the
Acid-base analysis quickly; may be pulsatile primary acid-base disturbance.
○ If severe hypoxemia identified, can run • Differential diagnoses for metabolic acidosis:
Overview and Goal known venous sample for comparison to increased AG
To assess acid-base status and respiratory func- be sure initial sample was indeed arterial ○ Lactate
tion. Arterial blood samples should be used • Can be difficult to obtain arterial sample ○ Ketones
to assess oxygenation, and venous or mixed • Bleeding and discomfort after arterial ○ Uremic acids
samples can be used to determine acid-base puncture are minor risks. ○ Toxins: ethylene glycol, salicylates, others
status. • Differential diagnosis for metabolic acidosis: Procedures and Techniques
Procedure normal AG
Indications • Arterial PaO 2 should equal approximately 5 ○ Renal tubular acidosis
−
• Ill patients with suspected ventilatory or times the inspired oxygen percentage (FIO 2 ). ○ HCO 3 loss through diarrhea
metabolic disturbances FIO 2 of room air is 21%. Healthy animal • Differential diagnoses for metabolic alkalosis
• Patients with suspected hypoxemia; arterial breathing room air should have PaO 2 of ○ Gastrointestinal (GI) obstruction: loss of
+
−
+
blood gas is gold standard for evaluation of ≈100 mm Hg. H , K , and Cl in vomitus
oxygenation ○ On room air, PaO 2 < 85 reflects hypox- ○ Furosemide
emia, PaO 2 < 60 reflects severe hypoxemia ○ Administration of sodium bicarbonate
Contraindications • Compare results to reference ranges. ○ Primary hyperaldosteronism (rare)
• Coagulopathy is a contraindication for ○ Hyperadrenocorticism (rare)
arterial puncture. • Differential diagnoses for respiratory acidosis
• Patients in severe respiratory distress may Canine Arterial Venous ○ Airway obstruction
not tolerate restraint for blood draw and pH 7.395 ± 0.03 7.352 ± 0.02 ○ Impairment of respiratory center
may decompensate. ○ Neuromuscular disease
PO 2 (mm Hg) 102.1 ± 6.8 55 ± 9.6
○ Severe pleural space disease
Equipment, Anesthesia PCO 2 (mm Hg) 36 ± 2.7 42.1 ± 4.4 ○ Severe primary pulmonary disease
−
• 1-mL heparinized syringe (with needle or HCO 3 (mmol/L) 21.4 ± 1.6 22.1 ± 2 • Differential diagnoses for respiratory alkalosis
indwelling arterial catheter) BE (mmol/L) −1.8 ± 1.6 −2.1 ± 1.7 ○ Hypoxemia
• Rubber stopper to cap needle ○ Pulmonary disease
• Blood gas analyzer ○ Central nervous system disease
More information about obtaining an arterial Feline Arterial Venous ○ Gram-negative sepsis
sample is available (p. 1058). ○ Liver disease
pH 7.34 ± 0.1 7.30 ± 0.08 ○ Hyperadrenocorticism
Anticipated Time PO 2 (mm Hg) 102.9 ± 15 38.6 ± 11 ○ Exercise
About 5 minutes ○ Stress and/or pain
PCO 2 (mm Hg) 33.6 ± 7 41.8 ± 9 • After determining primary disturbance, evalu-
Preparation: Important HCO 3 (mmol/L) 17.5 ± 3 19.4 ± 4 ate for presence of compensatory changes.
−
Checkpoints BE (mmol/L) −6.4 ± 5 −5.7 ± 5 ○ A change in the respiratory or metabolic
• Determine if arterial sample required (to component typically induces an opposite
assess oxygenation) or if venous sample is compensatory response in attempt to
adequate • Determine the primary disturbance in acid- return pH to normal.
• Venous samples: jugular venipuncture or base status: pH < 7.35 = acidemia, pH > ○ The absence, presence, and degree of
sample from central catheter is ideal for 7.45 = alkalemia compensatory response gives an indication
global acid-base status • If acidemia is present, determine if respiratory of chronicity of disturbance.
○ Can determine acid-base status, and or metabolic ○ Compensatory responses require sufficient
PvCO 2 can be used to assess ventilatory ○ PaCO 2 > 45 mm Hg = respiratory time
status ○ Base excess (BE) < −4 mmol/L (or ■ Acute respiratory disorders: 15 minutes
−
• Arterial samples HCO 3 < 21 mmol/L) = metabolic ■ Chronic respiratory disorders: 7 days
○ Can determine acid-base status; PaCO 2 • If alkalemia is present, determine if respira- ■ Long-standing respiratory acidosis: 30
can be used to assess ventilatory status, tory or metabolic days
and PaO 2 is used to assess oxygenation. ○ PaCO 2 < 35 mm Hg = respiratory ■ Metabolic disorders: 24 hours
−
○ BE > 4 mmol/L (or HCO 3 > 27 mmol/L) ○ Overcompensation never occurs (i.e., pH
Possible Complications and = metabolic change reflects primary disorder)
Common Errors to Avoid ○ NOTE: for cats, the above listed normal
−
• Exposure to room air can cause decreased values for BE or HCO 3 should be
PCO 2 and increased PO 2. substituted.
○ No air bubbles should be present in • Calculate the anion gap (AG) if electrolyte
sample. values are available. AG is the calculated
• Sample should be run immediately or placed difference between unmeasured cations (UC)
on ice to stop continued cellular metabolism, and unmeasured anions (UA).
+
+
−
−
decreased pH, increased CO 2 , and increased ○ AG = [(Na ) + (K )] − [(HCO 3 ) + (Cl )]
lactate = UA − UC
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