Metabolism

ABGs in Disease: Respiratory Disorders and Myocardial Infarction

Background

1. Arterial Blood Gas (ABG) analysis is most valuable for respiratory and perfusion illnesses
   • Less valuable in metabolic illnesses, where Venous Blood Gas analysis is adequate (e.g., DKA, sepsis, etc.)
2. Arterial sampling is more difficult and more painful than venous sampling
   • Pulse oximetry and ABG oxygen saturation measurements are more equivalent
3. Information obtained from an ABG (see also ABG General Info)
   • pH
   • Partial pressure of arterial oxygen (PaO2)
   • Partial pressure of arterial carbon dioxide (PaCO2)
   • Serum bicarbonate (HCO3)
     • Note: HCO3 obtained from an ABG is a calculated value and may be inaccurate
       • Consider measured HCO3 obtained from a basic metabolic panel

Acute Respiratory Failure

1. See also
   • Peak Flows and ABGs
     • Peak flow reference ranges chart (View image)
   • Breathing
2. PaO2 < 60 mmHg (hypoxia)
3. PaCO2 > 45 mmHg (hypercapnia)

Asthma

1. ABG has no clear relationship to Pulmonary Function Tests
2. Patients with severe airway obstruction may have a normal ABG
3. In children, ABG may increase respiratory effort secondary to agitation
4. In fatigue however, may have normal to increased PaCO2 and decreased PaO2

Chronic Obstructive Pulmonary Disorder

1. Baseline is critical: patient may have high baseline
   • Deviation more important than actual number
2. Renal compensation will occur for chronic respiratory insufficiency
   • High PaCo2, high HCO3
3. Hypoxemia results in hyperventilation and respiratory alkalosis
4. Over time CO2 is retained and ventilatory acidosis occurs, followed by renal compensation
   • Change in bicarbonate of 10 mEq/L [10 mmol/L]
   • Changes pH by 0.15
5. pH turns toward normal in long-standing COPD
6. Acute or Chronic ventilatory failure denoted by
   • Increased PaCO2
   • Decreased pH
   • Increased HCO3
7. Clinical appearance (ie., fatigue)
8. Aggressively attempting to lower PaCO2 may decrease respiratory drive

Dyspnea

1. Presence of abnormal A-a gradient is consistent with any V/Q mismatch
   • Pulmonary Embolism (high V/Q) (View image)
   • Pneumonia (low V/Q)
   • Aspiration (low V/Q)
   • Pulmonary Edema (low V/Q)
2. Pulmonary Embolism (PE)
   • Presence of PaO2 < 80 mmHg is supportive evidence for PE
   • Presence of abnormal A-a gradient is supportive evidence for PE
   • About 50% of PEs have a PaO2 > 80 mmHg
   • ABGs may be normal in up to 50% of angiogram-proven PEs
3. Pneumonia
   • Presence of high A-a gradient may indicate Pneumocystis jiroveci pneumonia
     • Formerly known as Pneumocystis carinii pneumonia (PCP)

Myocardial Infarction (MI)

1. Presence of acidosis in Acute MI is a marker for increased mortality
2. Bicarbonate administration NOT routinely recommended
3. Hypoxemia may result from CHF

Related Topics

• ABG Lab
• Arterial Blood Gas Analysis General Info

References

1. Kellum JA. Disorders of acid-base balance. Crit Care Med. Nov 2007;35(11):2630-2636
2. Adrogué HJ, Madias NE. Management of life-threatening acid-base disorders-- first of two parts. N Engl J Med. Jan 1, 1998;338(1):26-34
3. Adrogué HJ, Madias NE. Management of life-threatening acid-base disorders-- second of two parts. N Engl J Med. Jan 8, 1998;338(2):107-111
4. Davis MD, Walsh BK, Sittig SE, Restrepo RD. AARC clinical practice guideline: blood gas analysis and hemoximetry: 2013. Respir Care. Oct 2013;58(10):1694-1703
5. Williams AJ. ABC of oxygen: assessing and interpreting arterial blood gases and acid-base balance. BMJ. 1998 Oct 31;317(7167):1213-1216
6. Shebl E, Mirabile VS, Sankari A, et al. Respiratory Failure. StatPearls [Internet]. Available at: https://www.ncbi.nlm.nih.gov/books/NBK526127/. [Accessed September 2022]
7. McKeever TM, Hearson G, Housley G, et al. Using venous blood gas analysis in the assessment of COPD exacerbations: a prospective cohort study. Thorax 2016;71(3):210-215

Contributor(s)

1. Parker, Todd A., MD, FACEP, FAAEM
2. Ho, Nghia, MD

Updated/Reviewed: September 2022