Respiratory Acidosis
Respiratory Acidosis
David Ray Velez, MD
Table of Contents
Definition
Definition: An Acid-Base Disorder with Acidosis (Decreased pH) Due to Inadequate Respiratory Elimination of pCO2
Arterial Blood Gas (ABG) Analysis
- General Levels in a Primary Respiratory Acidosis:
- pH < 7.35
- pCO2 > 45 mmHg
- *See Arterial Blood Gas (ABG) Analysis
Causes
Hypoventilation – The Primary Mechanism of All Causes of Respiratory Acidosis
Primary Causes
- Neuromuscular Disease
- Asthma or COPD
- Obstructive Sleep Apnea (OSA)
- Interstitial Lung Disease
- Opioid or Sedative Use
- Malignant Hyperthermia
Can Be Induced by Inadequate Ventilation When Mechanically Ventilated
Physiologic Changes of Acidosis
Pulmonary Changes
- Stimulated Respiratory Drive and Increased Respiratory Rate
- Right-Shift of the Oxygen-Hemoglobin Dissociation Curve (Decreased Affinity)
Cardiovascular Changes
- Net Increase in Cardiac Output – Acidosis Directly Decreases Contractility but Sympathoadreal Effects Cause Increased Preload, Heart Rate, and Contractility
- Increased Risk of Arrhythmia
- Decreased Systemic Vascular Resistance (SVR) and Arterial Vasodilation
- Decreased Responsiveness to Catecholamines
Hematologic Changes
- Coagulopathy from Impaired Clotting Factor Function
- Decreased RBC Rheology (Flow) – Contributes to Rouleaux Formation
- Impaired Platelet Aggregation
Renal Changes
- Decreased Bicarbonate Secretion
- Diuresis
Electrolyte Changes
- Hyperkalemia – From Intracellular Shifts
- Hypercalcemia – From Decreased Calcium Binding to Albumin and Increased Renal Wasting
Neurologic Changes
- Cerebral Vasodilation and Increased Intracranial Pressure (ICP)
- Increased Cerebral Blood Flow
- Nausea and Vomiting
Acuity/Compensation
Respiratory Acid-Base Disorders are Compensated by Renal Changes in Bicarbonate Reabsorption (Slow Process Over Days-Weeks)
Acuity/Compensation
- Acute Respiratory Acidosis – High pH Changes are Seen with Minimal Compensation
- Only Slight Compensation Occurs Over Minutes-Hours from Cellular Buffering of Plasma Proteins
- Chronic Respiratory Acidosis – pH More Normalized Due to Renal Compensation Over Days-Weeks with Increased Reabsorption of Bicarbonate and Excretion of Acid
Expected Change in Bicarbonate and pH (For Every 10 mmHg Increased in pCO2 Above 40 mmHg)
- Acute Respiratory Alkalosis:
- HCO3 Increases 1 mEq/L Above 24
- pH Decreases 0.08 Below 7.40
- Chronic Respiratory Alkalosis:
- HCO3 Increases 3-4 mEq/L Above 24
- pH Decreases 0.03 Below 7.40
- Mnemonic: “1-2-3-4-5 Rule”
- Expected Change in HCO3 (mEq/L) for Every 10 mmHg Change in pCO2 Due to Respiratory Acid-Base Disorders
| Acute | Chronic | |
| Respiratory Acidosis (↑ CO2) | ↑ 1 | ↑ 4 |
| Respiratory Alkalosis (↓ CO2) | ↓ 2 | ↓ 5 |
Treatment
Treatment
- Primarily Managed by Treatment of the Underlying Cause
- Correct Hypercapnia Gradually – Rapid Alkalization of CSF Can Cause Seizures