Patient Data Evaluation and Recommendations · 6 min read
Normal Values and Definitions
Arterial Blood Gas Normal Values
pH
Normal arterial pH ranges from 7.35 to 7.45. Values below 7.35 indicate acidemia. Values above 7.45 indicate alkalemia. The pH reflects the overall acid-base status resulting from respiratory and metabolic components.
PaCO2
Normal partial pressure of carbon dioxide in arterial blood ranges from 35 to 45 mmHg. PaCO2 is the respiratory component of acid-base balance. Elevated PaCO2 indicates respiratory acidosis from hypoventilation. Decreased PaCO2 indicates respiratory alkalosis from hyperventilation.
PaO2
Normal partial pressure of oxygen in arterial blood ranges from 80 to 100 mmHg at sea level breathing room air. PaO2 decreases with age according to the formula: expected PaO2 = 104 - (0.27 × age). PaO2 below 80 mmHg indicates hypoxemia. Below 60 mmHg (SaO2 about 90%), hypoxemia is moderate to severe and supplemental oxygen is usually needed.
HCO3
Normal bicarbonate concentration ranges from 22 to 26 mEq/L. HCO3 is the metabolic component of acid-base balance. Elevated HCO3 indicates metabolic alkalosis or compensation for chronic respiratory acidosis. Decreased HCO3 indicates metabolic acidosis or compensation for chronic respiratory alkalosis.
Base Excess
Normal base excess ranges from -2 to +2 mEq/L. Positive base excess indicates metabolic alkalosis. Negative base excess indicates metabolic acidosis. Base excess quantifies the metabolic component independent of respiratory changes.
SaO2
Normal arterial oxygen saturation ranges from 95% to 100%. SaO2 reflects the percentage of hemoglobin binding sites occupied by oxygen. The oxyhemoglobin dissociation curve relates PaO2 to SaO2. At PaO2 of 60 mmHg, SaO2 is approximately 90%.
| Parameter | Normal Range | Abnormal Means |
|---|---|---|
| pH | 7.35-7.45 | Below 7.35 acidemia; above 7.45 alkalemia |
| PaCO2 | 35-45 mmHg | High = hypoventilation; low = hyperventilation |
| HCO3 | 22-26 mEq/L | High = metabolic alkalosis or compensation; low = metabolic acidosis |
| Base excess | -2 to +2 mEq/L | Negative = base deficit (metabolic acidosis) |
| PaO2 (room air) | 80-100 mmHg | Below 80 hypoxemia; below 60 usually needs oxygen |
| SaO2 | 95-100% | About 90% at a PaO2 of 60 mmHg |
Acid-Base Interpretation
Step 1: Assess pH
Determine whether acidemia (pH < 7.35) or alkalemia (pH > 7.45) is present. Normal pH may indicate compensation or mixed disorder.
Step 2: Identify Primary Disorder
If pH is low and PaCO2 is high, respiratory acidosis is present. If pH is low and HCO3 is low, metabolic acidosis is present. If pH is high and PaCO2 is low, respiratory alkalosis is present. If pH is high and HCO3 is high, metabolic alkalosis is present.
Step 3: Assess Compensation
The body compensates for primary disorders to normalize pH. Respiratory compensation for metabolic disorders occurs within minutes to hours. Metabolic compensation for respiratory disorders takes 3 to 5 days to reach maximum effect.
For metabolic acidosis, expected PaCO2 = (1.5 × HCO3) + 8 ± 2 (Winter's formula). For metabolic alkalosis, expected PaCO2 = (0.7 × HCO3) + 21 ± 2. For acute respiratory acidosis, expected HCO3 change = 1 mEq/L per 10 mmHg increase in PaCO2. For chronic respiratory acidosis, expected HCO3 change = 3.5 mEq/L per 10 mmHg increase in PaCO2. For acute respiratory alkalosis, expected HCO3 change = 2 mEq/L per 10 mmHg decrease in PaCO2. For chronic respiratory alkalosis, expected HCO3 change = 5 mEq/L per 10 mmHg decrease in PaCO2.
Step 4: Calculate Anion Gap
Anion gap = Na - (Cl + HCO3). Normal anion gap is 8 to 12 mEq/L. Elevated anion gap indicates accumulation of unmeasured anions from lactic acidosis, ketoacidosis, renal failure, or toxins (methanol, ethylene glycol, salicylates).
Oxygenation Assessment
Oxygen Content
Total oxygen content of blood (CaO2) equals oxygen bound to hemoglobin plus dissolved oxygen. CaO2 = (1.34 × Hb × SaO2) + (0.003 × PaO2). Normal CaO2 is approximately 20 mL O2/dL blood.
P/F Ratio
The PaO2/FiO2 ratio assesses oxygenation efficiency. Normal P/F ratio is about 400 to 500. Using the Berlin definition with PEEP of at least 5 cmH2O, ARDS is mild at 201 to 300, moderate at 101 to 200, and severe at 100 or below.
A-a Gradient
The alveolar-arterial oxygen gradient assesses gas exchange efficiency. Calculate alveolar PO2 using the alveolar gas equation: PAO2 = FiO2 (PB - PH2O) - (PaCO2/0.8). At sea level breathing room air: PAO2 = 0.21 (760 - 47) - (PaCO2/0.8). A-a gradient = PAO2 - PaO2. Normal A-a gradient = (age/4) + 4 or approximately 2.5 + (0.21 × age). Elevated A-a gradient indicates V/Q mismatch, shunt, or diffusion impairment.
Oxygen Index
Oxygen index = (FiO2 × mean airway pressure × 100) / PaO2. Used in mechanically ventilated patients to assess oxygenation relative to ventilator support. Oxygen index above 25 indicates severe oxygenation impairment.
Pulmonary Function Test Values
Spirometry
Forced vital capacity (FVC) is the total volume exhaled during a forced expiration from total lung capacity. Forced expiratory volume in 1 second (FEV1) is the volume exhaled in the first second. FEV1/FVC ratio distinguishes obstructive from restrictive patterns. Normal FEV1/FVC ratio is 70% to 80% or greater. FEV1/FVC below 70% indicates obstruction. Reduced FVC with normal or elevated FEV1/FVC ratio suggests restriction.
Lung Volumes
Total lung capacity (TLC) is the volume of air in the lungs after maximum inspiration. Normal TLC is approximately 6 L in adult males. Residual volume (RV) is the volume remaining after maximum expiration. Normal RV is approximately 1.2 L. Functional residual capacity (FRC) is the volume remaining after normal expiration. Increased TLC and RV indicate hyperinflation from air trapping. Decreased TLC indicates restriction.
Diffusing Capacity
Diffusing capacity for carbon monoxide (DLCO) measures gas transfer across the alveolar-capillary membrane. Reduced DLCO indicates emphysema, interstitial lung disease, pulmonary vascular disease, or anemia. Increased DLCO occurs with pulmonary hemorrhage, polycythemia, or increased pulmonary blood flow.
Hemodynamic Parameters
Blood Pressure
Normal systolic blood pressure is 90 to 120 mmHg. Normal diastolic blood pressure is 60 to 80 mmHg. Mean arterial pressure (MAP) = diastolic + 1/3 (systolic - diastolic). Normal MAP is 70 to 105 mmHg.
Heart Rate
Normal adult heart rate is 60 to 100 beats per minute. Bradycardia is heart rate below 60. Tachycardia is heart rate above 100.
Central Venous Pressure
Normal CVP ranges from 2 to 6 mmHg. Elevated CVP suggests volume overload, right heart failure, or cardiac tamponade. Low CVP suggests hypovolemia.
Pulmonary Artery Pressure
Normal pulmonary artery systolic pressure is 15 to 30 mmHg. Normal pulmonary artery diastolic pressure is 4 to 12 mmHg. Mean pulmonary artery pressure above 25 mmHg at rest indicates pulmonary hypertension.
Pulmonary Artery Wedge Pressure
Normal PAWP ranges from 6 to 12 mmHg. Elevated PAWP indicates left heart failure, mitral valve disease, or volume overload. PAWP above 18 mmHg suggests cardiogenic pulmonary edema.
Cardiac Output
Normal cardiac output is 4 to 8 L/min. Cardiac index (CI) normalizes for body size: CI = CO/BSA. Normal CI is 2.5 to 4.0 L/min/m2.
| Measurement | Normal | High Suggests |
|---|---|---|
| CVP | 2-6 mmHg | Fluid overload, right heart failure, tamponade |
| PAP (systolic/diastolic) | 15-30 / 4-12 mmHg | Pulmonary hypertension, hypoxemia, PE |
| PAWP | 6-12 mmHg | Left heart failure, mitral disease; above 18 suggests cardiogenic edema |
| Cardiac output | 4-8 L/min | Low: pump failure or hypovolemia |
| Cardiac index | 2.5-4.0 L/min/m2 | Low: inadequate perfusion |
| MAP | 70-105 mmHg | Below 65 threatens organ perfusion |
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