1.2 Diagnostic Procedures & ABG Interpretation
Key Takeaways
- ABG normal values: pH 7.35-7.45, PaCO2 35-45 mmHg, PaO2 80-100 mmHg, HCO3 22-26 mEq/L, SaO2 95-100%, base excess -2 to +2.
- Use the fixed 5-step method: read pH, read PaCO2, read HCO3, name the primary disorder, then grade compensation.
- The component whose abnormality matches the pH direction is the PRIMARY disorder; the opposing component shows compensation.
- Expected acute compensation: PaCO2 changes ~10 mmHg shifts pH ~0.08, and acute respiratory changes move HCO3 ~1 mEq/L per 10 mmHg CO2.
- PFT: FEV1/FVC below 70% defines obstruction; a reduced FVC with a normal ratio defines restriction (confirm with TLC).
- Obstructive severity by FEV1 percent predicted: mild 60-79%, moderate 40-59%, severe under 40%.
- A-a gradient = PAO2 minus PaO2; an elevated gradient localizes hypoxemia to the lung (shunt, V/Q mismatch, diffusion defect).
- On chest film the endotracheal tube tip belongs 3-5 cm above the carina; too low produces right-mainstem intubation and left-lung collapse.
ABG Analysis — the Highest-Yield Skill
Arterial blood gas interpretation threads through nearly every TMC domain: oxygen titration, ventilator changes, and acid-base calls all hinge on it. Memorize the reference set, then apply one repeatable algorithm.
| Parameter | Normal Range | Measures |
|---|---|---|
| pH | 7.35-7.45 | Net acid-base status |
| PaCO2 | 35-45 mmHg | Respiratory (ventilation) |
| PaO2 | 80-100 mmHg (room air) | Oxygenation |
| HCO3 | 22-26 mEq/L | Metabolic (renal) |
| SaO2 | 95-100% | Hemoglobin saturation |
| Base excess | -2 to +2 mEq/L | Metabolic indicator |
The 5-Step Method
- pH — below 7.35 = acidemia, above 7.45 = alkalemia. A normal pH may still hide a fully compensated disorder.
- PaCO2 — above 45 = respiratory acidosis (hypoventilation); below 35 = respiratory alkalosis (hyperventilation).
- HCO3 — below 22 = metabolic acidosis; above 26 = metabolic alkalosis.
- Primary disorder — the component whose direction matches the pH is primary. pH 7.28 with PaCO2 58 = primary respiratory acidosis.
- Compensation — uncompensated: one component abnormal, pH abnormal. Partially compensated: both abnormal, pH still abnormal. Fully compensated: both abnormal, pH back within 7.35-7.45.
A quick sanity rule: an acute 10 mmHg rise in PaCO2 lowers pH about 0.08; an acute 10 mmHg fall raises pH about 0.08. If the measured pH shift is far larger than CO2 alone predicts, a metabolic process is also present.
Pattern Recognition
| Disorder | pH | PaCO2 | HCO3 | Classic Cause |
|---|---|---|---|---|
| Acute respiratory acidosis | Low | High | Normal | Opioid overdose, neuromuscular failure |
| Chronic respiratory acidosis | Low-normal | High | High | COPD, obesity hypoventilation |
| Respiratory alkalosis | High | Low | Normal | Anxiety, pain, hypoxemia, PE, early sepsis |
| Metabolic acidosis | Low | Normal/low | Low | DKA, lactic acidosis, renal failure |
| Metabolic alkalosis | High | Normal/high | High | Vomiting, NG suction, diuretics |
| Combined acidosis | Very low | High | Low | Cardiac arrest, severe shock |
For metabolic acidosis, classify by anion gap = Na+ - (Cl- + HCO3-); normal is 8-12. A high gap (DKA, lactate, toxins) versus a normal gap (diarrhea, renal tubular acidosis) is a recurring exam distinction.
The A-a Gradient
The alveolar-arterial oxygen gradient localizes hypoxemia:
- PAO2 = (FiO2 x (Pbar - 47)) - (PaCO2 / 0.8); at sea level Pbar - 47 = 713.
- A-a gradient = PAO2 - PaO2; normal on room air is under 10-15 mmHg and widens with age (estimate age/4 + 4).
- An elevated gradient means an intrapulmonary problem — shunt (ARDS, pneumonia), V/Q mismatch (PE, COPD), or diffusion impairment (fibrosis).
- A normal gradient with hypoxemia points to hypoventilation or a low inspired FiO2 — the lung itself is transferring oxygen normally.
A practical companion is the PaO2/FiO2 ratio (P/F ratio), which grades oxygenation impairment without a calculation: a ratio at or below 300 with bilateral infiltrates meets the Berlin definition of acute respiratory distress syndrome, 200-300 is mild, 100-200 moderate, and below 100 severe ARDS. Because it needs only the ABG and the delivered FiO2, the P/F ratio is the number the exam expects you to quote when titrating PEEP and FiO2 on a ventilated patient.
Pulmonary Function Testing
PFTs classify chronic lung disease and appear on the TMC as result tables you must read at a glance.
| Measurement | Definition | Normal |
|---|---|---|
| FVC | Total volume forcibly exhaled after full inspiration | >80% predicted |
| FEV1 | Volume exhaled in the first second | >80% predicted |
| FEV1/FVC | Fraction of FVC exhaled in 1 second | >0.70 |
| PEF | Peak expiratory flow | >80% predicted |
| FEF25-75% | Mid-expiratory flow | Sensitive early small-airway marker |
| DLCO | Diffusing capacity for carbon monoxide | Low in emphysema/fibrosis, high in alveolar hemorrhage |
Obstructive vs Restrictive
| Parameter | Obstructive | Restrictive |
|---|---|---|
| FEV1/FVC | Decreased (<0.70) | Normal or increased |
| FVC | Normal or decreased | Decreased (<80%) |
| TLC | Increased (air trapping) | Decreased |
| RV | Increased | Decreased |
| Examples | COPD, asthma, bronchiectasis, CF | Fibrosis, kyphoscoliosis, obesity, neuromuscular |
Obstruction is graded by FEV1 percent predicted — mild 60-79%, moderate 40-59%, severe <40%. A clinically important bronchodilator response is a 12% AND 200 mL improvement in FEV1 or FVC after a beta-agonist, which supports an asthma diagnosis.
Chest Radiograph Basics
Dense tissue is white (radiopaque); air is black (radiolucent).
| Finding | Appearance | Conditions |
|---|---|---|
| Consolidation | White opacity, may show air bronchograms | Pneumonia, pulmonary edema, ARDS |
| Pneumothorax | Black field, absent lung markings | Spontaneous, traumatic, barotrauma |
| Pleural effusion | Basal white density, meniscus sign, blunted costophrenic angle | CHF, infection, malignancy |
| Hyperinflation | Flattened diaphragms, increased AP diameter | COPD, severe asthma |
| Atelectasis | Volume loss, trachea pulled TOWARD it | Mucus plug, postoperative |
| ET tube | Tip 3-5 cm above carina (~aortic knob) | Verify on every post-intubation film |
A tube tip below the carina enters the right mainstem bronchus, ventilating only the right lung and collapsing the left — a classic stem in which the correct action is to withdraw the tube a few centimeters and recheck breath sounds.
ABG on room air: pH 7.52, PaCO2 28 mmHg, HCO3 24 mEq/L, PaO2 105 mmHg. The interpretation is:
PFT results: FEV1 45% predicted, FVC 82% predicted, FEV1/FVC 0.55. This pattern indicates:
A hypoxemic patient has a widened A-a gradient. This finding most strongly suggests:
On a post-intubation chest film the endotracheal tube tip should sit:
ABG: pH 7.36, PaCO2 60 mmHg, HCO3 34 mEq/L. This represents: