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1.2 Diagnostic Procedures & ABG Interpretation

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CRT/RRT Respiratory Therapist Exam Exam

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1 Introduction: NBRC TMC Exam Overview

NBRC TMC Exam Overview Credential Pathway: CRT to RRT Study Strategies for the TMC Exam Content Domains & Weight Distribution

2 Chapter 1: Patient Data Evaluation

1.1 Patient History & Physical Assessment 1.2 Diagnostic Procedures & ABG Interpretation 1.3 Hemodynamic Monitoring & Waveform Analysis 1.4 Laboratory Values & Diagnostic Data

3 Chapter 2: Airway Management & Oxygen Therapy

2.1 Airway Anatomy & Assessment 2.2 Airway Management Devices & Procedures 2.3 Oxygen Therapy Devices & Delivery 2.4 Humidification & Aerosol Therapy

4 Chapter 3: Mechanical Ventilation

3.1 Ventilation Fundamentals 3.2 Initial Ventilator Settings 3.3 Ventilator Monitoring & Waveform Analysis 3.4 Weaning & Liberation from Mechanical Ventilation

5 Chapter 4: Pharmacology & Equipment

4.1 Respiratory Pharmacology 4.2 Equipment & Troubleshooting 4.3 Specialty Procedures

6 Chapter 5: Infection Control, Ethics & Professional Practice

5.1 Infection Control & Prevention 5.2 Emergency Procedures & Resuscitation 5.3 Professional Practice & Ethics

1.2 Diagnostic Procedures & ABG Interpretation

Key Takeaways Explain This Topic AI

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.

Last updated: June 2026

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.