A patient has the following ABG: pH 7.52, PaCO2 28 mmHg, HCO3 24 mEq/L, PaO2 105 mmHg. What is the interpretation?

Respiratory alkalosis, uncompensated. pH is high (alkalosis), PaCO2 is low (respiratory cause), and HCO3 is normal (no metabolic compensation). This is uncompensated respiratory alkalosis, commonly caused by hyperventilation due to anxiety, pain, hypoxemia, or early sepsis. The PaO2 is normal, ruling out hypoxemia as a trigger.

A patient's PFT results show: FEV1 45% predicted, FVC 82% predicted, FEV1/FVC ratio 0.55. This pattern is MOST consistent with:

Moderate obstructive disease. An FEV1/FVC ratio below 0.70 indicates an obstructive pattern. The FEV1 of 45% predicted classifies this as moderate obstruction (40-59% = moderate). The FVC is relatively preserved at 82%, which is typical of obstructive disease. This pattern is commonly seen in COPD or chronic asthma.

An elevated A-a gradient in a hypoxemic patient MOST likely indicates:

An intrapulmonary cause such as V/Q mismatch or shunt. An elevated A-a gradient indicates that the lungs are not effectively transferring oxygen from the alveoli to the blood. This points to intrapulmonary causes of hypoxemia such as V/Q mismatch (COPD, PE), intrapulmonary shunt (ARDS, pneumonia), or diffusion impairment (fibrosis). Hypoventilation and low FiO2 cause hypoxemia with a normal A-a gradient.

On a chest X-ray, the tip of an endotracheal tube should be positioned:

3-5 cm above the carina. The ideal ET tube tip position is 3-5 cm above the carina, approximately at the level of the aortic knob on CXR. If the tube is too high, it may slip out or cause vocal cord damage. If too low, it may enter the right mainstem bronchus (right mainstem intubation), causing left lung atelectasis.

A patient has the following ABG: pH 7.48, PaCO2 30 mmHg, HCO3 20 mEq/L, PaO2 55 mmHg. The MOST likely interpretation is:

Respiratory alkalosis with partial metabolic compensation and hypoxemia. The pH is high (alkalosis), PaCO2 is low (respiratory cause = hyperventilation), and HCO3 is slightly low (metabolic compensation by the kidneys excreting HCO3). Since the pH is still abnormal, this is partially compensated respiratory alkalosis. The PaO2 of 55 indicates hypoxemia, which is likely the stimulus for hyperventilation.

1.2 Diagnostic Procedures & ABG Interpretation | CRT/RRT Respiratory Therapist Exam | Free Exam Prep

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%
Use the 5-step approach: check pH, check PaCO2, check HCO3, determine primary disorder, assess compensation
Respiratory acidosis: pH <7.35 with PaCO2 >45 (hypoventilation); respiratory alkalosis: pH >7.45 with PaCO2 <35 (hyperventilation)
Metabolic acidosis: pH <7.35 with HCO3 <22 (diabetic ketoacidosis, renal failure, lactic acidosis); metabolic alkalosis: pH >7.45 with HCO3 >26 (vomiting, diuretics)
PFT interpretation: FEV1/FVC ratio <0.70 indicates obstructive disease; FVC <80% predicted with normal ratio indicates restrictive disease
Spirometry measures: FVC (total exhaled volume), FEV1 (volume in first second), FEV1/FVC ratio, PEF (peak expiratory flow)
Chest X-ray basics: white (opaque) = fluid/consolidation, black (lucent) = air, silhouette sign, air bronchograms
The A-a gradient helps distinguish intrapulmonary from extrapulmonary causes of hypoxemia

Diagnostic Procedures & ABG Interpretation

Arterial blood gas (ABG) analysis is the single most important diagnostic skill for the TMC exam. It appears in virtually every content domain and is essential for managing ventilated patients, titrating oxygen therapy, and assessing acid-base status.

ABG Normal Values

Parameter	Normal Range	What It Measures
pH	7.35 - 7.45	Overall acid-base balance
PaCO2	35 - 45 mmHg	Respiratory component (ventilation)
PaO2	80 - 100 mmHg	Oxygenation (on room air)
HCO3	22 - 26 mEq/L	Metabolic component (kidneys)
SaO2	95 - 100%	Oxygen saturation of hemoglobin
Base Excess (BE)	-2 to +2 mEq/L	Metabolic component indicator

5-Step ABG Interpretation Method

Step 1: Evaluate the pH

pH < 7.35 = Acidemia (acidosis)
pH > 7.45 = Alkalemia (alkalosis)
pH 7.35-7.45 = Normal (but may still have a compensated disorder)

Step 2: Evaluate PaCO2 (Respiratory Component)

PaCO2 > 45 mmHg = Respiratory acidosis (hypoventilation)
PaCO2 < 35 mmHg = Respiratory alkalosis (hyperventilation)
PaCO2 35-45 mmHg = Normal respiratory component

Step 3: Evaluate HCO3 (Metabolic Component)

HCO3 < 22 mEq/L = Metabolic acidosis
HCO3 > 26 mEq/L = Metabolic alkalosis
HCO3 22-26 mEq/L = Normal metabolic component

Step 4: Determine the Primary Disorder

The component that matches the direction of pH change is the primary disorder
Example: pH 7.30 (acidosis) + PaCO2 55 (acidosis) = Primary respiratory acidosis

Step 5: Assess Compensation

Uncompensated: Only one component is abnormal, pH is abnormal
Partially compensated: Both components are abnormal, pH is still abnormal
Fully compensated: Both components are abnormal, pH is normal (7.35-7.45)

Common ABG Patterns

Condition	pH	PaCO2	HCO3	Clinical Example
Respiratory acidosis (acute)	Low	High	Normal	Drug overdose, neuromuscular disease
Respiratory acidosis (chronic)	Low-normal	High	High	COPD, obesity hypoventilation
Respiratory alkalosis	High	Low	Normal	Anxiety, hypoxemia, PE, pain
Metabolic acidosis	Low	Normal	Low	DKA, renal failure, lactic acidosis
Metabolic alkalosis	High	Normal	High	Vomiting, NG suctioning, diuretics
Combined resp + met acidosis	Very low	High	Low	Cardiac arrest, septic shock

The A-a Gradient

The alveolar-arterial (A-a) gradient helps distinguish causes of hypoxemia:

Formula: A-a gradient = PAO2 - PaO2

Where PAO2 = (FiO2 x 713) - (PaCO2 / 0.8)

Normal A-a gradient: < 10 mmHg (increases with age; approximate normal = age/4 + 4)
Elevated A-a gradient: Indicates intrapulmonary problem (V/Q mismatch, shunt, diffusion impairment)
Normal A-a gradient with hypoxemia: Suggests hypoventilation or low FiO2 as the cause

Pulmonary Function Testing (PFTs)

PFTs are essential for diagnosing and classifying respiratory diseases. The TMC exam tests your ability to interpret PFT results and distinguish between obstructive and restrictive patterns.

Key Spirometry Measurements

Measurement	Definition	Normal Value
FVC	Total volume forcefully exhaled after maximal inhalation	>80% predicted
FEV1	Volume exhaled in the first second of FVC maneuver	>80% predicted
FEV1/FVC Ratio	Percentage of FVC exhaled in the first second	>0.70 (70%)
PEF	Maximum expiratory flow rate	>80% predicted
FEF25-75%	Average flow during middle 50% of FVC	Sensitive early airway disease marker

Obstructive vs. Restrictive Patterns

Parameter	Obstructive	Restrictive
FEV1/FVC	Decreased (<0.70)	Normal or increased
FVC	Normal or decreased	Decreased (<80% predicted)
FEV1	Decreased	Decreased
TLC	Increased (air trapping)	Decreased
RV	Increased	Decreased
Examples	COPD, asthma, bronchiectasis, CF	Pulmonary fibrosis, scoliosis, obesity, neuromuscular disease

Severity Classification for Obstructive Disease (based on FEV1 % predicted):

Severity	FEV1 % Predicted
Mild	60-79%
Moderate	40-59%
Severe	<40%

Chest X-ray Interpretation Basics

Respiratory therapists should be able to identify common CXR findings:

Finding	Appearance	Conditions
Consolidation	White (opaque) area	Pneumonia, pulmonary edema
Pneumothorax	Black (lucent) area without lung markings	Spontaneous, traumatic, iatrogenic
Pleural effusion	White area at bases with meniscus sign	CHF, infection, malignancy
Hyperinflation	Flattened diaphragms, increased AP diameter	COPD, severe asthma
Atelectasis	White area with volume loss, tracheal shift toward	Post-operative, mucus plug, tumor
Air bronchograms	Air-filled bronchi visible within consolidation	Pneumonia, ARDS
ET tube position	Tip should be 3-5 cm above the carina	Verify placement on every CXR

CRT/RRT Respiratory Therapist Exam

1Introduction: NBRC TMC Exam Overview

2Chapter 1: Patient Data Evaluation

3Chapter 2: Airway Management & Oxygen Therapy

4Chapter 3: Mechanical Ventilation

5Chapter 4: Pharmacology & Equipment

6Chapter 5: Infection Control, Ethics & Professional Practice