ABG Interpretation and Respiratory Nursing

Respiratory assessment skills — ABG interpretation, oxygen device selection, breath sound identification, and chest tube management — are foundational nursing competencies tested on virtually every nursing exam and the NCLEX. This guide covers these skills with the clinical specificity needed to apply them at the bedside and answer exam questions confidently.

ABG Interpretation: Step-by-Step

Normal ABG Values

• pH: 7.35–7.45 (acidosis <7.35; alkalosis >7.45)
• PaCO2: 35–45 mmHg (respiratory parameter — CO2 is an acid)
• HCO3: 22–26 mEq/L (metabolic parameter — bicarb is a base)
• PaO2: 80–100 mmHg (oxygenation)
• SaO2: 95–100%

The 4-Step Framework

1. Is the pH acidotic or alkalotic? <7.35 = acidosis. >7.45 = alkalosis
2. Look at PaCO2: If the PaCO2 matches the pH direction (high CO2 + low pH, or low CO2 + high pH) → respiratory problem
3. Look at HCO3: If the HCO3 matches the pH direction (low bicarb + low pH, or high bicarb + high pH) → metabolic problem
4. Compensation: The system not causing the problem will try to correct the pH. Partial = pH still abnormal. Fully compensated = pH within normal range despite abnormal CO2 and HCO3

ROME mnemonic: Respiratory Opposite (pH and CO2 move in opposite directions in respiratory disorders). Metabolic Equal (pH and HCO3 move in the same direction in metabolic disorders).

Common ABG Examples

• pH 7.28, CO2 58, HCO3 24: Respiratory acidosis, uncompensated. Cause: COPD exacerbation, hypoventilation, opioid overdose
• pH 7.50, CO2 30, HCO3 23: Respiratory alkalosis, uncompensated. Cause: anxiety/hyperventilation, pain, mechanical ventilation set too high
• pH 7.30, CO2 38, HCO3 17: Metabolic acidosis, uncompensated. Cause: DKA, renal failure, severe diarrhea, lactic acidosis
• pH 7.48, CO2 42, HCO3 31: Metabolic alkalosis, uncompensated. Cause: vomiting (loss of HCl), NG suction, excess antacids, hypokalemia

Oxygen Delivery Devices: Flow Rates and Indications

• Nasal cannula: 1–6 L/min. FiO2 ~24–44%. Each 1 L/min adds ~4% FiO2. Comfortable for mild hypoxemia, allows eating and talking
• Simple face mask: 6–10 L/min. FiO2 ~35–55%. Minimum 6 L/min required to flush exhaled CO2 from mask
• Non-rebreather mask (NRB): 10–15 L/min. FiO2 ~60–80%. Reservoir bag must stay inflated. Used for severe acute hypoxemia
• Venturi mask: Variable flow, precise fixed FiO2 (24%, 28%, 31%, 35%, 40%, 60%). Best for COPD where exact FiO2 control is critical — use 24–28% to maintain SpO2 88–92%
• High-flow nasal cannula (HFNC): Up to 60 L/min heated, humidified O2. Near-100% FiO2 possible. Used for severe hypoxemia to delay or avoid intubation

NCLEX critical point — COPD and oxygen: Target SpO2 88–92% in COPD, not the usual 95–100%. Patients with chronic CO2 retention may rely on hypoxic drive for respiratory stimulus. Excessive oxygen suppresses this drive and can cause respiratory depression. Use Venturi mask for precision.

Breath Sounds: What to Listen For

• Crackles (rales): Crackling on inspiration. Fluid in alveoli — pulmonary edema, pneumonia, heart failure. Fine crackles = small airways; coarse = large airways
• Wheezes: High-pitched continuous sound, mainly on expiration. Narrowed airways — asthma, bronchospasm, anaphylaxis
• Rhonchi: Low-pitched, rattling. Secretions in large airways. Clears or changes with coughing (distinguishes from wheeze)
• Stridor: High-pitched, harsh sound on inspiration. Upper airway obstruction — laryngeal edema, croup, foreign body. Airway emergency
• Pleural friction rub: Grating, leathery sound synchronous with breathing. Pleural inflammation (pleuritis, early effusion)
• Diminished/absent: Pleural effusion, pneumothorax, atelectasis, mucous plug

Chest Tube Management

• Water seal chamber: Gentle fluctuation (tidaling) with respirations = patent tube. Absent tidaling = lung re-expanded (good) or tube kinked/clotted (bad) — assess clinically
• Continuous bubbling in water seal: Air leak. Check all connections first. Persistent after connections checked = leak from patient
• Suction control chamber: Gentle continuous bubbling = correct suction level. Vigorous bubbling does not mean more suction — it means evaporation
• Drainage: Mark and document each shift. >100 mL/hr bright red blood = active hemorrhage → notify provider
• If chest tube disconnects: Submerge distal end in sterile water (water seal) or apply petroleum gauze. Never clamp unless ordered — risk of tension pneumothorax

Airway Suctioning Priorities

• Indications: Audible secretions, decreased SpO2, visible secretions in ETT, high-pressure ventilator alarm, decreased breath sounds
• Pre-oxygenate: 100% O2 for 30–60 seconds before suctioning
• Technique: Apply suction only while withdrawing catheter. No longer than 10–15 seconds per pass
• Frequency: Only when clinically indicated — routine scheduled suctioning causes mucosal trauma

NCLEX Tips: Respiratory Assessment Skills

• ROME: Respiratory Opposite, Metabolic Equal — simplest ABG interpretation framework
• Venturi mask = most precise O2 delivery. Best choice when exact FiO2 matters (COPD)
• Stridor = upper airway emergency — call for help immediately, prepare airway equipment
• Chest tube absent tidaling: assess whether lung has re-expanded (bilateral breath sounds, improving SpO2) or tube is obstructed
• Never clamp a chest tube without a provider order — tension pneumothorax risk
• COPD oxygen target 88–92% SpO2, not 95–100%

For NCLEX-ready notes on all major respiratory conditions — asthma, COPD, pneumonia, PE, pneumothorax, and ARDS — including clinical pharmacology and assessment priorities, see the Lectures Note Respiratory Bundle.

Official Resources and Further Reading

External References

• Global Initiative for Chronic Obstructive Lung Disease (GOLD) — evidence-based COPD management guidelines and staging criteria
• American Thoracic Society (ATS) — respiratory disease clinical practice guidelines and patient resources

Related Articles on Lectures Note

• Respiratory Nursing Study Guide — diseases, medications, and NCLEX interventions for the respiratory system
• Critical Care Nursing Study Notes — mechanical ventilation, ARDS, and ICU respiratory management

Frequently Asked Questions: Respiratory Assessment

What does ROME stand for in ABG interpretation?

Respiratory Opposite, Metabolic Equal. In respiratory disorders, pH and PaCO2 move in opposite directions (respiratory acidosis = pH low, CO2 high). In metabolic disorders, pH and HCO3 move in the same direction (metabolic acidosis = pH low, HCO3 low).

Which oxygen device delivers the highest FiO2?

A properly fitted non-rebreather mask at 15 L/min delivers 60–80% FiO2. High-flow nasal cannula at maximum settings can deliver near 100% FiO2 but requires specialized equipment. The NRB is the highest-output device available at the bedside without HFNC.

What does continuous bubbling in the water seal chamber mean?

An air leak. First check all connections for gaps. If connections are secure and bubbling continues, the air leak is coming from the patient — the lung may not be fully sealed. Notify the provider. Do not clamp the tube.