Oxygenation depends on four linked steps. A problem at any step produces hypoxemia (low oxygen in arterial blood) or hypoxia (low oxygen at the tissues).
| Step | What happens | Example of failure |
|---|
| Ventilation | Air moves in and out of the lungs | Opioid overdose, neuromuscular weakness, airway obstruction |
| Diffusion | O₂ and CO₂ cross the alveolar-capillary membrane | Pulmonary edema, fibrosis |
| Perfusion | Blood flows past ventilated alveoli | Pulmonary embolism, shock |
| Transport | Hemoglobin carries O₂ to tissues | Anemia, carbon monoxide poisoning |
Ventilation-perfusion (V/Q) mismatch is the most common cause of hypoxemia. Because CO₂ diffuses about 20 times more easily than O₂, early V/Q mismatch lowers PaO₂ while PaCO₂ stays normal or even falls (the client hyperventilates). A rising PaCO₂ means ventilation itself is failing.
Breathing is driven mainly by CO₂ (through pH) acting on central chemoreceptors in the medulla; peripheral chemoreceptors in the carotid and aortic bodies respond to low PaO₂.
Risk factors for impaired oxygenation: smoking, obesity, immobility, surgery (especially chest or upper abdominal), older age (weaker cough, stiffer chest wall), chronic lung and heart disease, decreased level of consciousness, and environmental exposures (secondhand smoke, biomass fuel, dust, mold).
Health history
- Dyspnea: onset, triggers, at rest or on exertion, orthopnea (needs to sit up to breathe), paroxysmal nocturnal dyspnea. Dyspnea is the client's subjective sensation of difficult breathing; rate it with a 0–10 scale.
- Cough: dry or productive, timing (morning cough in chronic bronchitis)
- Sputum: amount, color, consistency, odor, blood, and time of day — clear/white (normal or viral), yellow-green (infection), rust-colored (pneumococcal pneumonia), pink frothy (pulmonary edema), bright red streaks (hemoptysis)
- Smoking history in pack-years (packs per day × years smoked), occupational and home exposures, medications, activity tolerance
Inspection
- Rate, depth, rhythm, effort; accessory muscle use, intercostal retractions, nasal flaring, tripod position, pursed-lip breathing
- Ability to speak full sentences (speaking only 2–3 words indicates severe distress)
- Chest shape: barrel chest (anteroposterior diameter close to lateral diameter) in COPD; kyphosis; pectus deformities
- Skin and mucous membranes: central cyanosis is best seen on the lips, tongue, and oral mucosa and reflects low arterial saturation; peripheral cyanosis (nail beds, earlobes, nose tip) reflects slow peripheral blood flow. In darker skin, inspect the oral mucosa, conjunctivae, and nail beds. Cyanosis is a late and unreliable sign — it may be absent in anemia.
- Clubbing (loss of the normal angle at the nail base) suggests chronic hypoxemia
Palpation
- Tracheal position: place a finger in the suprasternal notch and gently move side to side; the trachea should be midline. Deviation suggests tension pneumothorax (shift away) or atelectasis (shift toward).
- Chest expansion: place thumbs at the 10th rib posteriorly, hands on the lateral chest; as the client inhales deeply, the thumbs should separate equally. Unequal movement suggests pneumothorax, pleural effusion, or splinting.
- Tactile fremitus increases with consolidation (pneumonia) and decreases with air or fluid in the pleural space
Percussion: resonance over normal lung; dullness over consolidation or effusion; hyperresonance in emphysema or pneumothorax.
Auscultation — use the diaphragm, compare side to side, move from top to bottom, and ask the client to breathe slowly through the mouth.
| Normal sound | Where heard | Features |
|---|
| Vesicular | Most of the peripheral lung fields | Soft, low-pitched; inspiration longer than expiration |
| Bronchovesicular | Near the main bronchi (1st–2nd intercostal spaces anteriorly, between scapulae) | Medium pitch; inspiration equals expiration |
| Bronchial (tracheal) | Over the trachea and manubrium | Loud, high-pitched; expiration longer than inspiration |
Bronchial sounds heard over the peripheral lung are abnormal and suggest consolidation.
| Adventitious sound | Description | Common causes |
|---|
| Crackles (fine or coarse) | Discontinuous popping, usually on inspiration | Pneumonia, heart failure, atelectasis, fibrosis |
| Wheezes | Continuous high-pitched musical sounds, often on expiration | Asthma, COPD, bronchospasm |
| Rhonchi (low-pitched wheezes) | Snoring sounds that may clear with coughing | Secretions in large airways |
| Stridor | High-pitched sound on inspiration, loudest over the neck | Upper airway obstruction — emergency |
| Pleural friction rub | Grating sound in both phases, localized | Pleuritis |
| Diminished or absent sounds | Reduced air entry | Atelectasis, effusion, pneumothorax, severe bronchospasm ("silent chest"), obesity |
Early vs. late signs of hypoxemia
| Early | Late |
|---|
| Restlessness, anxiety, confusion | Cyanosis |
| Tachycardia, tachypnea | Bradycardia |
| Rising BP | Hypotension |
| Dyspnea, accessory muscle use | Decreased level of consciousness, dysrhythmias |
| Test | Key points |
|---|
| Pulse oximetry (SpO₂) | The most common noninvasive, continuous method; check the waveform and site; false readings with poor perfusion, motion, nail polish; falsely normal in carbon monoxide poisoning; unreliable in methemoglobinemia (reads near 85%); may overestimate saturation in darker skin (occult hypoxemia; FDA 2025 draft guidance addresses this) |
| Arterial blood gases (ABGs) | Invasive. PaO₂ is the most direct measure of oxygenation; also gives pH, PaCO₂, HCO₃⁻ |
| Capnography (EtCO₂) | Noninvasive trend of ventilation; normal about 35–45 mmHg; detects hypoventilation earlier than SpO₂ |
| Chest X-ray | Infiltrates, effusion, pneumothorax, tube position |
| Pulmonary function tests / spirometry | FEV₁, FVC, FEV₁/FVC ratio; FEV₁/FVC < 0.70 indicates obstruction; peak expiratory flow tracks asthma control |
| Sputum culture and sensitivity | Collect early morning, after mouth rinse with water, before antibiotics |
| CBC | Hemoglobin (O₂-carrying capacity), WBC (infection) |
| Sleep study (polysomnography) | For suspected obstructive sleep apnea |
Normal ABG values (arterial, adult at sea level)
| Value | Normal |
|---|
| pH | 7.35–7.45 |
| PaCO₂ | 35–45 mmHg (4.7–6.0 kPa) |
| HCO₃⁻ | 22–26 mEq/L (mmol/L) |
| PaO₂ | 80–100 mmHg (10.7–13.3 kPa) |
| SaO₂ | 95–100% |
Before an ABG from the radial artery, perform an Allen test (or confirm collateral flow per policy); afterward apply firm pressure for at least 5 minutes (longer with anticoagulants), send the sample for analysis within about 30 minutes (plastic syringes stay at room temperature; ice only if the lab requires it, e.g., glass syringe or delayed analysis), and note the oxygen delivery and FiO₂ on the requisition.
Quick ABG interpretation
- pH < 7.35 = acidosis; > 7.45 = alkalosis.
- PaCO₂ moving opposite to pH = respiratory cause.
- HCO₃⁻ moving in the same direction as pH = metabolic cause.
- The other value moving to correct the pH = compensation (full if pH normal, partial if not).
Oxyhemoglobin relationship: an SpO₂ of about 90% corresponds to a PaO₂ of about 60 mmHg; below this point, saturation falls steeply with small drops in PaO₂.
Assessment findings guide the team's response:
- Target oxygen saturation is prescribed by the provider — unless the provider sets another target, commonly 92–96% or 94–98% for most acutely ill adults, and 88–92% for clients at risk of hypercapnic respiratory failure (e.g., severe COPD, obesity hypoventilation, neuromuscular disease).
- Treatment addresses the failing step: bronchodilators for bronchospasm, diuretics for pulmonary edema, antibiotics for pneumonia, anticoagulation for pulmonary embolism, blood transfusion for severe anemia, and ventilatory support for failing ventilation.
- See Promoting Oxygenation for oxygen devices and airway care.
- Airway first. Stridor, inability to speak, or absent breath sounds are emergencies — call for help and stay with the client.
- Check SpO₂ immediately in any client reporting dyspnea; this is the fastest objective data point. Then assess rate, effort, and breath sounds, and apply oxygen as ordered or per protocol.
- Position upright (high-Fowler's or tripod) to improve lung expansion unless contraindicated.
- Assess the whole picture: level of consciousness, heart rate, skin color, and trend — not the SpO₂ number alone.
- For an intubated client with absent breath sounds bilaterally, suspect a displaced tube (esophageal placement); absent sounds on the left only suggest the tube has advanced into the right mainstem bronchus. Check tube depth at the lip, use capnography, and call for help.
- Identify nursing diagnoses from cues (see Promoting Oxygenation): ineffective airway clearance (secretions, weak cough), ineffective breathing pattern (retractions, abnormal rate), impaired gas exchange (abnormal ABGs, hypoxemia), activity intolerance (dyspnea on exertion only).
- Document findings with location (e.g., "coarse crackles right lower lobe, clear after coughing").
- Report new or worsening shortness of breath, chest pain, blood in sputum, or confusion.
- Stop smoking and avoid secondhand smoke; reduce indoor pollutants (smoke from cooking fuel, mold, dust).
- Use home pulse oximeters correctly: warm hands, remove nail polish, stay still, and read only with a steady waveform; follow the target range given by the provider.
- Stay current on vaccinations (influenza, pneumococcal, COVID-19, and RSV where recommended).
- Keep a symptom and peak-flow diary if prescribed.
| Red flag | Concern |
|---|
| Stridor, drooling, inability to speak | Upper airway obstruction |
| Sudden dyspnea, pleuritic chest pain, tachycardia | Pulmonary embolism or pneumothorax |
| Tracheal deviation with absent breath sounds and hypotension | Tension pneumothorax |
| Pink frothy sputum, crackles, orthopnea | Acute pulmonary edema |
| Drowsiness, headache, falling respiratory rate in a client with COPD on oxygen | Hypercapnia (CO₂ narcosis) |
| SpO₂ falling despite increasing oxygen | Worsening respiratory failure |
| New restlessness or confusion | Early hypoxemia — never assume it is only anxiety |
- Bronchial sounds are normal only over the trachea; vesicular sounds are normal over most lung fields
- Central cyanosis: lips and oral mucosa; peripheral cyanosis: nail beds and extremities
- Restlessness and confusion are early signs of hypoxemia; cyanosis is late
- Pulse oximetry = most common noninvasive method; ABG = invasive and most direct (PaO₂)
- SpO₂ is falsely normal in carbon monoxide poisoning
- Diminished breath sounds = decreased air entry (atelectasis, effusion, pneumothorax)
- Stridor = upper airway obstruction — emergency
- Barrel chest = COPD with air trapping
- Early V/Q mismatch: low PaO₂ with normal or low PaCO₂
- Chest expansion: thumbs at the 10th rib should separate equally
- Sputum assessment: amount, color, consistency, odor, blood, timing
- Allen test before radial ABG; firm pressure at least 5 minutes afterward
Country Notes
United States
- ABG reports and nursing references use mmHg; some point-of-care analyzers also display kPa.
- Home pulse oximeters are sold over the counter; teach clients that they are screening tools and may be less accurate in darker skin.
Philippines
- Household smoke from wood or charcoal cooking fuel and occupational dust are important non-smoking exposures to include in the history.
- Tuberculosis remains common; ask about chronic cough of 2 weeks or more, night sweats, weight loss, and hemoptysis, and refer for TB screening according to national program guidance.