Respiratory Assessment and Oxygen Therapy

Mastering respiratory assessment and oxygen therapy is a cornerstone of nursing practice, as pulmonary compromise can rapidly escalate into life-threatening failure. Your ability to accurately assess a patient's breathing and administer appropriate oxygen therapy directly impacts outcomes, from managing chronic obstructive pulmonary disease (COPD) to stabilizing acute respiratory distress.

Foundational Components of Respiratory Assessment

A systematic respiratory assessment begins with inspection, a deliberate visual examination. You observe the patient's general appearance for signs of distress, such as a tripod position or pursed-lip breathing. Note the skin color for cyanosis, particularly in the lips, nail beds, and mucous membranes. Assess the chest wall for symmetrical expansion, use of accessory muscles (neck and shoulder muscles), and any abnormal shape like barrel chest. The rate, rhythm, and depth of respirations are critical; tachypnea (rapid breathing) or bradypnea (slow breathing) are significant findings.

Following inspection, auscultation of breath sounds provides direct auditory information about airflow through the tracheobronchial tree. Using the diaphragm of your stethoscope, listen over symmetrical lung fields from apices to bases. Normal vesicular breath sounds are soft and low-pitched, heard over most lung fields. Bronchial breath sounds are louder, higher-pitched, and normally heard only over the trachea; their presence in peripheral lung fields can indicate consolidation, as in pneumonia. Adventitious (abnormal) sounds include crackles (fine, discontinuous sounds often from fluid, as in heart failure), wheezes (high-pitched, musical sounds from narrowed airways, as in asthma), and rhonchi (low-pitched, snoring sounds often from secretions).

Diagnostic Measurements: SpO2 and Arterial Blood Gas

While inspection and auscultation are clinical skills, objective data is provided by pulse oximetry interpretation and arterial blood gas (ABG) analysis. Pulse oximetry measures the percentage of hemoglobin saturated with oxygen (SpO2). A normal value is typically 95-100%. It is a vital continuous monitor but has limitations: it can be falsely elevated in carbon monoxide poisoning, is unreliable in poor peripheral perfusion, and does not provide information on carbon dioxide levels or pH.

An arterial blood gas (ABG) analysis gives a complete picture of respiratory and metabolic function. It measures:

• pH (acidity): Normal 7.35-7.45
• PaCO2 (partial pressure of carbon dioxide): Normal 35-45 mm Hg
• PaO2 (partial pressure of oxygen): Normal 80-100 mm Hg on room air
• HCO3- (bicarbonate): Normal 22-26 mEq/L

Interpreting an ABG requires a stepwise approach. First, assess the pH to determine acidemia (<7.35) or alkalemia (>7.45). Next, look at the PaCO2 to see if the respiratory component (acid) explains the pH shift. Then, evaluate the HCO3- to determine if the metabolic component (base) is involved. For example, in an acute COPD exacerbation, you would expect a respiratory acidosis: low pH, high PaCO2, with a normal or slightly elevated HCO3- as compensation begins.

Evaluating Work of Breathing and Selecting Oxygen Devices

Work of breathing (WOB) evaluation synthesizes your observations. Increased WOB is the physiological effort required to move air into and out of the lungs. Signs include tachypnea, nasal flaring, intercostal or suprasternal retractions, and marked use of accessory muscles. It is a critical indicator of respiratory fatigue, often preceding a decline in SpO2 or ABG values.

The goal of oxygen therapy is to correct hypoxemia (low blood oxygen) while minimizing complications. Selection depends on the required FiO2 (fraction of inspired oxygen, from 21% room air to 100%) and patient comfort. Devices are categorized as low-flow or high-flow systems.

Low-flow systems deliver oxygen at a rate less than the patient's inspiratory flow rate, so room air is entrained, diluting the oxygen. Common devices include:

• Nasal cannula: Delivers 24-44% FiO2 at 1-6 L/min. Low concentration, well-tolerated for chronic needs.
• Simple face mask: Delivers 40-60% FiO2 at 5-8 L/min (requires >5 L/min to flush CO2). Used for short-term, moderate oxygen needs.
• Non-rebreather mask: A reservoir bag with a one-way valve, delivering 80-95% FiO2 at 10-15 L/min. It is the highest-concentration low-flow device, used for acute, severe hypoxemia.

High-flow systems deliver a precise FiO2 at flow rates meeting or exceeding the patient's inspiratory demand. The high-flow nasal cannula (HFNC) delivers warmed, humidified oxygen at up to 60 L/min and an FiO2 up to 100%. It provides positive airway pressure, improves oxygenation and ventilation, and is often used in acute hypoxemic respiratory failure.

Nursing Considerations for Patients with Chronic Lung Disease

Administering oxygen to patients with chronic lung disease, such as COPD, requires specific caution. These patients often have a "hypoxic drive," where their primary stimulus to breathe is low oxygen levels (hypoxemia) rather than high carbon dioxide levels (hypercapnia). The longstanding fear was that excessive oxygen would abolish this drive, causing hypoventilation and dangerous CO2 retention (hypercapnic respiratory failure). Current evidence supports titrating oxygen to a target SpO2 of 88-92% in this population, avoiding the historical practice of withholding oxygen from a hypoxic patient. Your role involves vigilant monitoring with ABGs, assessing for somnolence or headache (signs of CO2 narcosis), and using Venturi masks when precise, low FiO2 delivery is critical.

Common Pitfalls

1. Relying Solely on Pulse Oximetry: A patient with severe anemia or carbon monoxide poisoning may have a normal SpO2 while being profoundly hypoxic at the tissue level. Always correlate SpO2 with clinical assessment and ABG results when the patient's status is unclear.
2. Incorrect Device Selection and Application: Placing a nasal cannula on a patient in severe respiratory distress provides inadequate oxygen. Conversely, using a non-rebreather mask without inflating the reservoir bag or at a low flow rate turns it into a simple mask, delivering a dangerously unpredictable and potentially lower FiO2. Ensure the reservoir bag is inflated and the oxygen flow is set to 10-15 L/min.
3. Over-oxygenating the COPD Patient: While the hypoxic drive is often overemphasized, blindly titrating oxygen to an SpO2 > 94% in a known CO2 retainer can suppress respiratory drive and worsen acidosis. Use a controlled oxygen device (like a Venturi mask) for precise delivery and target the 88-92% saturation range.
4. Neglecting Humidification: Delivering oxygen at flows greater than 4 L/min via nasal cannula or any flow via a mask for prolonged periods dries mucous membranes, causing discomfort and impairing mucociliary clearance. Always add a humidifier for long-term therapy or high-flow delivery.

Summary

• A thorough respiratory assessment integrates inspection, auscultation of breath sounds, interpretation of pulse oximetry and arterial blood gas values, and evaluation of work of breathing to form a complete clinical picture.
• Oxygen delivery devices range from low-flow (nasal cannula, simple face mask, non-rebreather mask) to high-flow systems like HFNC, each with specific flow rates, FiO2 ranges, and clinical applications.
• Key nursing considerations for patients with chronic lung disease include titrating oxygen to a target SpO2 of 88-92%, monitoring for signs of CO2 narcosis, and using precise delivery devices to avoid suppressing the hypoxic drive.
• Always treat the patient, not the monitor: clinical assessment supersedes any single number. Proper device application and understanding of its limitations are as important as selecting the right device.