4.1 Respiratory Failure & ABG Interpretation

Respiratory Failure & ABG Interpretation

Respiratory failure exists when the lungs cannot maintain adequate gas exchange -- delivering oxygen, clearing carbon dioxide, or both. On the CCRN you are rewarded for classifying the failure type first, because the type dictates the intervention.

Type I vs Type II Respiratory Failure

Type I (hypoxemic) respiratory failure is a PaO2 below 60 mmHg with a normal or low PaCO2. The defect is oxygenation, driven by ventilation-perfusion (V/Q) mismatch or true shunt -- alveoli that are perfused but not ventilated (pneumonia, pulmonary edema, ARDS, pulmonary embolism). Because shunted blood bypasses functioning alveoli, raising FiO2 corrects hypoxemia poorly when the shunt fraction is high; alveolar recruitment with PEEP is required.

Type II (hypercapnic) respiratory failure is a PaCO2 above 50 mmHg with an acute respiratory acidosis (pH < 7.35). The defect is ventilation -- the patient cannot move enough air to clear CO2. Causes include COPD and asthma, opioid or sedative overdose, neuromuscular weakness (Guillain-Barre, myasthenia gravis), and chest-wall restriction. The fix is to raise minute ventilation: reverse the cause, support with noninvasive positive-pressure ventilation (BiPAP), or intubate.

Recognizing Failure at the Bedside

Early signs are tachypnea, accessory-muscle use, and restlessness or confusion from hypoxia; late signs are somnolence and CO2 narcosis, then cyanosis. SpO2 tracks PaO2 along the oxyhemoglobin dissociation curve -- an SpO2 near 90% corresponds to a PaO2 of about 60 mmHg, the threshold of hypoxemic failure, so a falling SpO2 below 90% demands action. A rising PaCO2 with a falling pH marks decompensation and the need to support ventilation.

A Systematic ABG Method

Read every arterial blood gas (ABG) the same five-step way.

1. pH -- acidemia (< 7.35) or alkalemia (> 7.45)? Normal is 7.35-7.45.
2. PaCO2 -- the respiratory component, normal 35-45 mmHg. High CO2 is acidotic, low is alkalotic.
3. HCO3 -- the metabolic component, normal 22-26 mEq/L. Low is acidotic, high is alkalotic.
4. Name the primary disorder -- which value moves in the SAME direction as the pH abnormality? Low pH + high CO2 = respiratory acidosis; low pH + low HCO3 = metabolic acidosis.
5. Compensation -- uncompensated: pH abnormal, only the primary system shifted. Partial: pH still abnormal, both systems shifted. Full: pH back inside 7.35-7.45 while CO2 and HCO3 both remain abnormal.

Use ROME -- Respiratory Opposite, Metabolic Equal: in respiratory disorders pH and PaCO2 move in opposite directions; in metabolic disorders pH and HCO3 move together.

Worked Examples

Example 1 -- pH 7.28, PaCO2 58, HCO3 25: acidotic pH, high CO2 that matches, normal HCO3 -> uncompensated (acute) respiratory acidosis from hypoventilation.

Example 2 -- pH 7.32, PaCO2 30, HCO3 15 in a septic patient with elevated lactate: acidotic pH, low HCO3 that matches -> metabolic acidosis; the low CO2 is partial respiratory compensation (pH still < 7.35).

Example 3 -- pH 7.36, PaCO2 60, HCO3 33 in stable COPD: near-normal pH with both CO2 and HCO3 elevated -> fully compensated chronic respiratory acidosis, the patient's baseline. Treat the patient, not the number, and avoid oxygen high enough to blunt the hypoxic drive.

Oxygenation Indices

• PaO2/FiO2 (P/F) ratio -- divide PaO2 by FiO2 as a decimal. PaO2 70 on FiO2 0.70 gives a P/F of 100. Normal is > 400; a value <= 300 (with PEEP >= 5) defines ARDS.
• Alveolar-arterial (A-a) gradient -- A-a = PAO2 minus PaO2, where on room air at sea level PAO2 is about 150 minus (PaCO2/0.8). Normal is roughly (age/4) + 4, or 5-15 mmHg in young adults. A widened A-a gradient with hypoxemia signals a lung problem (shunt, V/Q mismatch, diffusion defect -- ARDS, PE, pneumonia). A normal A-a gradient with hypoxemia signals hypoventilation or low inspired oxygen.

The A-a gradient separates a primary lung problem from a ventilation/drive problem. A massive PE can preserve a near-normal P/F early yet markedly widen the A-a gradient because of dead-space physiology.

Common traps: treating a chronically compensated COPD gas as an emergency; expecting FiO2 alone to fix a large shunt; and calling a gas "compensated" when the pH is still outside 7.35-7.45 (that is only partial compensation). Always correlate the number with the patient -- work of breathing, mental status, and SpO2 trend -- before escalating.

Mixed Disorders and Oxygen Delivery

Some ICU gases show two primary disturbances at once -- for example a COPD patient (respiratory acidosis) who is also vomiting (metabolic alkalosis), or a septic patient with both metabolic acidosis and respiratory alkalosis. Suspect a mixed disorder when compensation is larger or smaller than the primary disturbance predicts. Calculating the anion gap (Na minus [Cl + HCO3], normal 8-12 mEq/L) uncovers an added high-gap metabolic acidosis (lactate, ketones, toxins) even when the pH looks compensated.

Oxygen delivery escalates from nasal cannula (1-6 L/min, roughly FiO2 24-44%) to simple and non-rebreather masks, then high-flow nasal cannula and noninvasive ventilation before intubation. Match the device to the failure type: hypoxemic (Type I) failure needs higher FiO2 and PEEP, while hypercapnic (Type II) failure needs ventilatory support such as BiPAP -- not simply more oxygen, which can worsen CO2 retention.