Physiology Questions for the CFRN Exam — Practice Free

Boyle's Law states that Pressure (P) and Volume (V) are inversely proportional (P1V1 = P2V2). As a flight nurse ascends, barometric pressure decreases, causing trapped gas volumes to expand. A pneumothorax will expand in volume as altitude increases, potentially converting a simple pneumothorax into a life-threatening tension pneumothorax. BCEN Exam Tip: Remember the mnemonic Boyle's Balloon. If pressure goes down (ascent), the balloon (volume) goes up. This is the most frequently tested gas law on the CFRN exam.

As ambient pressure decreases during ascent, the air inside the ETT cuff expands (Boyle's Law). This expansion can exert excessive pressure on the tracheal wall, leading to ischemia. Management: 1. Use a cuff manometer to maintain pressure between 20-30 cmH2O throughout the flight. 2. Alternatively, replace the air in the cuff with sterile saline (non-compressible) prior to takeoff, which eliminates volume changes. Exam Strategy: BCEN often tests the saline vs. air preference. While saline is a classic answer, real-time monitoring with a manometer is the modern gold standard. Avoid releasing air blindly as it risks aspiration.

Even a small 10% pneumothorax will expand by approximately 20-25% at a cabin altitude of 5,000-8,000 feet due to Boyle's Law. This expansion can lead to rapid clinical deterioration, hypoxia, and tension physiology that is difficult to manage in a cramped cabin. Clinical Pearl: Always vent the pleural space before flight if air is present. If a chest tube is placed, ensure it is connected to a one-way valve (e.g., Heimlich valve) or a flight-certified drainage system. Distractor Alert: Do not rely on close observation alone; the BCEN expects proactive management of trapped gas.

Trapped gas in the stomach or intestines will expand as barometric pressure drops (ascent). In a patient with an ileus or bowel obstruction, this expansion can cause: 1. Significant abdominal distention. 2. Compression of the diaphragm, leading to decreased tidal volumes and respiratory distress. 3. Increased risk of vomiting and aspiration. CFRN Key Point: Gastric decompression (OG/NG tube) is a must-do intervention for almost all flight patients, especially those being intubated or with abdominal trauma, to mitigate the effects of Boyle's Law.

The IABP uses helium, which is subject to Boyle's Law. As altitude increases, the helium volume expands. Clinical Management: 1. Most modern transport IABPs (e.g., Maquet Cardiosave) have automated atmospheric pressure compensation that purges and refills the balloon during ascent/descent. 2. If the Autofill fails, the nurse must manually trigger a purge/fill cycle to ensure the balloon volume remains appropriate for the aorta. Exam Tip: BCEN focuses on the need for purging the system during altitude changes to maintain trigger accuracy and prevent balloon over-inflation.