CRNA Diagnostics Flashcards: 60 Practice Questions & Answers

Pulse oximetry uses 660 nm (Red light) and 940 nm (Infrared light). Deoxyhemoglobin (reduced Hb) absorbs more light at 660 nm, while Oxyhemoglobin (O2Hb) absorbs more at 940 nm. The pulse oximeter measures the ratio of pulsatile (AC) to non-pulsatile (DC) light absorption at both wavelengths to calculate the R-value. Exam Strategy: NBCRNA frequently tests these specific numbers. Remember: Red = 660 (Deoxy), Infrared = 940 (Oxy). A shift in the ratio changes the calculated saturation based on pre-programmed calibration curves derived from healthy volunteers.

The most likely cause is rebreathing of CO2, typically due to exhausted CO2 absorbent (soda lime) or a faulty expiratory valve. An elevated baseline (Phase I > 0) is pathognomonic for rebreathing. Troubleshooting: The first step is to increase the Fresh Gas Flow (FGF) to 10 L/min or more. This converts the system to a non-rebreathing circuit, bypassing the absorbent. If the baseline returns to zero, the absorbent or the valves are the culprit. Exam Tip: NBCRNA distinguishes between exhausted absorbent (baseline elevation) and sampling line contamination (which can also cause baseline elevation but usually with a distorted waveform).

The facial muscles (Corrugator Supercilii) and the diaphragm are more resistant to NMBAs and recover faster than the Adductor Pollicis (AP). If monitoring only the facial muscles, the CRNA may overestimate recovery, as the AP (and more importantly, the upper airway dilator muscles) may still be significantly blocked. Exam Tip: Always use the AP for recovery/extubation assessment because it is the canary in the coal mine for airway muscles; use facial muscles for onset/intubation assessment as they mirror the diaphragm.

NMBAs can cause a sudden, significant decrease in the BIS value. This occurs because the monitor's algorithm often misinterprets EMG activity (30-300 Hz) as high-frequency Beta waves (wakefulness). Eliminating muscle tension removes this noise, revealing the true, lower hypnotic BIS. **Exam Tip:** If the BIS drops immediately after Vecuronium or Rocuronium without changes in anesthetic delivery, it is likely due to EMG suppression rather than a change in anesthetic depth.

Dexmedetomidine at clinical doses (0.2–0.7 mcg/kg/hr) is generally compatible with EP monitoring. While it may cause a slight, dose-dependent decrease in SSEP/MEP amplitude, it is much less depressive than volatile agents and is often used as an adjunct to TIVA. Exam Tip: Avoid high-dose boluses of Dexmedetomidine during critical monitoring phases, as transient signal loss can occur.