Question

Arterial oxygen saturation at the forehead, chest and limbs can be measured with a:

• A. Transcutaneous reflectance oximeter
• B. Percutaneous transmittance oximeter
• C. Transcutaneous transmittance oximeter
• D. Blood gas analyzer

Hint:

Oximeter configuration choice depends heavily on anatomy: - Transmittance Mode: Limited to narrow appendages like fingers, toes, and earlobes. - Reflectance Mode: Can be applied to flat, thick bony areas like the forehead or chest. This mode is widely used in fetal scalp monitoring during labor and in athletic forehead bands or patches.

Answer 1

The Correct Option is A

Concept: Pulse oximetry measures arterial oxygen saturation ($\text{SpO}_2$) non-invasively by leveraging the distinct light absorption profiles of oxygenated hemoglobin ($\text{HbO}_2$) and deoxygenated hemoglobin ($\text{Hb}$). There are two optical configurations used to capture this data across skin surfaces:
• Transmittance Oximetry: The light source and the photodetector are placed directly opposite each other on a thin, transilluminable anatomical structure (like a finger, toe, or earlobe). Light passes entirely through the tissue layer to reach the detector.
• Reflectance Oximetry: The light source and the photodetector are mounted adjacent to one another on the same planar surface. The light penetrates the upper tissue layers, scatters off internal structures and vascular beds, and reflects back up to be captured by the adjacent photodetector.

Step 1: Analyzing the anatomical measurement sites.
The question specifies measurement at flat, thick, or bone-backed anatomical surfaces:
• Forehead: Supported directly by the frontal bone of the skull. It is physically impossible to pass light through the head to a detector on the opposite side.
• Chest: A thick muscular and bony barrier (sternum/ribcage) that completely blocks light transmission.
• Limbs: Thick muscular structures where direct light transmission is highly attenuated. Because light cannot pass completely through these sites, transmission-based oximetry cannot be used. Instead, we must capture backscattered light from the underlying tissue using a reflectance configuration.

Step 2: Distinguishing terminology options.

• Transcutaneous means passing or measuring through the intact skin surface non-invasively, which fits standard pulse oximetry methods.
• Percutaneous implies an invasive procedure that breaks or punctures the skin barrier (such as a needle or catheter probe). Oximeters used on the skin surface are non-invasive, making "percutaneous" incorrect.
• Blood gas analyzer provides an accurate measure of oxygen saturation ($\text{SaO}_2$), but it requires drawing an arterial blood sample and running an in-vitro biochemical analysis. It cannot directly measure saturation continuously at the forehead or chest. Therefore, measurements at the forehead, chest, and limbs rely on a Transcutaneous reflectance oximeter, which aligns with Option (A).