HESI A2 Reading Comprehension Practice Test

A pulse oximeter is the small clip that goes on a fingertip in a clinic or on a hospital ward, and it answers one question: what share of the hemoglobin in a person’s arterial blood is carrying oxygen? Hemoglobin is the protein inside red blood cells that picks oxygen up in the lungs and carries it to the tissues. A reading of 95 percent or higher is usually called normal. A reading below 90 percent is low, and clinicians call that hypoxemia. The device does not draw blood. It shines two kinds of light through the fingertip, red light and infrared light, and measures how much of each comes out on the other side. The two forms of hemoglobin absorb those wavelengths differently. Hemoglobin carrying oxygen absorbs more infrared light and lets more red light pass; hemoglobin that has given up its oxygen absorbs more red light. The oximeter compares the two amounts and turns the comparison into a percentage, which is what the number on the screen, labeled SpO2, reports. Only the pulsing part of the signal is used. Arterial blood surges with every heartbeat, while skin, bone and the blood in veins do not pulse, so the device subtracts the steady part of the light and keeps only the part that changes. That subtraction is why the reading is called pulse oximetry. It is also why a cold hand or a weak pulse can make the number unreliable: with less pulsing to measure, the device is estimating from a smaller signal. Several ordinary things can push a reading off. Nail polish, especially dark polish, can block the light. Cold fingers narrow the small vessels and reduce the pulse the sensor needs. Motion, bright overhead light and a clip that does not fit cause the same kind of trouble. When a reading does not match how a patient looks, a nurse repeats it on another finger or on the other hand, and a clinician may confirm it with a blood sample analyzed in a laboratory. A pulse oximeter also cannot report how well a person is breathing. It gives an oxygen saturation and a pulse rate. It does not give the carbon dioxide level in the blood or the depth of each breath, and a person whose breathing has slowed can hold a normal saturation for a while before the number falls. That limit is the reason a normal reading never replaces an assessment of the person wearing the clip.

Question 1 of 10. Which sentence best states the main idea of the passage?

Hint: A main idea has to cover both halves: how the number is made and what the number cannot show.