I&M: Monitoring of Pulse Oximetry, NIBP, and Temperature

Question 1

Pulse Oximetry

Answer 1

Pulse oximeter can be placed on fingers, toes, nose, ear. The more peripherally the pulse oximeter is placed on patient, the longer the delay in detecting an event causing oxygenation change. For example, if placed on finger or toe, delay may be between 30-60s.

The LEDs alternate on and off at 400Hz and has a measurable off phase to detect ambient lighting (artifact).

Photodetector measures absorption of both pulsatile (arterial) and non-pulsatile (venous) components at both wavelengths. Pulsatile portion is only 1-2% of total.

Accuracy of monitor decreases as saturation falls below 70%. Meanwhile, when PaO2 values exceed 90mmHg, small changes in saturation are associated with large changes in PaO2. This makes it hard distinguish between high but safe levels of arterial O2 from excessively elevated levels. (see Hg-O2 dissociation graph)

Question 2

Two components of pulse oximetry

Answer 2

• Pulse plethysmography = detects change in cylindrical volume of artery and displays a graphic representation of waveform on monitor.
• Infrared spectroscopy = detects absorption of light by oxyhemoglobin and deoxyhemoglobin. Oxyhemoglobin absorbs 940nm infrared light while deoxyhemoglobin absorbs 660nm red light.

Question 3

Factors that limit accuracy of pulse oximeter

Answer 3

1. Movement or vibration
2. Electromagnetic interference (cell phone)
3. Hypertension, vasoconstriction
4. Hypotension, hypovolemia
5. Hyperdynamic venous circulation (tricuspid valve regurgitation)
6. IV dyes (e.g. methylene blue)
7. Dark nail polish
8. Ambient light **one of biggest factors

Question 4

Factors that causes failure to determine oxygen saturation

Answer 4

Pulse oximeter requires adequate pulsations in order to distinguish arterial blood from venous blood and tissue light.

1. Hypotention
2. Hypovolemia
3. Hypothermia
4. Low CO
5. Peripheral vascular disease
6. Raynaud’s phenomenon
7. External pressure
8. Proximal BP cuff
9. Improper positioning
10. Valsalva maneuver often seen in laboring patients
11. Cardiopulmonary bypass

Question 5

Factors that causes delayed detection of hypoxic events

Answer 5

1. Lag time increased with low perfusion
2. Device algorithms used to prevent false alarms may increase delay in detecting hypoxic events
3. Pulse oximeter may respond to weak/noisy signal by holding onto old values
4. Increasing time over which pulse signals are averaged (signal processing) may increase delay time
5. Pulse oximeter may not detect other species of hemoglobin like carboxyhemoglobin, sulfhemoglobin, fetal hemoglobin, and methemoglobin

Question 6

Carboxyhemoglobin

Answer 6

Formed when Hb is exposed to CO and has similar absorption spectrum to oxyhemoglobin –> over-readSpO2 by percentage of COHb present

Question 7

Patient complications caused by pulse oximeter

Answer 7

1. Corneal abrasions (don’t use dominant fingers/hand)
2. Pressure and ischemic injuries (frequent examination of site and moving probe to different sites)
3. Burns
4. Bruns associated with MRI
5. Electric shock

Question 8

Body temperature

Answer 8

Core body temperature is ~37’C. Temperature decreases as move away from core. Value is not so much important as temperature trend–> warmer, cooler, staying the same? Most common measurements are on skin, esophageal, nasal, and rectal.

Question 9

Classic

Devices to measure temperature

Answer 9

Glass tube filled with fluid that expands as temperature rises (mercury)

Question 10

Thermistor

Devices to measure temperature

Answer 10

Consists of semiconductor whose resistance changes as temperature changes

Question 11

Thermocouple

Devices to measure temperature

Answer 11

Consists of two dissimilar metals connected at both ends. One metal is kept at known reference temperature while the other is exposed to temperature to be measured. A potential difference is generated proportional to temperature difference.

Question 12

Liquid crystal displays

Devices to measure temperature

Answer 12

Made of material that changes color as temperature changes and can be placed on skin.

Question 13

Infrared

Devices to measure temperature

Answer 13

Consists of a series of thermocouples (thermopile) that detect the infrared radiation emitted. Thermopile generates a potential difference proportional to temperature difference.

Question 14

Noninvasive blood pressure

Answer 14

NIBP involves inflating an occlusive cuff around a limb until cuff pressure is greater than systolic pressure and then letting cuff deflate until flow returns to limb.

There are four measurement techniques: auscultation, oscillotonometer, oscillometer, volume clamp

Question 15

Blood pressure cuff size

Answer 15

Cuff size is imperative: cuff width should be 40% of mid circumference of limb; length should be 2x this width.

If a cuff is too narrow –> falsely high BP
If cuff is too wide –> falsely low BP

Question 16

Sphygmomanometer

Answer 16

Uses a combination of pneumatic cuff, inflating bulb, release valve, and mercury manometer.

Question 17

Auscultation

Answer 17

Cuff is placed on upper arm and Korotkoff sounds are listened to over brachial artery. When pulse is first heard (turbulent flow) is systolic pressure. When pulse sound disappears is diastolic pressure.

Disadvantages: poor correlation to invasive BP measurement; overestimates low BP and underestimates high BP

Question 18

Complications of intermittent NIBP monitors

Answer 18

1. Damage to underlying tissues
2. Neuropathies (damage to nerves)
3. Compartment syndrome (excessive pressure build up in an enclosed space of the body)
4. Mechanical problems such as leaking cuff, hose, or connector
5. Artifacts (intrinsic and extrinsic motion)
6. Air embolism (gas bubble in vascular system)
7. Iatrogenic harm