Plethysmography

Question 1

What are the benefits or advantages of pulse oximetry?

Answer 1

Inexpensive

Portable

Allows early detection of desaturation

Probe may be placed in various places

Question 2

What are the negatives and or inaccuracies of pulse oximetry?

Answer 2

Susceptible to artifact
Risk of burns in poor perfusion states
Unable to detect pulsatile blood flow in hypothermic/vasoconstricted patients.
Some nail polish pigments interfere with accurate estimates
During high FiO2 delivery, V/Q abnormalities may be masked
(PaO2 of 100 mmHg & 500 mmHg both read at 100%)
Doesn’t measure respiratory rate

Question 3

How does pulse ox work?

Answer 3

A sensor containing a light source (2 or 3 LEDs) & a light detector transilluminate perfused tissues. Oxygenated versus deoxygenated hgb are detected in this manner via changes in light absorption. The ratio of light absorption provided a saturation.

Question 4

Whose law is the basis of pulse oximetry?

Answer 4

Beer-Lambert Law:

the intensity of transmitted light passing through a vascular bed decreases exponentially as a function of the concentration of absorbing substances & the distance from the source of the light to the detector

I_t = I_i x e^-DCa

Question 5

What are the absorption wavelengths utilized in pulse oximetry?

Answer 5

Oxygenated hgb = 940 nm
Deoxygenated hgb = 660 nm

Question 6

What is the isobestic point?

Answer 6

It is when Iso is your Best choice…

OR:

A spectroscopic wavelength at which the absorbance of two substances, one of which can be converted into the other, is the same.

Question 7

What are the lag times for finger? Ear? Toe?

Answer 7

Finger: 20-35 sec

Ear: 20 sec

Toe: 40-70 sec

Question 8

SaO2 of 90% equals PaO2 of ______ mmHg.

Answer 8

60 mmHg

Question 9

When does cyanosis occur?

Answer 9

SpO2 of 70% (PaO2 of 40 mmHg)

Question 10

Why does cyanosis occur?

Answer 10

Conformational changes occur in hgb molecules depending on the state of oxygen binding. When hgb is full of O’s, it reflects a lighter red wavelength, when hgb if devoid of O’s, it reflects a dark red/blue wavelength.

(This is why to exhibit cyanosis, one must have adequate amounts of hgb.)

Question 11

Does acidosis shift the oxyhemoglobin dissociation curve to the left or the right?

Answer 11

Right

Question 12

Does this aid or hinder oxygen offloading?

Answer 12

Aids (decreased affinity for oxygen, thus pushing it’s lazy-bum ass off the Blood Train into the surrounding tissues)

Question 13

SaO2 of 70% = PaO2 of _____ mmHg

Answer 13

40 mmHg