Hemoximetry

Question 1

What state must Heme be in to bind 02?

Answer 1

Heme must be in the ferrous state (Fe+2) to bind 02. Not in the ferrous state = it will NOT bind heme and will be non-functional

Question 2

Equation for Hgb binding O2

Answer 2

Hb + O2 = HbO2

Question 3

Factors influencing affinity of O2 for Hgb

Answer 3

CO2, pH, temp, and 2,3 DPG

Question 4

Functional O2 Saturation

Answer 4

only the functional Hgbs (oxy and deoxy)

Question 5

Hemoximetry

Answer 5

the measurement of hemoglobin

Question 6

SO2%

Answer 6

hemoglobin oxygen saturation

Question 7

Fractional Oxygen Saturation

Answer 7

(%O2Hb or FO2Hb) includes the dyshemoglobins in the denominator and is written as:

Question 8

When are functional and fractional O2 saturations the same?

Answer 8

Only when the concentrations of carboxy- and methemoglobin are zero

Question 9

What happens in the presence of lethal levels of COHb?

Answer 9

The “functional” saturation can be normal at 98% and the “fractional” will reflect dangerously diminished oxygen-carrying capacity of the blood.

Question 10

Hemoximeters are used to measure:

Answer 10

1) total Hb concentrations
2) O2 sat of blood

Question 11

Complex (multi wave) hemoximeters measure:

Answer 11

1) Carboxyhemoglobin (HbCO)
2) Methemoglobin (MetHb)

Question 12

Carboxyhemoglobin

Answer 12

HbCO - carbon monoxide bound to Hgb

Question 13

Methemoglobin (MetHb)

Answer 13

Ferric (Fe+3) hemoglobin

Question 14

Spectra of OxyHgb and DeoxyHgb from 500 to 900 nm

Answer 14

Question 15

Which form of hemoglobin is least read and which is the reddest in the absorption spectra?

Answer 15

Least red - reduced hemoglobin

Most red - caboxyhemoglobin (cherry red!)

Question 16

Basics of Hemoximeter Operation

Answer 16

1) blood aspirated into instrument, hemolyzed, and placed in a light cell
2) hemoximeter measures the absorbance of light of the lubstance in the light cell (cuvette) using the principles of spectrometry

Question 17

What is the visible absorbance of blood due to?

Answer 17

Most of the visible absorbance of blood is due to the various forms of Hgb. The remaining absorbance (plasma) is ow and ignored.

Question 18

Beer-Lambert Law

Answer 18

For any compound, absorbance (A) is porportional to the thickness of the cuvette (light path L) and the concentration (c) of the compound.

Question 19

molar extinction coefficient

Answer 19

a tany wavelenght the light absorbed by a particular compound is porportional to the constant molar extinction coefficient (E).

Absorption spectrums for Hgb, deoxyHgb, and Carboxy Hgb are constants so we can use this to calculate the proportion of Hgb.

Question 20

How many wavelengths needed to measure single substances such as Hgb?

Answer 20

For pure single substances like Hgb, only one wavelength is needed to measure it. However, blood has more than one form of Hgb!

Question 21

How can you measure multiple Hgb forms?

Answer 21

By making measurements are more than 2 wavelengths and knowing the extinction coefficients for each form of Hgb at those wavelengths and knowing the ratios fo absorbances for the pure forms.

Question 22

How do you measure two forms of Hgb?

Answer 22

by using two wavelengths (pulse oximeters measure only two wavelengths)

Simple hemoximeters (Radiometer OSM-2) determine only Hgb and HbO2 at 506.5 nm and 600 nm.

Question 23

Ratio of absorbance of pure deoxyHgb at 506.5 and 600nm (506.5/600)

Answer 23

1.43

Therefore, a measured absorbance ratio of 1.43 indicates 100% deoxyHgb

Question 24

Ratio fo absorbance of pure OxyHgb at 506.5 and 600nm

Answer 24

5

Therefore, a measured absorbance ratio of 5 indicates 100% oxyHgb

Question 25

What do measured ratios between 1.43 and 5 indicate?

Answer 25

Measured ratios between 1.43 and 5 indicate the presence of both forms of each deoxyHgb and oxyHgb and the proportion of each.

Question 26

Why is measurement at 506.5 nm used to determine total Hgb (Hb + O2Hb)?

Answer 26

because both forms of Hgb absorb the same or have the same extinction (absorption) coefficient

Question 27

Causes of false readings of Hgb measurement

Answer 27

• presence of Hgb in forms other than Hgb and HgbO2
• foreign substances

Question 28

Advantages of Pulse Oximetry

Answer 28

1) noninvasive
2) continuous measurements

Question 29

Pulse Oximeters and Arterial Blood

Answer 29

Pulse oximeters estimate the functional saturation of arterial blood & cannot measure the total Hgb because of tissue absorbance.

Question 30

How many wavelengths do pulse oximeters use?

Answer 30

Two - therefore they cannot distinguish COHgb and Met-Hgb

Question 31

Why is the ear a good place for a pulse ox?

Answer 31

Because the LED easily passes thorugh the ear tissue. Bone and thicker tissues block light.

Question 32

Design of a Pulse Ox

Answer 32

Pulse oximetry works by applying a sensor to a pulsating arteriolar vascular bed. The sensor contains a dual light source and photodetector.

Question 33

Do pulse oximeters measure venous or arterial blood?

Answer 33

Arterial blood pulsates, NOT venous! So we know the oxygen saturation of arterial blood. Lab samples are superior to pulse ox because it measures the whole blood instead of just the part that pulsates.

Question 34

What two wavelengths does the pulse oximeter use for %O2 Hgb measurements?

Answer 34

660nm (red) and 940nm (infrared - not visible to us)

Question 35

Principles of the Pulse Oximeter Graph

Answer 35

As red (to infrared) increases there is less oxyHgb in relation to deoxyHgb