Current brands that are available and their theoretical advantages & disadvantages

1. Bentley/Baxter Oxysat

a) Can monitor SaO2 & SvO2 (not simultaneously)

b) Require cuvette

c) Response time: 10 sec

2. Medtronic

a) Can monitor SaO2 & SvO2 (not simultaneously?)

b) Can monitor Hct (by optical measurement)

i) Require initial setting input

c) Require cuvette

d) Response time?

e) Poor feel buttons

3. CDI

a) LCD

b) Hct

c) Offers trend

Methods of measuring oxygen saturation by: Reflectance; Transmission & various wavelengths used to measure Hb & Oxygen saturation

1. Transmission [spectrophotometric] oximetry

a) Involves shining radiation through a sample & determining the quantity of radiation absorbed

b) The wavelength of radiation is chosen so that it is absorbed by the compound of interest in the sample

c) The absorption of radiation by a sample of interest increases as the concentration increases

d) At sufficiently low concentrations, the absorption is proportional to the concentration

e) The light absorbed by blood depends on the quantities of oxyHb & deoxyHb present and the wavelength of light

f) The absorbance of oxyHb in red light (at 660 nm) is less than that of deoxyHb — accounts for its red colour

g) The absorbance of deoxyHb in infrared light (at 940 nm) is less than oxyHb

h) Comparison of the absorbances at these wavelengths enables the oximeter to determine oxygen saturation

i) The difference between the absorption at 800 nm (isobestic point & reference point) and 650 nm is proportional to the degree of saturation

j) Method based on the fact that if a mixture of oxyHb & deoxyHb is read at two wavelengths, one where there is a significant difference between the oxy & deoxyHb, the other at the isobestic point, the % saturation can be derived

k) Baxter OxySat: operating Wave lengths approx 900 nm & 660nm

Limitations on saturation monitors: accuracy & range; Time response; Blood flow

1. Bentley/Baxter Oxysat

a) Blood flow rate: 0.25 - 7.0 L/min

b) Response time 10 sec

c) Accuracy

i) ± 2% between 60% & 100%

ii) ± 4% between 40% & 60%

iii) < 60% unspecified

d) Hematocrit range: automatic compensation from 15% to 40% Hct

2. Medtronic Biotrend

a) Blood flow rate: 0.25 - 7.0 L/min

b) Response time 12 sec (subjectively even longer)

i) Relevant during coming off partial CPB

c) Range:

i) Hct: 16 - 38%

ii) O2 Sat: 40 - 99%

iii) Exceeding these limits will result in flashing display

Effects of MetHb & CarboxyHb?

Calibration of in line O₂ saturation device

Baxter/Bentley OxySat

· Probe itself is source of light

· Test cells used to measure a reference value of O2 saturation

· Calibration test is not an absolute value reference; it indicates that readings are repeatable & reliable

· Ensure serial number on probe matches serial number on test cell

· Ensure 20 min warm up period

· Ensure measurement cell is clean

· Acceptable readings ± 2%

· If out of range, do not use meter, send to biomed or distributor

Medtronic Biotrend

· Fibre optic cable to reflect light (source of light within machine)

· Self calibrating

· Calibration performed automatically every time a set up is initiated

· Connect sensor to CALPORT on BioTrend instrument

· Press CAL for 5 sec

· Calibration will commence