Chris Thompson  

OXYGEN MONITORING

1. Monitoring Oxygen Delivery

Oxygen Supply

  • Hospital line supply failure system
  • Local line pressure gauges
  • Cylinder contents
  • Oxygen supply failure alarm

Gas delivery system - electronic or manual

Regular visual checks of rotameter position / FGF settings are important.

Rotameter interlocks

  • Ulco lever system
  • Ohmeda chain
  • Both limit the lowest possible Oxygen concentration to around 21% by some mechanical means.

Backbar Oxygen analysers

CIG GEM system. Ultrasonic real-time gas analysis.

In-Circuit Oxygen Analysers

Ideal unit would be easy to calibrate and have useful alarms, ie the low O2 alarm should be on at all times. Location is sually inspiratory but may be in the expiratory limb.

Oxygen analysis methods:

Polarographic

Platinum electrode with buffer solution (KCl) so that a small current can travel across polarised electrodes; the amount is proportional to the oxygen concentration. Oxygen is changed to Chlorine and the KCl eventually becomes KOH and won't work. Useful like around 3 to 18 months. Slow response, usually 30 seconds. Buffer wears out. Must be polarised all the time (requires battery); if not will drift when first connected.

Fuel Cell

Creates electricity when in contact with Oxygen. Doesn't need continuous electricity supply to maintain sensor. Otherwise like polarographic analysers in performance

Paramagnetic

ie GE, recent Dräger. Can have very rapid response, unlike all others except mass spectrometer. Reliable.

Visual

For a crude idea of what gas is what, it's worth knowing that Nitrous but not oxygen causes refraction of light something like what happens over a road on a hot day. \

2. Assessing Oxygenation in the Patient

Cyanosis

In ideal circumstances can detect bluish tinge of hypoxia at SpO2 of 91 or less (PaO2 around 60mmHg). Note that Carbon Monoxide binds to Hb and looks like Oxygenated Hb, so your eye (and the pulse oximeter) will be too high by the amount of COHb.

Pallor in anaemia makes cyanosis much harder to detect.

Good light is absolutely necessary. All blood looks red in tungsten (incandescent) light, which is a red/orange colour. Fluorescent tubes have a strong green colour, and very liitle red light, so all blood looks kind of grey.

The eye's ability to detect colour is poor in dimly-lit areas, particularly if the light reflects off green drapes or green operating room walls. Bright sunlight, halogen bulbs, or true daylight fluorescent tubes are required for early detection of cyanosis.

"Doctor, the blood looks a little BLUE, you know!"

Usually venous blood or oozing from very vasoconstricted areas, but never to be ignored!

Clinical symptoms of hypoxia

Nausea, confusion, drowsiness, brain damage.

Acidosis, organ dysfunction, vasodilation, hypotension, dysrhythmias, eventual cardiac arrest.

Hyperventilation is weak and abolished by anaesthetic agents, although hypoxia enhances hyperventilation from CO2 retention.

ALL TOO LATE and nonspecific to be clinically useful.

Pulse Oximetry

Almost always right! Simple, continuous. Short delay only (20 - 30 seconds)

Includes COHb in measurement, ie over-estimates sat if CO present.

Reads 85% if 100% MetHb so not much use here either.

Less accurate if perfusion is poor. Under-reads if there is a venous pulsation that is synchronous with the arterial pulse, ie in Tricuspid Incompetance, but this is usually only a roblem if monitoring off the ear or the face.

Uses two transmitting LED's (one infra-red (940nm) and one red (650nm)) which flash on and off alternately, and one receiving light dependent transistor. The ac light absorbance at one frequency divided by that at the other frequence give a ratio (the `R' value), which is looked up on a table to read saturation.

NO calibration is possible.

Continuous mixed venous oxygen saturation

Available on some SwanGanz catheters.

Indication of cardiac output, oxygen consumption, etc.

Transcutaneous oxygen analysers

Only really suitable for neonates

Indwelling arterial oxygen analysers

Not generally available but will be one day I guess.

Blood Gases

Gold standard if samples properly kept on ice and machine correctly calibrated; long time-lag from when you take the sample to when you get the answer, though. Useful to check clinical/pulse oximeter readings. Not confused by Carbon Monoxide, ie reads true. Also gives CO2, base excess, etc.

 


Last updated Tuesday, April 13, 2010
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