Chp. 16: Anesthetic Gas Monitoring

Question 1

Dalton’s Law of Partial Pressure

Answer 1

The total pressure exerted by a mixture of gases is equal to the arithmetic sum of the partial pressures of each gas in the mixture.

Question 2

What is the partial pressure of oxygen and isoflurane for a patient administered 2% isoflurane in 100% oxygen at sea level?

Answer 2

Partial pressure O2 = 0.98 x 760mmHg = 744.8 mmHg

Partial pressure iso = 0.02 x 760mmHg = 15.2 mmHg

Question 3

Partial pressure

Answer 3

Quantifies the number of molecules of gas that are present in absolute terms, e.g. mmHg.

• Used in IR analysis of respiratory gases

Question 4

Volume percent (vol %)

Answer 4

Separates molecular components of a gas mixture into individual portions, and is expressed as a ratio, or proportion of the gases in the mixture

Question 5

What is the vol % of isoflurane for a patient administered 2% isoflurane in 100% oxygen?

Answer 5

2 vol % or 0.02

Question 6

Convert between vol % and mmHg for 2% isoflurane in 100% oxygen at sea level

Answer 6

O2 vol% = 98 vol% = 0.98 x 760mmHg = 744.8 mmHg

Iso vol % = 2 vol% = 0.02 x 760mmHg = 15.2 mmHg

Question 7

What is transit time in a gas sampling system?

Answer 7

Aka “lag time”

Accounts for delay that occurs as the gas sample travels to the analyzer. Applies ONLY to sidestream monitors.

Question 8

The response time of a sidestream system is dependent upon what?

Answer 8

The inner diameter and length of the sampling line and the gas sampling rate.

Question 9

What is “rise time” in gas sampling systems?

Answer 9

Time it takes an analyzer to react to a change in gas concentration, generally defined as the time it takes the analyzer to move from 10% to 90% of a sudden change in concentration.

Question 10

What is the advantage of sidestream over mainstream gas analysis?

Answer 10

All respiratory gases of interest can be measure simultaneously.

Question 11

What are the risks of sidestream gas analysis in small patients?

Answer 11

• If FGF exceeds the patient expiratory flow rate, the sample obtained will be artificially low due to dilution with fresh gas
• The sampling flow rate of the monitor may exceed the patient’s FGF rate, creating the potential for negative pressures within the system

Question 12

What wavelengths are included in the IR spectrum?

Answer 12

0.40um to 40um

Question 13

How does a dispersive IR analyzer work?

Answer 13

It passes IR radiation through a gas sample and then separates the radiation into component wavelengths, which are ordered sequentially. A plot of absorbance vs. wavelength is constructed. The gas mixture measured will absorb one or more components of the radiation spectrum emitted.

*As long as the gases in the mixture have distinct absorption peaks, the composition of the mixture can be analyzed and quantified based on the amount of absorbance measured for each wavelength.

Question 14

How does a non-dispersive IR analyzer work?

Answer 14

The radiation emitted is filtered and only bands of a specific wavelength are allowed to pass through the gas mixture. The wavelengths allowed to pass through the filter correspond to the absorption peak of the gas of interest.

*Most respiratory gas monitors today are non-dispersive and agent-specific

Question 15

What are the peak absorption wavelengths for CO2, N2O, and the potent volatile anesthetics?

Answer 15

CO2 = 4.2-4.4um
N2O = 4.4-4.6um
Volatile anesthetics = 3.3um and 8-12um

Question 16

What are the components of an IR analyzer?

Answer 16

Source of radiation (a blackbody radiator that absorbs all thermal energy and does not reflect light, appearing black), narrow-band filter, sample cell, and a detector

Question 17

What are the advantages of IR analyzers?

Answer 17

Do not alter anesthetic agents, the sampled gas can be returned to the breathing system for waste gas scavenging.

Question 18

How do IR analyzers fare at high respiratory rates?

Answer 18

Tend to underestimate inspired concentrations and overestimate expired concentrations

Question 19

What are the disadvantages of IR analysis?

Answer 19

• Concentrations of nonpolar molecules (eg. O2 and N) cannot be measured, erroneous readings may result due to cross-interference or overlapping of absorption bands (some may be inaccurate when high concentrations of N2O are present)
• Methane in animals with large intestinal fermentation compartments can have erroneously high readings of anesthetic agents with short IR wavelength analyzers due to methane
• Collision broadening can cause erroneous readings

Question 20

What historical technologies were used for respiratory gas analysis?

Answer 20

Mass spectrometry, Raman spectroscopy, photo acoustic spectrometry, piezoelectric analysis, refractometry

Question 21

What are the applications of volatile agent monitoring?

Answer 21

• End-tidal inhalant partial pressure closely approximates alveolar inhalant partial pressures
• The anesthetist can titrate FGF and vaporizer settings more precisely