How do we monitor anaesthesia? -- capnography

Question 1

What info will we gain from capnography machine?

Answer 1

• Inspired CO2 as a number.
• Expired CO2 as a number.
• RR as a number.
• Capnograph as a wave formation.

Question 2

Forms of capnography device?

Answer 2

• As part of multiparameter.
• Handheld.

Question 3

How do I set up a capnography machine?

Answer 3

• Check machine plugged in / batteries OK.
• Untangle all cables and wires etc.
• Select a connector.
• Ensure the connector / sample line in good order.
• Attach connector to the breathing system ready to use.
• Turn on the machine.
• Once patient breathes, get a capnogram.

Question 4

Types of capnography device.

Answer 4

Side stream or mainstream.

Question 5

1. What does side stream capnography do?
2. Advantages of side stream capnography.
3. Disadvantages of side stream capnography.

Answer 5

1. Takes a small sample from the adaptor, down the sample line, back to the machine where it is analysed, bringing up a numerical display on the screen of the device.
2. Cheaper.
   Less likely to break.
   Easy to replace if it does break.
3. Slight time delay w/ readings.
   Takes some FGF requirement (negligible in some animals).
   Sample line can get easily damaged and needs changing regularly.

Question 6

1. What does mainstream capnography do?
2. Advantages of mainstream capnography.
3. Disadvantages of mainstream capnography.

Answer 6

1. There is a light source and a sensor inside the connector between the ET tube and the breathing system. The gases are analysed at this point so the result is instantaneous w/ no need for sample line.
   2, Real time results.
   No need for sample line so no taking of requirement of FGF.
2. Very expensive to buy.
   Can be damaged easily which can be pricey – sensor = £800 and v delicate.
   Can add drag to the breathing system.

Question 7

How does capnography work?

Answer 7

Continuous sampling of CO2 in both inspired and expired gases.
CO2 molecules absorb infrared light energy at spec. wavelengths w/ amount of energy absorbed directly related to CO2 conc.
There is an infrared light beam source that passes through the expired gas and to the sensor.
The amount of light absorbed by the CO2 is reflected by the difference between the light from the source and the light that hits the sensor.

Question 8

1. What is phase I of a capnograph?
2. What is phase II of a capnograph?
3. What is the alpha angle?

Answer 8

1. Inspiratory baseline. V low levels of CO2. Should always be at 0.
2. The start of expiration. Occurs when we have anatomic dead space and alveolar gas from the alveoli and the bronchioles leaving the airway.
3. The angle between phase II and III. Can be used to assess ventilation perfusion of the lungs. Ventilation-perfusion mismatching may be represented w/ angle >90 degrees.

Question 9

1. What is phase III of the capnograph?
2. What is the beta angle?
3. What is phase 0 of the capnograph?

Answer 9

1. Alveolar plateau – where that last bit of alveolar gas is sampled from.
2. The angle between phase III and phase 0. Can be used to assess rebreathing. Rebreathing indicated by angle 90 degrees.
3. Inspiratory down stroke and the beginning of the next expiration.

Question 10

1. What is normal end tidal CO2 in dogs?
   – and in cats?
2. What is normal inspiratory CO2?

Answer 10

1. 35-45mmHg.
   – 28-35mmHg.
2. 0.

Question 11

1. Why might end tidal CO2 be high?
2. Why might end tidal CO2 be low?
3. Define tidal volume.

Answer 11

1. Hypoventilation.
   - Reduced RR.
   - Reduced TV.
2. • Hyperventilation.
     - Low CO.
     - Decreased metabolic rate.
     - Hypothermia.
     - Pulmonary embolism.
     - leak in the sampling line.
     - Poor sampling technique (dilution from FGF).
     - Leak in the breathing system.
3. The amount of air that moves in and out during one respiratory cycle.

Question 12

1. What can cause high inspiratory CO2 in non-rebreathing system?
2. What can cause high inspiratory CO2 a rebreathing system?

Answer 12

1. Too low FGF.
   Too much dead space.
2. Exhausted absorbent (soda lime).
   Faulty / sticky valves.

Question 13

Benefits of capnography.

Answer 13

• Non-invasive.
• Easy to set up.
• Easy to use.
• Very efficacious way of monitoring the ventilatory ability of the patient.
• Can give info on CO.

Question 14

Limitations of capnography?

Answer 14

• Dead space (connectors and adaptors).
• Requires ETT/mask.
• The operator. Needs exposure, experience and training.

Question 15

1. What does it mean where there is a stepped inspiratory phase on capnograph?

Answer 15

1. Cardiac oscillations. This is pulmonary arterial pulsations which are normal in large breed / large chested dogs.

Question 16

What does it mean when the capnograph trace is not well formed and the plateau is shorter and complexes are smaller?

Answer 16

Leaky endotracheal tube.

Question 17

What does it mean when the capnograph trace has a gradual increase in the expiratory phase and a normal sharp drop in the inspiratory phase?

Answer 17

Bronchospasm e.g. asthma, anaphylaxis, obstruction (mucus plug, debris, ETT kink, ETT against tracheal wall). Increased expiratory airway resistance

Question 18

What does it mean when the capnograph has a small dip in the plateau before the inspiratory phase?

Answer 18

Bucking. Likely the patient is on a mechanical ventilator and has had some neuromuscular blocks, and are beginning to regain some muscle function and fighting against the ventilator.

Question 19

What does it mean when the capnograph trace line does not go back down to 0 after the inspiratory phase?
– What other info could tell us this?

Answer 19

The patient is rebreathing CO2.
– The number displayed for INCO2 is not 0.