Monitoring

Question 1

What are the 5 phases of a normal capnogram?

Where is EtCO2 measured?

Answer 1

Phase IV: early inspiration - CO2 free gaz starts entering the airway - decrease in CO2

Phase 0: inspiration - NO CO2 should be measured

Phase I: early expiration, emptying of anatomic dead space - NO CO2 should be measured

Phase II: rapidly changing mixture of alveolar and dead space gas - steep increase in CO2

Phase III: alveolar plateau - slight increasing slope

Max concentration at the end of Phase III = EtCO2

Question 2

What can cause in increased PaCO2 - EtCO2 gradient?

Answer 2

• Increased dead space ventilation - PTE, low cardiac output, alveolar overdistention from PEEP
• One-lung intubation
• Obstruction (obstructive pulmonary disease, tube obstruction)

Question 3

Describe the abnormalities in this capnograph

Answer 3

Increased inspired CO2 (elevated baseline), decreased slope of phase IV, increasing EtCO2

–> increased apparatus dead space, rebreathing

• PaCO2-ETCO2 gradient will be normal or decreased

Question 4

What capnograph changes can be expected with an obstruction to expiration?

Answer 4

• Phase II –> decreased slope
• Phase III –> increased slope
• Decreased EtCO2

Question 5

What can a low, non-zero end tidal reading indicate in regards to an equipment issue?

Answer 5

Leak

Question 6

True or false: in herper/hypoventilation, the CO2 waveform is normal.

Answer 6

True, however, there will be an increase/decrease of ETCO2

• normal PaCO2-ETCO2 gradient

Question 7

What changes on the capnograph can be expected in patients with bronchoconstriction?

Answer 7

Prolonged expiration –> Decreased slope of phase II, increased slope of phase III

Question 8

What is going on in this capnograph tracing?

Answer 8

cardiac oscillations

Question 9

What is this abnormality of the capnograph called and what causes it?

Answer 9

Curare cleft = spontaneous inspiratory effort in the mechanically ventilated patient

Question 10

What physical law is oximetry based on

Answer 10

Beer-Lambert (the concentration of a substance is proportional to its transmission of light)

Question 11

What is the difference between functional and fractional hemoglobin saturation (SO2)? Which one reflects pulmonary function best

Answer 11

Fractional SO2 takes into account the dyshemoglobins

Functional SO2 = [HbO2/(HbO2+HHb)] * 100

Fractional SO2 = [HbO2/(HbO2+HHb+COHb+MetHb)] * 100

Functional SO2 reflects pulmonary function (but not necessarily O2 delivery to tissues)

Question 12

What wavelengths are used by the pulse oximeter? Which one does HbO2 / HHb / COHb / MetHb absorb the most?

Answer 12

• Red = 660 nm -> deoxyhemoglobin
• Infrared = 940 nm -> oxyhemoglobin

COHb absorbs more in red (660 nm)
MetHb absorbs the same at both wavelengths

Question 13

How will SpO2 readings be in the presence of MetHb / COHb

Answer 13

• MetHb -> absorbs red similar to HHb and infra-red close to HbO2 -> will read in between and plateau at 85%
• COHb ->absorbs red with same coefficient as HbO2 -> read as HbO2 -> falsely high SpO2

Question 14

For what range of SpO2 is pulse oximetry best correlated with PaO2

Answer 14

80-97% (most linear portion of dissociation curve)

Question 15

What technology do most capnometers use

Answer 15

Infrared absorption

(other methods = mass spectrometry and Raman scattering)

Question 16

List causes of increased and decreased EtCO2

Answer 16

See picture

Question 17

What type of monitoring allows calculation of dead space

Answer 17

Volumetric capnography

Question 18

Label this graph of volumetric capnography

Answer 18

Phase I = elimination of volume in the airways, no CO2
Phase II = gas coming from regions in transition between anatomic and alveolar gas compartments
Phase III = pure alveolar gas compartment

Z = airway / anatomical dead space
Y = alveolar dead space
X = alveolar ventilation / CO2 elimination

Question 19

List causes of errors of pulse oximetry

Answer 19

• Movement artifact (non-pulsatile signal interpreted as pulsatile)
• Presence of venous pulse (heart failure)
• Decreased pulsatile signal: hypoperfusion, too high pressure from probe
• External light (increases non-pulsatile signal)
• Presence of absorbers other than HbO2 and HHb (MetHb, COHb)
• Pigmented skin

Question 20

List pros and cons of mainstream vs sidestream capnography

Answer 20

1. Mainstream
   - Increases dead space
   - Puts weight on circuit / ET-tube
   - Marked condensation
   - Faster response time
2. Sidestream
   - Samples gas from patient ; CO2 will be diluted if low tidal volume or high fresh gas flow
   - Delay in response
   - Sampling line can obstruct
   - Minimal dead space
   - Light

Question 21

What can cause a decrease in the slope of phase IV in a capnogram

Answer 21

• Leak
• Obstruction on inspiration -> default in inspiratory valve
• Slow response time (sidestream capnograph)

Question 22

What can cause a decrease in the slope of phase II in a capnogram

Answer 22

• Obstruction -> mucus plug, kink, bronchoconstriction, stuck expiratory valve
• Slow response time (sidestream capnograph)

Question 23

What are the 2 types of pulse oximeter probes

Answer 23

• Reflectance probe (= linear probe)
• Transmittance probe (= regular clip probe)

Question 24

What are the determinants of anesthetic depth?

Answer 24

1. Amount of anesthetic drug in the brain
2. Magnitude of surgical stimulation
3. Underlying conditions that have synergistic CNS depressant effects

Question 25

Why is cyanosis a late sign of hypoxemia?

Answer 25

Signals the presence of deoxygenated hemoglobin in the observed tissue.

Absolute concentration of unoxygenated hemoglobin of 5 g/dL to manifest sufficient cyanosis

If a dog has a normal hemoglobin concentration of 15g/dl, in order to manifest cyanosis (5g/dl of deoxygenated hemoglobin = 1/3 of 15g/dl), arterial blood saturation would need to be decreased to 67% (100% - 1/3). At this level, according to the oxyhemoglobin dissociation curve, PaO2 is about 37mmHg  severe hypoxemia