Monitoring Flashcards
What are the 5 phases of a normal capnogram?
Where is EtCO2 measured?
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
What can cause in increased PaCO2 - EtCO2 gradient?
- Increased dead space ventilation - PTE, low cardiac output, alveolar overdistention from PEEP
- One-lung intubation
- Obstruction (obstructive pulmonary disease, tube obstruction)
Describe the abnormalities in this capnograph
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
What capnograph changes can be expected with an obstruction to expiration?
- Phase II –> decreased slope
- Phase III –> increased slope
- Decreased EtCO2
What can a low, non-zero end tidal reading indicate in regards to an equipment issue?
Leak
True or false: in herper/hypoventilation, the CO2 waveform is normal.
True, however, there will be an increase/decrease of ETCO2
- normal PaCO2-ETCO2 gradient
What changes on the capnograph can be expected in patients with bronchoconstriction?
Prolonged expiration –> Decreased slope of phase II, increased slope of phase III
What is going on in this capnograph tracing?
cardiac oscillations
What is this abnormality of the capnograph called and what causes it?
Curare cleft = spontaneous inspiratory effort in the mechanically ventilated patient
What physical law is oximetry based on
Beer-Lambert (the concentration of a substance is proportional to its transmission of light)
What is the difference between functional and fractional hemoglobin saturation (SO2)? Which one reflects pulmonary function best
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)
What wavelengths are used by the pulse oximeter? Which one does HbO2 / HHb / COHb / MetHb absorb the most?
- Red = 660 nm -> deoxyhemoglobin
- Infrared = 940 nm -> oxyhemoglobin
COHb absorbs more in red (660 nm)
MetHb absorbs the same at both wavelengths
How will SpO2 readings be in the presence of MetHb / COHb
- 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
For what range of SpO2 is pulse oximetry best correlated with PaO2
80-97% (most linear portion of dissociation curve)
What technology do most capnometers use
Infrared absorption
(other methods = mass spectrometry and Raman scattering)
List causes of increased and decreased EtCO2
See picture
What type of monitoring allows calculation of dead space
Volumetric capnography
Label this graph of volumetric capnography
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
List causes of errors of pulse oximetry
- 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
List pros and cons of mainstream vs sidestream capnography
- Mainstream
- Increases dead space
- Puts weight on circuit / ET-tube
- Marked condensation
- Faster response time - 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
What can cause a decrease in the slope of phase IV in a capnogram
- Leak
- Obstruction on inspiration -> default in inspiratory valve
- Slow response time (sidestream capnograph)
What can cause a decrease in the slope of phase II in a capnogram
- Obstruction -> mucus plug, kink, bronchoconstriction, stuck expiratory valve
- Slow response time (sidestream capnograph)
What are the 2 types of pulse oximeter probes
- Reflectance probe (= linear probe)
- Transmittance probe (= regular clip probe)
What are the determinants of anesthetic depth?
- Amount of anesthetic drug in the brain
- Magnitude of surgical stimulation
- Underlying conditions that have synergistic CNS depressant effects
Why is cyanosis a late sign of hypoxemia?
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
