5. Measurement of Oxygen, CO2 & Anaes Flashcards
(31 cards)
Capnograph 4 phase
Gap Paco2 & ETCo2
Increased gap?
- Baseline - Inspiration
- Rapid upstroke - expiration
- COPD - sloped up as unit empty diff / obstruction - Plateau at end expiraton - mix alveolar gas
- Rapid decline - Inspiration
Usually 0.4- 0.7
PP gradient Gas exhchange
If reverse - CO2 Not leave alveoli
V/Q mismatch
Transcutaneous method of O2 measure 1. Define 2. Describe how 3. Risks & minimised What temperature
- Contin measurement of capillary PO2~~ pAo2
- Skin heathed 44’C
Vessels dilate, O2 usage negligable w/ total o2
= skin o2 approx art
O2 diffuse thru skin - electrode skin
Polgraphic electrode Plat or gold cathode & silver anode - electroylte soln
45’ localised burn - skin temp thermistor
controls heating power
second thermistor as safety - switch off if malfunction
Move 3-6 hourly
Pulse oximetry
inaccurate - with what
high
low
unaffected
Accuracy
Curve when sats plotted ppo2
4 causes of Right shift hb dissoc curve
NIV - meas O2 sats & HR
Reduced by causes of perfusion reduction
70-100% accuracy +/-2%
CO overstimates
inacc low
- methaem
methylen blue
icg
bili sulp hb fetal hb - dont alter accuray
PPO2 vs Sat curve - sigmoidal Right shift Reduced pH Increased temperature Partial pressure of carbon dioxide and 2,3-diphosphoglycerates.
300hz
red 660
ir 940
Sats probe
requires
Whats a plethysmograph
Pulsatile flow
measures changes in volume with time within an organ - trace from finger represent change volume
How does a sats probe work
Measure relative absorb 2 WL 660 red 940 IR differentiate oxy deoxy Ratio absorb 2 lights calibrated nomogram convert to sats
Methaemoglobin
Phsiological effects
absorb light
what cause sats to run
more IR than either O2Hb or HH
Absorbance of red light ~ HHb,
patients with significant methemoglobinaemia can appear cyanotic.
red and IR light equally well at both wavelengths used in conventional pulse oximetry
high MetHb levels will cause the SpO2 to trend toward 85%
Capnography based on
Infrared absorption
CO2 absorb IR @
4.28 µm
Etco2 is normally how much higher or lower than PACo2
ETCO2 0.6-0.7 kPa lower than the arterial partial pressure.
Mainstream analyser
the patient’s breathing system and therefore increases dead space but allows rapid analysis. The IRMATM mainstream probes (manufactured by the company Masimo) are comercially available. They can measure CO2, N2O and five volatile anaesthetic agents (halothane, enflurane, isoflurane, sevoflurane and desflurane).
Sidestream
Small diameter tubing that samples gases at 150-200 mL/min. It can cause sampling delay but multiple gases and anaesthetic vapours can be analysed from the same sample.
Oxygen crit temp
-118.6
annot be liquefied no matter how much pressure is applied.
O2 @ altitude
PP falls
Fi same
Clark electrode
Platinum cathode
silver anode
K OH or K Cl
O2
slow response time
Requires a battery
halothane - false high
Galvanic fuel cell
gold cathode
a lead anode.
slow response time
O2
Nitrous oxide - reacts lead - N - alter pressure damage
not affected by halothane
both liverate Electrons at anode
pH electrode
Glass electrode - responds H+
Measuring - bulb - pH sense glass - silver/sliver chloride in buffer
potential diff dep ph in & out
Ref -merc/merc cl - KCl buffer soln - semi perm membrane reduces contam protein
Blood sample sep H+ ion sens glass - gradient accross = potential difference
Create circuit - pot diff displayed
Both maintained at 37
Linear output 60mV per unit pH
Pulse Ox principle
nm absorbeds
Differential absorption light Oxy & Deoxy
Oxy absorbs @ 940 (IR) Deoxy 660 (red)
590 805 Isobestic
590
Explain how pulse ox works
The light absorbed by non-pulsatile tissues is constant (DC),
non-constant absorption (AC) is the result of arterial blood pulsation
The DC and AC components at 660 and 940 nm are then analysed by the microprocessor and the result is related to the arterial saturation.
0ulse oximeter provides a continuous non-invasive measurement of the arterial oxygen saturation.
The light emitting diodes (LEDs) produce beams of red and infrared light at 660 nm and 940 nm respectively (not 640 and 960 nm), which travel through a finger (toe, ear lobe or nose) and are then detected by a sensitive photodetector.
Oxygenated blood abosorbs at
940
Deoxy absorbs at
660
Store abg sample cold can lead to
False Low ongoing metabolism in white blood cells) or a falsely high reading d/t bubbles dissolving
What is the base excess
base excess is the amount of strong acid required to return the pH of 1 litre of blood to 7.40 at a PaCO2 of 5.3 kPa and37°C
excessive heparin errors on abg
w PaCO2 and low PaO2, though the pH is little affected).
Side stream analser
flow min min
how long
syphon gas at 150ml min
flow < than above - slow transit time
effects rise time
High flow - incorrect - decreased pressure in sample line
<2m in length
