ph & po2 monitoring during mechanical ventilation: abg ... · vbg is probably very close to abg...
TRANSCRIPT
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pH & pO2 monitoring during mechanical ventilation: ABG, VBG,
or etCO2?
Josh Farkas MD, MS
Division of Pulmonary & Critical Care, UVM
@PulmCrit
blog: PulmCrit.org
no conflicts of interest 💰
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goals
• monitoring oxygenation: pO2 vs. pulse oximetry?
• ventilation: what is our target pH?
• VBG vs. ABG?
• etCO2 strengths & weaknesses
• putting it together: algorithm
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goals
• monitoring oxygenation: pO2 vs. pulse oximetry?
• ventilation: what is our target pH?
• VBG vs. ABG?
• etCO2 strengths & weaknesses
• putting it together: algorithm
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tissue oxygen delivery = 13.4 (cardiac output)(hemoglobin)(oxygen saturation)
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tissue oxygen delivery = 13.4 (cardiac output)(hemoglobin)(oxygen saturation)
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limitations of pulse oximetry
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goals
• monitoring oxygenation: pO2 vs. pulse oximetry?
• ventilation: what is our target pH?
• VBG vs. ABG?
• etCO2 strengths & weaknesses
• putting it together: algorithm
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pH vs. pCO2
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respiratory alkalosis
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respiratory acidosis
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Garg SM 2014 PMID 25249747
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permissive hypercapnia
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Yang C et al. Emergency Medicine 2018
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contraindications to permissive hypercapnia
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7.5~7.2?
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7.5~7.2?
Mediquations App
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7.5~7.2?
pCO2 64 mm? pCO2 32 mmBicarbonate 24 mEq/L
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7.5~7.2
pCO2 93 mm? pCO2 46 mmBicarbonate 35 mEq/L
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goals
• monitoring oxygenation: pO2 vs. pulse oximetry?
• ventilation: what is our target pH?
• VBG vs. ABG?
• etCO2 strengths & weaknesses
• putting it together: algorithm
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CO2: direct comparisons
venous pCO2
arte
rial
pC
O2
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ΔpCO2 vs. ΔO2%
ΔO2% = (arterial O2% - venous O2%)
ΔpCO
2=
(ve
no
us
pC
O2–
arte
rial
pC
O2)
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VBG is probably very close to ABG
Venous O2% > 75%
VBG may be a bit off compared to ABG- venous pH may be low- venous pCO2 may be high
VBG may be unreliable
Venous O2% 50-75% Venous O2% <50%
What is the venous oxygen saturation?
Use of venous oxygen saturation to qualify reliability of VBGs
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goals
• monitoring oxygenation: pO2 vs. pulse oximetry?
• ventilation: what is our target pH?
• VBG vs. ABG?
• etCO2 strengths & weaknesses
• putting it together: algorithm
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arterial pCO2 > etCO2difference ~ dead space
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arterial pCO2 > etCO2
• high etCO2 (60 mm)
• low etCO2 (20 mm)
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narrow-gap patient
• normal lungs• intubated for non-respiratory reason
• no history of lung disease
• normal chest x-ray
• good capnograph
(PaCO2-etCO2) < ~15 mm
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7.5~7.2?
pCO2 64 mm? pCO2 32 mmBicarbonate 24 mEq/L
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30mm 40mm 50mm
etCO2 32mm
range of possible PaCO2 values
60mm 70mm
target pCO2 values to achieve pH between 7.2-7.5
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limitations of etCO2
• APRV 😰
• rapidly evolving lung disease
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30mm 40mm 50mm
etCO2 32mm
range of possible PaCO2 values
60mm 70mm
target pCO2 values to achieve pH between 7.2-7.5
saving grace of etCO2
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ideal monitoring: etCO2 & minute ventilation
etCO2 minute ventilation
etCO2 minute ventilation
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ideal monitoring: etCO2 & minute ventilation
etCO2 minute ventilation
etCO2 minute ventilation
minute ventilation (liters/min)
en
d t
idal
CO
2
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goals
• monitoring oxygenation: pO2 vs. pulse oximetry?
• ventilation: what is our target pH?
• VBG vs. ABG?
• etCO2 strengths & weaknesses
• putting it together: algorithm
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Approach to: intubated patient
Vampire pathway
ABG, repeat PRN.
(Also follow etCO2)
No
Need ABG to monitor oxygenation? - Unreliable pulse oximetry waveform- Methemoglobinemia/carboxyhemoglobinemia
Yes
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Approach to: intubated patient
Vampire pathway
ABG, repeat PRN.
(Also follow etCO2)
No
Need ABG to monitor oxygenation? - Unreliable pulse oximetry waveform- Methemoglobinemia/carboxyhemoglobinemia
Yes
Criteria for etCO2-only monitoring[1] Precise CO2 control unnecessary- No ICP elevation- Not hemodynamically unstable with pulmonary HTN- Not pregnant
[2] etCO2 expected to be reliable- Good waveform- No chronic lung disease- No acute lung disease (intubated for non-pulmonary reason)- Chest X-ray shows normal-appearing lungs
Anesthesiologist pathway(No ABG/VBG!)
- Use oxygen saturation to monitor O2
- Use etCO2 to monitor CO2
- Follow minute ventilation
Yes
No
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Approach to: intubated patient
vampire pathway
ABG, repeat PRN.
(Also follow etCO2)
No
need ABG to monitor oxygenation? - Unreliable pulse oximetry waveform- methemoglobinemia/carboxyhemoglobinemia
Yes
VBG pathway
- oxygen saturation to monitor O2
- initial VBG to correlate & confirm the accuracy of etCO2. - subsequently monitor CO2 using minute ventilation & etCO2
- repeat VBG if substantial change or data not making sense (avoid lots of VBGs)
criteria for etCO2-only monitoring[1] precise CO2 control unnecessary- no ICP elevation- not hemodynamically unstable with pulmonary HTN- not pregnant
[2] etCO2 expected to be reliable- good waveform- no chronic lung disease- no acute lung disease (intubated for non-pulmonary reason)- chest X-ray shows normal-appearing lungs
anesthesiologist pathway(no ABG/VBG!)
- use oxygen saturation to monitor O2
- use etCO2 to monitor CO2
- follow minute ventilation
Yes
No
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electronic handout
- https://emcrit.org/squirt/resp2019/
- link will be posted on @PulmCrit