These are not blog posts.
They are unfinished thoughts and decision frames that help me think during ER shifts.
They are unfinished thoughts and decision frames that help me think during ER shifts.
For a long time, I believed that severe metabolic acidosis by itself was a reason to intubate.
Over time, I have realised that this can actually be bad physiology and in some situations, dangerous medicine.
A quiet shout-out to Laryngoscope as a Murder Weapon by Scott Weingart, which completely changed the way I think about airways in acidotic patients.
This is what I have learned.
Do You Really Have to Intubate?
The mantra is simple
Avoid intubation if at all possible.
Patients with severe metabolic acidosis survive by breathing fast and deep.
They are blowing off carbon dioxide to compensate for the acid load in their body.
That respiratory alkalosis is their only form of compensation.
When we intubate, we take that away instantly.
If we do not keep up with their minute ventilation, their acidosis worsens and they can crash very quickly.
A few important points to remember:
- These patients are often already breathing at their maximum capacity
- Matching that minute ventilation on a ventilator is extremely difficult
- Losing compensation can lead to rapid hypotension and cardiovascular collapse
When should you intubate despite this?
Only when there is no safe alternative.
The two main indications are:
- Respiratory failure, most commonly from fatigue
- Look carefully at the ABG
- A rising PaCO₂ or a secondary respiratory acidosis tells you compensation is failing
- Airway compromise
- Reduced consciousness
- Loss of protective reflexes
If neither is present, pause before reaching for the laryngoscope.
Is There a Role for BiPAP or NIPPV?
Yes, and it is often underused.
If the patient can tolerate it, a short trial of NIPPV while you treat the underlying cause can be extremely helpful.
Even when oxygenation is not the issue:
- BiPAP supports breathing while maintaining spontaneous ventilation
- Minute ventilation is preserved
- It buys you time to correct the metabolic problem
- It is excellent for pre-oxygenation
Most importantly, it allows you to measure the patient’s spontaneous minute ventilation.
That number becomes your target if intubation becomes unavoidable.
What About Bicarbonate?
In most cases, it will not help.
The reaction looks like this:
H⁺ + HCO₃⁻ ⇌ H₂CO₃ ⇌ H₂O + CO₂
Any benefit from bicarbonate depends entirely on the patient’s ability to exhale the carbon dioxide that is generated.
In severe metabolic acidosis:
- Patients are already ventilating at their maximum
- They cannot blow off additional CO₂
- Giving bicarbonate may fail to improve pH
- It may even worsen acidosis
Without the ability to eliminate CO₂, bicarbonate is usually poor physiology.
Minimise Apnea Time.
After paralysis for RSI, there is always a brief period of apnea.
This leads to a rise in PaCO₂.
In most patients, this does not matter.
In patients with severe metabolic acidosis, even a small rise in PaCO₂ can worsen acidemia enough to cause cardiovascular collapse.
This makes minimising apnea time absolutely essential.
Key principles
- Pre-oxygenate with NIPPV (Recommended by BJA).
- Match the patient’s minute ventilation, even when bagging
- Consider delayed sequence intubation using ketamine
- Use a short-acting paralytic when appropriate
Once you sedate and paralyse these patients, their ability to compensate is gone. At that point, their physiology is entirely in your hands.
Advanced Pre-oxygenation: VAPOX
Ventilator-assisted pre-oxygenation combines NIPPV with apneic oxygenation.
A typical approach involves:
- Non-invasive ventilation in S/T mode for about three minutes
- Nasal cannula at 15 L per minute running throughout apnea
- The ventilator switching to pressure control once apnea occurs
- Intubation after adequate paralysis
- Nasal oxygen continuing until the tube is placed
- Transition to conventional ventilation afterward
This technique requires a ventilator with a biphasic design and an open valve system capable of both non-invasive and invasive ventilation through the same circuit.
It is promising but requires experience, close monitoring, and further study.
Post-Intubation Ventilator settings:
The rule here is simple.
Match the patient’s minute ventilation.
If a patient with severe metabolic acidosis requires intubation, you must ventilate aggressively enough to maintain their pre-intubation PaCO₂.
If you do not, acidosis will worsen.
Winter’s Equation Is Your Anchor
Expected PaCO₂
= (1.5 × HCO₃⁻) + 8 (±2)
This tells you what the PaCO₂ should be if respiratory compensation is appropriate.
Set your ventilator to achieve this value and prevent further acidemia.
Ventilator calculation
Goal minute ventilation
= (Current PaCO₂ × Current minute ventilation) ÷ Desired PaCO₂
Recheck ABGs frequently and adjust as needed.
Quick Reference for Minute Ventilation
Target PaCO₂ | Approximate Minute Ventilation |
40 mmHg | 6 to 8 L |
30 mmHg | 12 to 14 L |
20 mmHg | 18 to 20 L |
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