We are there for the good of the patient, not for the good of the protocol, not for the good of the medical director, and not for the good of the company.

- Rogue Medic

Pulse Oximetry vs. Waveform Capnography – Which is better?

 

 

Pulse Oximetry vs. Waveform Capnography – Which is better?

Choose only one?
 

No.
 

Why would I?

In a situation where I had to choose only one, I would pinch myself to awaken from this National Registry nightmare of artificial choices. There is no one best choice.
 

The best choice depends on what we are trying to do.

If the patient is receiving paralytics during an intubation attempt, does waveform capnography matter?

No, but the pulse oximetry is important in determining how long it is safe to continue to attempt to intubate before ventilating the patient again (whether with a BVM or a crichothyrotomy).

With use of preoxygenation and passive apneic oxygenation, patients may tolerate apnea for extended periods without desaturation. Apneic oxygenation dramatically changes the way we approach airway management.

Does it matter if the patient is not breathing, if the patient’s oxygen saturation is in the high 90s?

Yes, because ventilation is the removal of CO2 (Carbon DiOxide). If the patient is already acidotic, even a brief period of apnea may kill the patient.

Does it matter if the cardiac arrest patient is receiving ventilation?

No.

Chest compressions appear to provide adequate ventilation without any use of the usual means of ventilation.

Does pulse oximetry have a place in the assessment of endotracheal tube placement?

Yes, but waveform capnography, while not 100% accurate maybe 99 44100%, is almost always the best means of tube confirmation.
 

 

The numbers do not match the waveforms. There are four pulses for three breaths. A heart rate of 79 and a respiratory rate of 12 are indicated on the screen.

For a heart rate of 79, the respiratory rate would be 60. For a respiratory rate of 12, the heart rate would be 15. A respiratory rate of 60 could be to compensate for a metabolic acidosis or many other medical conditions, but these would not be expected to produce an accurate reading of 35 mm/Hg.

A heart rate of 15 is kind of slow for a human and should get our attention, but the context is important. What is going on with the patient?

It could be that the display presents the waveform at different rates, so that there is more of the capnograph to see.
 

 

Is this a problem?

The answer depends on knowing what is going on with the patient.

An oxygen saturation of 98% may drop pretty quickly, if the patient has not been preoxygenated. Is this following a paralytic during an intubation attempt? Is the patient being ventilated with a BVM after being disconnected from a ventilator for suctioning, or bronchoscopy, or . . . ? If the patient has sleep apnea the display would look different, but that is another form of intermittent apnea that does not necessarily require any emergency medical intervention.
 

 

We generally want to maintain an oxygen saturation of 94% or better. Is 93% a problem?

 

 

 

 

 

 

 

Is 83% a problem?

 

 

 

 

 

 

 

Is 58% a problem?

83% is looking a lot better.

Again, the answers depend on context.

Isolated hypoxia is not a problem, but the patients we assess tend not to have isolated hypoxia. what is normal oxygenation for the patient? The hypoxia is generally an indicator that something else is wrong – something that is probably not going to be made all better just because we turn up the oxygen and make the numbers pretty. 100% does not mean all better.
 

 

Is 55 mm/Hg a problem?

 

 

 

 

 

 

 

Is 85 mm/Hg a problem?

Why is the CO2 elevated?

Treating the numbers is not the same as treating the patient.

Added 01/16/2013 @ 12:00 There is an interesting Facebook post on a patient with an oxygen saturation of 58% here. Thanks to Vince DiGiulio for the link.

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Comments

  1. Tim, the best part of the post is how you keep drilling home the “whats going on with the patient”. Sure numbers on the monitor can mean something, but they should be taken into consideration with everything else happening – the patient, our treatments, non treatments etc.
    I hate when people just stare at the monitor and base everything on what they see on it instead of what they see with the patient. My favorite is when the first thing a provider does before anything else is slap the Pulse Ox on a patients finger and stare eagerly at the readout. Good post.

  2. Emcrit postcast on Pluse Ox lag/latency. and use of finger vs other probe sites .

    http://emcrit.org/podcasts/oxygen-physiology/

  3. The on screen representations of pulse wave form and respiratory wave form are close, but I’m not aware of any direct correlation. The disparity is even more pronounced when you print it out.

    O2 saturation is incredibly insensitive of oxygenation status. It lags about three minutes behind what is actually going on with the patient. By the time it drops to what we could consider a dangerous level, the damage has likely been done.

    TA disparity between SPO2 and ETCO2 should tell us that there is a ventilation problem with the patient. It’s not uncommon for a patient to have a SPO2 of 100%, but be in acute respiratory arrest.

    The pulse waveform with SPO2 is there to help correlate the SPO2 number with the waveform to help judge accuracy. The wave form of the ETCO2 is there to help you evaluate CO2 retention although it is not diagnostic in and of it self. Younger asthmatics can have elevated ETCO2 readings and be acutely ill but have a square waveform early on.

    As you say, evaluate the patient, but you also have to be able to understand and evaluate the capnography data.

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