pH on the gas, should it influence you cardiac arrest prognostication??

It’s a common scenario in multiple ED’s every single day;

The pre hospital cardiac arrest has arrived in your resus. They’re undergoing ALS, the team has run through their 4H’s and 4T’s. So far they haven’t been able to achieve a ROSC and are at a tipping point. They’re wondering whether continuing with this resuscitation could be of benefit, or if it is likely to be futile.

‘I know’ says the team leader ‘lets just get a gas, if the pH is really low then we should stop.’

It seems to make sense, a low pH must be associated with a bad outcome mustn’t it?? But then you think about those diabetics who have a pH of 6,9 that were sat up talking to you whilst you got their insulin and fluids hooked up, they didn’t have a bad outcome. And if you are going to be concerned about a low pH – how low is low?

The gas is taken off the patient in arrest, your colleague goes over to the analyser and it’s on a scheduled rinse for the next 10 minutes, typical! The gas will need to be run up to ITU.  That leaves the team with a long delay before getting the result back and no one seems happy to call an end to the arrest without seeing that the pH. Is this justified or a false cognitive safety blanket?

It’s fair to say that there is a paucity of quality evidence on the topic and as has been said before  – a lack of quality evidence does not necessarily mean a lack of quality of the test. But when you’re considering a significant decision in a patients’  clinical management, why would you pick something thats not proven, especially when you have others tools in your armoury.

One of the better papers out there on this topic was published by Schultz el al in 1996  in Resuscitation. The paper looked retrospectively at 266 patients undergoing CPR and evaluated features that may be associated with survival, these included age, gender, co-morbidities, duration and the initial pH obtained.

They showed no correlation between survival and the initial pH.

In the analysis they used a pH of 7.2 as the boundary between a high and a low pH. Some may argue that this would not be the optimum choice but again what else exists out there proving pH’s prognostic utility? One could argue the clinician bias involved in such a non-blinded scenario adds more weight to the conclusion that pH has no utility (in this study).

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As mentioned earlier there appears to be a lack of evidence to counter this paper.

Several case reports also exist on survival following a grossly acidotic arrest, again poor quality evidence but cases such as Fumeraux et al described with a patient surviving following a pH of 6.54.

Rather than using pH to aid your decision making wouldn’t you be better using the information we have from the history that we know predict a better chance of ROSC, such as a witnessed arrest, bystander CPR, a shockable rhythm etc etc. Then if looking for objective evidence intra arrest using proven strategies such as  ETCO2 and cardiac echo.

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A systematic review from Blyth el al. from 2012 showed utility in cardiac echo intra arrest, with the association of

cardiac motility and ROSC having relatively useful positive likelihood ratio (LR) of  4.26 and a negative LR of 0.18

At these values USS doesn’t seem to allow an independent binary decision to be made about continuation/futility of an arrest but when considering the echo in conjunction with other factors it can be very helpful

So next time you find yourself managing a patient in cardiac arrest and your considering prognosing based on pH, just think twice and do you need that false blanket but rather have you utilised your proven tools.

Simon L


Further Resources

1. Schultz, Scot C., et al. “Predicting in-hospital mortality during cardiopulmonary resuscitation.” Resuscitation 33.1 (1996): 13-17.

2. Fumeaux, T., et al. “Survival after cardiac arrest and severe acidosis (pH= 6.54).” Intensive care medicine 23.5 (1997): 594-594.

3. Sasson, Comilla, et al. “Predictors of survival from out-of-hospital cardiac arrest a systematic review and meta-analysis.” Circulation: Cardiovascular Quality and Outcomes 3.1 (2010): 63-81.

4. Blyth, Lacey, et al. “Bedside focused echocardiography as predictor of survival in cardiac arrest patients: a systematic review.” Academic Emergency Medicine19.10 (2012): 1119-1126.


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