Another study on airway management in cardiac arrest was just published in JAMA. The study was done in Denmark, where all intubations elective and emergency are done by anaesthestetists. It was a retrospective study, where they matched intubated patients with historical arrest patients not intubated at that same point in resus time. Intubation lowered the pt’s chance of survival to discharge with CPC 1-2. So, another study against intubation in cardiac arrest. Or?
This is hard, especially for us anaesthetists. We’re in the oxygen delivery business. Our job is to deliver oxygen through A, the Airways, into B Breathing, the lungs, and then into C, the circulation, to get that oxygen all the way out to the target organs and cells.
So, we need a functioning airway and lungs as well as a functioning circualtion. And an intubated, secure airway must be better than an unintubated one. Many studies have discussed how intubation take focus away from compressions and circulation, and therefore gives the patient more CPR downtime. Most intubations I see in Scandinavia are done either during continuous compressions, or with a very brief pause while passing the tube between the chords. I don’t think this is where the big problem is. The standard 30:2 circulation downtime during the breathing cycle for bag/mask ventilation is often longer than the brief pause for intubation.
A tube down the trachea gives us a secured and always open airway, with good protection against aspiration. This tube is not the problem in itself. But it gives us the means to generate extremely high intrathoracic volumes and pressures, and to ventilate as often and as much as we want. This is what causes the problems we see with intubation in cardiac arrest. Not the intubation itself.
Positive pressure hyperventilation
I think the real problem is the following positive pressure ventilation. Through a cuffed tube, you can easily generate high intrathoracic pressures. These pressures will hinder venous return and preload. This will give worse cerebral flow and oxygenation as well as worse coronary flow and can reduce chances of ROSC as well as survival. And very often I see normo- to hyperventilation during CPR.
Oxygenation in arrest
We’re thinking about oxygenation in arrest patients all wrong. This is a patient in cardiac arrest, and even with optimal compressions, in an extreme low-flow state, so the amount of red cells passing through the lungs per minute is very low. So, you need very little extra oxygen to saturate those red cells. Oxygenation of the few hemoglobins passing through the lungs close to alveoli is not the problem, getting the oxygen out to the target organs is.
Still, we see low PaO2 levels during CPR, probably due to atlectasis and shunts in the pulmonary circulation. PEEP is the standard measure to keep the lungs open, but higher PEEP will definitely also hinder venous return and preload in arrest/low flow states.
The real conclusion
From the studies we have now, we can confidently say that intubation with or without CPR pauses still harms the patient with a standard ventilation strategy, which often is hyperventilation by hand. Ventilation should be low and slow.
Ventilation and compressions
And if you time ventilation with CPR, when’s the best time to squeeze the bag?
During relaxation, when it’s easier to get air into the lungs with low pressures, but also the cycle when blood is supposed to flow into the thorax, and the heart be perfused? During compressions, when inhalation will cause even higher intrathoracic pressures, but is also the time when compressions will hinder venous return anyway, and then let the relaxation phase be the venous return phase as per usual compression cycle? Or doesn’t compression cycle sync matter? The main point is probably just to ventilate infrequently.
I’ve tried searching for animal experiments, as that’s probably the closest we’ll get to monitoring CPR physiology in a controlled fashion. There are a few studies out there, and we’ll discuss some of them in a future post. If anyone has good links to articles on ventilation and its impact on circualtion during CPR, please post in comments.
For now, let’s stop doing studies on intubation in cardiac arrest, and rather focus on studies optimising ventilation during cardiac arrest instead.