As I dig deeper and deeper into the studies used to support the new AHA Guidelines, I keep facing the same issue over and over. The cited researchers and AHA advisory board do not seem to know what hyperventilation is. I talked about this subject in my Blog of earlier this year, What Happened to Ventilation?
Hyperventilation is “clinical reduction in PCO2 caused by increasing the alveolar minute volume of ventilation”. Just Google or Wikipedia the term hyperventilation or hyperventilation syndrome. You will get over a million hits. The abbreviated definition of Hyperventilation from answers.com is,
hy·per·ven·ti·la·tion (hī’pər-vĕn’tl-ā‘shən) n.
Abnormally fast or deep respiration, which results in the loss of carbon dioxide from the blood…
Think of the term above “deep respirations” as increasing tidal volumes. Hyperventilation is much more than just a faster ventilation rate. It can be achieved by increasing rate, increasing volume, or by increasing both. It is all about how much gas goes in & out of the lungs each minute, called Minute Volume.
Minute Volume is the total amount or volume, of ventilation gas that you deliver in one minute. You can easily calculate this number (usually expressed in LPM) by multiplying the ventilation volume times the ventilation rate. When you increase the amount of gas going in and out of the lungs enough to lower the level of carbon dioxide in the blood (PCO2), this is true hyperventilation.
So why does everyone seem to think it only means “bag faster”? Today most clinicians now assume incorrectly, that the only way you can increase minute volume is by increasing ventilation rate. This was what many of us were taught in Paramedic school. This is not true. You can hyperventilate, even while slowing down ventilation rates. Let me share an example.
Which of the following two combinations of Rate and Tidal Volume would be an increased minute volume over the other? Which is more likely to produce a lower PCO2?
A) 10 x 1000cc per breath or
B) 20 x 400cc per breath
If you answered A on both, you are correct. 10LPM is more Minute Volume than 8LPM. With choice A you increase minute volume by 20% over option B.
This brings me to my concerns with the AHA cited study, “Death by hyperventilation: a common and life-threatening problem during cardiopulmonary resuscitation”. There are two important conclusions I reached after reading it. Neither support the authors conclusion stated in the title. This study compared different ventilation rates using only one type of device.
In the first phase of this study project, they observed rescuers performing actual CPR and learned that most rescuers used a rapid ventilation rate over an average 1-second inspiratory time. What this study proved of interest to me, was to confirm my long held belief that very few have followed the last set of AHA Guidelines. They called for a long slow breath, delivered over a 1.5 – 2.0 second inspiratory time. Few have been taught to do this, and even fewer actually do it well. Maybe if we had been doing it right, we would not now be faced with these drastic changes to the Guidelines.
- So my first question is, why have so many professional emergency medical responders completely ignored the last set of ventilation guidelines?
Reading more of this article, I learn that “The average percentage of time a positive pressure was recorded in the lung was 47.3%.” All methods of artificial ventilation are positive pressure delivery devices except one – the old iron lung. So what can we really conclude from this? Only that the test subjects did not get a decent breath delivered on more than half of their rescuers ventilation attempts.
- So my next question is, if you do not deliver a measurable positive pressure breath 52.7% of the time, would not a better title for this study really be,
“Death by hypoventilation: a common and life-threatening problem during cardiopulmonary resuscitation using a BVM”.
Since this study did not measure the minute volume delivered at the three different ventilation rates compared (of 12, 20, & 30 Breaths per Minute), we have no real way of knowing that the more rapid rate of ventilation produced a higher minute volume. We cannot conclude from the evidence that hyperventilation was induced by the most rapid rate, which produced the worst outcomes. So the study does not prove that hyperventilation is bad.
I wish they had measured minute volumes at these different ventilation rates. I also wish they had correlated the ventilation frequency to the percentage of positive pressure breaths. Did they have a recorded positive pressure in the lung more often at the slower rates or at the faster rates? We might have learned that slower ventilation rates could help to improve minute volumes by extending the duration of each breath (inspiratory time). At the slower rate you also extend the time between breaths (expiratory time), which may enhance exhaled air volumes and reduce the “stacking” of breaths. Remember, for it to count – the air must go both in and out!
The study did prove two things to me; first that rapid rate ventilation with a one particular type of device (the BVM) is bad. It also proved that rescuers do not know how to use a BVM correctly. This is not really new news. So how the author or the AHA concludes that this study proves hyperventilation is bad is completely beyond me. I do not think they know what hyperventilation really is.