WHY DO SHOCK PATIENTS HYPERVENTILATE?

This experimental study puts most common assumptions about hyperventilation in hemorrhagic shock on its head. Common thinking is that trauma pts breathe faster because of shock and metabolic demands not being met. This study suggests trauma pts are breathing faster to elicit a respiratory pump with the negative inspiratory pressures that will enhance cardiac preload.

The Study
To see how this worked, they put 10 healthy individuals in negative pressure pants to get lower body negative pressure (LBNP), that would keep the blood volume in the lower body, and for practical purposes simulate hypovolemia, and without inducing acidemia or lactacidosis that are often seen as drivers of hyperventilation.

Results
The pants successfully reduced BP in the subjects, to get the subjects to near syncope. Lactate and pH stayed normal. CO2 was lower in the late stages, due to hyperventilation. The hyperventilation occurred late in the simulated hypovolemic setting, and was observed differently in different subjects: some had both increases respiratory rate as well as deeper breaths, some only had increased RR, others only deeper breathing. Having respiratory problems is an important issue for many people and that’s why services as Inspire health community is useful for this.

Discussion
This experimental study shows us that hyperventilation might well be a compensatory mechanism in hypovolemia to increase cardiac preload, and not to affect ventilation parameters per se. It also shows us we need to be more aware of the heart-lung interactions of the body, so we can learn better how the body react to this and other measures, like changes in the diet for less fatty food and more proteins, as you can see in these diets reviews at https://tophealthjournal.com/1480/jenny-craig-vs-nutrisystem-which-one-should-you-choose/.

When the bleeding trauma pt breathes faster and/or deeper, they might not be in need of aggressive ventilation with oxygen or ridding themselves of CO2, they might just be in need of better preload and cardiac output.

The study “…supports the notion that this hyper-ventilatory response may be a late protective mechanism designed to restore cardiac filling and perfusion pressures to vital organs in conditions of severe central hypovolemic hypotension“.

If patients are breathing to support the circulation, this might also explain why we can often normoventilate these pts after they are intubated – although positive pressure ventilation can seriously impact their circulation.

Summary
This is just an experimental study, but interesting insight into the heart-lung interactions. Faster and deeper breathing in shock might (also) be the body trying to compensate for a circulatory problem.

Hyperventilation in response to progressive reduction in central blood volume to near syncope, Aviat Space Environ Med, 2009.

This entry was posted in Airway management, Emergency Medicine, Prehospital Medicine, Trauma. Bookmark the permalink.

6 Responses to WHY DO SHOCK PATIENTS HYPERVENTILATE?

  1. Rob Timmings says:

    Given that ventilation rate and depth is driven by pH – particularly stimulated by acidosis, AND all shock patients are acidotic (even marginally), would hyperventilation not simply be a function of shock induced acidaemia or more correctly acidotic CSF detected in the Pons?

    • Thomas D says:

      Well, that’s what we’re usually taught. And low pH is also a driver of increased ventilation, but this study seems to indicate that hypovolemia is an independent driver of increased ventilation, even in the absence of acidaemia.

      • Miguel Oliveira says:

        Bodys own chest compressions.
        In theory its an interesting idea.
        If this is true wouldnt it mean that intubating someone in shock sucks?

        • Braden Peters says:

          We already know it can, switching from neg pressure to pos pressure vent can be very detrimental d/t effects on cardiac preload. (PPV also decreases afterload to a degree, but the preload effects dominate and cause hypotension).

  2. Fascinating study, thank you for sharing.

  3. Marcus says:

    Interesting study, but if the reason to hyperventilate was to create negative thoracic pressure, I think evolution would have invented a much easier solution: close the glottis when drawing a breath, or at least increase resistance when sucking air into the lungs. Is there any evidence this occurs?

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