How Can Hyperventilation Decrease Venous Return To The Heart And Reduce Blood Flow During Cpr?
During CPR, delivering too many breaths per minute (hyperventilation) can decrease venous return to the heart and reduce blood flow during chest compressions. why?
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October 9, 2009 @ 6:44 pm
Joel – That is an excellent question. Because venous return consists of nonpulsatile flow, it is drive primarily by pressure gradients. During normal breathing, the diaphragm flattens and generates low (negative) intrathoracic pressures. This is what pulls air into our lungs, but it also pulls blood into the chest and therefore back to the heart (increased venous return).
During positive pressure ventilation, instead of pulling air into the lungs by generating negative intrathoracic pressure, the patient has air PUSHED into the lungs by a high-pressure source. This can be a ventilator, a bag-valve mask, or someone administering rescue breaths. Positive pressure ventilation INCREASES intrathoracic pressure (the same way you would increase pressure in a balloon as you blow it up) and therefore DECREASES venous return to the heart.
When chest compressions are administered, negative intrathoracic pressures are actually generated during the release phase. When the chest is compressed, the pressure in the thorax increases (the way it would in a balloon that is being squeezed), but when it is released, the sternum springs up and creates the negative intrathorcaic pressure, pulling blood into the heart and increasing venous return.
In a landmark study by Keith Lurie and colleagues in 2004, they demonstrated that professional rescuers have a tendency to administer more breaths per minute than recommended by the American Heart Association at the time. They also demonstrated that the more breaths delivered per minute, the higher the mean intrathoracic pressure. Excessive ventilation inhibited venous blood flow back to the heart by allowing insufficient time for the development of negative intrathoracic pressure between compressions.
October 9, 2009 @ 8:02 pm
Because as you give the breaths, it increases the blood o2 and c02 by a vast amount. The chest compressions help to move the blood through the body. When there are more breaths, and less compression, the blood has less of a chance to disappate the c02 via the alvioli.