In an effort to improve the dismal outcomes for patients after cardiac arrest, a novel technology has recently been developed that functions to maximize circulation of blood to the vital organs during ardiopulmonary resuscitation.
Use of this new device, called the impedance threshold device (ITD), enhances the changes in intrathoracic pressures during cardiopulmonary resuscitation (CPR) to double both blood flow to the heart and blood pressures, improve circulation to the brain, and significantly increase the chances for survival after an out-of-hospital cardiac arrest.Additionally, recent clinical studies of this device punctuate the difficulty of performing CPR correctly; these studies have demonstrated that even mildly excessive ventilation rates and incomplete chest wall recoil during CPR can be lethal. Importantly, use of the ITD has enhanced understanding of the essential elements needed to perform effective CPR and improve survival rates after cardiac arrest. While not a panacea, use of the ITD in efficient emergency medical systems increases the likelihood for survival after out-of-hospital cardiac arrest.
Conventional manual CPR (sCPR) is inherently inefficient, and provides only marginally adequate coronary perfusion pressures due to the sub-optimal pressure gradient between the aorta, the right atrium, and left ventricle. During the decompression (or passive relaxation) phase of CPR, a small intrathoracic vacuum (relative to atmospheric pressure) develops with each chest wall recoil, promoting blood flow back to the heart. Myocardial perfusion predominantly occurs during this key decompression phase of CPR. The difference between the diastolic aortic and the right atrial pressures (coronary perfusion pressure) is a critical determinant of CPR efficacy.
With sCPR,much of the potential hemodynamic benefit of this intrathoracic vacuum is lost by the passive influx of inspiratory gas. Alternatively, the ITD, which is a small, lightweight device containing pressure-sensitive valves, selectively impedes the influx of air during chest wall decompression, providing an augmented amplitude and duration of vacuum within the thorax.When used with both sCPR and active compression–decompression (ACD) CPR, the ITD prevents respiratory gases from entering the lungs during the decompression phase of CPR. By harnessing the kinetic energy of the chest wall recoil, thereby augmenting the ‘bellows-like’ action of the chest with each compression–decompression cycle, the ITD draws more venous blood back into the heart.This results in increased cardiac preload and, thus, increased cardiac output, improved blood pressure, and enhanced vital organ perfusion. The ITD also lowers intracranial pressure during the decompression phase of CPR, which, combined with increased cardiac output, results in greater cerebral perfusion. Because of the pressure-sensitive valves, the ITD offers no resistance to active ventilation by a rescuer or to patient exhalation of respiratory gases. Clinical and animal studies, described in more detail below, have shown that the combination of an ITD with either sCPR or ACD CPR results in markedly higher blood flow to the heart and brain compared with either method alone.