Reducing ventilation frequency combined with an inspiratory impedance device improves CPR efficiency in swine model of cardiac arrest
Introduction
The basic premise that frequent ventilations are a necessity to maintain tissue oxygenation during cardiopulmonary resuscitation (CPR) has been challenged by recent animal and human studies [1], [2], [3], [4], [5], [12], [13]. These new investigations have shown that adequate oxygenation can be maintained with chest compressions alone without ventilation for some limited period of time and that there is no haemodynamic compromise with that approach. When chest compressions are stopped to deliver a breath, coronary perfusion is interrupted and consequently falls until the next series of compressions [17]. An additional cause and explanation for the negative hemodynamic consequences of ventilation is that frequent positive pressure ventilations may result in higher intrathoracic pressures and thereby impede venous return to the heart during the decompression phase of CPR. As such, the frequency of ventilation preventing venous return of blood to the heart during the chest wall decompression phase directly may alter coronary perfusion pressure (CPP) and cardiac output during CPR [8].
The importance of changes in intrathoracic pressure during CPR recently has been highlighted by studies of a new device called an inspiratory impedance threshold device (ITD). This helps to pump more blood back to the heart during cardiac arrest by enhancing negative intrathoracic pressure during the decompression phase of CPR, thereby enhancing blood return [6], [9], [10], [11], [14], [15]. In view of the increased efficiency of CPR with the ITD, we hypothesized that the cardiopulmonary interactions associated with blood flow and ventilation may be improved further by reducing the frequency of ventilations when using the ITD. A reduction in the ventilation frequency would result in less overall positive intrathoracic pressure, enable more time for venous blood flow back to the heart and provide more time per minute for the ITD to increase circulation [6], [7], [10].
To test the hypothesis we investigated the effects of two different compression: ventilation ratios, with and without an ITD, on the coronary perfusion pressure, mean arterial pressure (MAP), and oxygenation. Compressions were performed continuously at a rate of 100 min−1. The delivery of each breath was initiated during the decompression phase of CPR. To ensure that the chest wall was allowed to recoil fully during the decompression phase, an automatic compression-release device was used. The results support the hypothesis that fewer ventilations per minute improved hemodynamics during CPR. The physiological benefits of fewer ventilations per minute can be further enhanced using the ITD, without compromising oxygen delivery.
Section snippets
Materials and methods
The study was approved by the Committee of Animal Experimentation at the University of Minnesota. The animals received care in compliance with the 1996 Guide for the Care and Use of Laboratory Animals by the National Research Council in a facility that was accredited by the American Association for Accreditation of Laboratory Animal Care. Anaesthesia was used in all surgical interventions to avoid all unnecessary suffering. Experiments were performed by a qualified team. The study was performed
Results
A total of 32 pigs randomized to four equal groups: group A received CPR with a compression:ventilation (C/V) ratio of 5:1, group B with a C/V ratio of 5:1 plus the ITD, group C with a C/V ratio of 10:1 and group D with a C/V ratio of 10:1 plus the ITD. Before induction of cardiac arrest, there were no statistically significant differences in weight, temperature, hemodynamic variables and arterial blood gases between the groups (Table 1).
Intrathoracic pressures during the decompression phase
Discussion
Cardiopulmonary interactions during CPR are influenced by the ratio of chest compressions to ventilations and the degree of negative intrathoracic pressure achieved during chest wall recoil. The results of this study demonstrate that both a reduction in the frequency of ventilation and use of an ITD enhance the decompression phase vacuum created by the chest wall recoil. This results in a significant improvement in the efficiency of CPR and in survival rates [10], [14].
At present, the
Conclusions
This study supports the hypothesis that the efficiency of cardiopulmonary resuscitative measures can be improved by optimizing the cardiopulmonary interactions that regulate vital organ blood flow during CPR. A reduction in the frequency of ventilations resulted in a lower intrathoracic pressure during CPR and this contributed to the increase in arterial and coronary perfusion pressures. Addition of an inspiratory impedance device resulted in a further decrease in intrathoracic pressures during
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