Minimally invasive cardiorespiratory monitoring and lung recruitment manoeuvres. Studies in animals and humans

Abstract

Background: The maintenance of adequate oxygen delivery is essential to preserve organ function. The circulatory and respiratory systems interact closely to accomplish this. Optimising the function of these systems is of fundamental importance in critically ill patients. This requires adequate monitoring techniques. Availability of non-invasive, bedside, continuous techniques for monitoring the effects of circulatory and respiratory interventions are limited. The aim of this thesis was to evaluate alternative techniques to monitor central hemodynamics and lung volume changes and to use these techniques to study the effects of lung recruitment manoeuvres.Methods: A transoesophageal echo-Doppler technique measuring descending aortic blood flow and a partial CO2 rebreathing technique for cardiac output determination were evaluated in unstable hemodynamic conditions and compared with cardiac output measured by thermodilution using pulmonary artery catheter. Electric impedance tomography was used to monitor global and regional lung volume changes and ventilation distribution during different recruitment manoeuvres in experimental acute lung injury (ALI).Results: The transoesophageal echo-Doppler rapidly and accurately followed marked changes in central blood flow during liver transplantation and experimental cardiac tamponade. Partial CO2 rebreathing technique showed good agreement with the reference technique, despite several possible sources of error. By continuously monitoring aortic blood flow, pronounced circulatory depression was detected during lung recruitment using rapid high airway pressure manoeuvres especially in endotoxin- induced ALI. Monitoring blood pressure underestimated the true circulatory impairment. These negative circulatory effects could be attenuated by prior volume expansion or by using a slow, lower pressure recruitment manoeuvre. This recruitment manoeuvre increased lung volume, compliance and improved gas exchange equally well as high-pressure manoeuvres. Electric impedance tomography revealed the changes in ventilation distribution from non-dependent to dependent areas induced by lung recruitment.Conclusions: The investigated minimally invasive monitoring techniques are useful for monitoring changes in central hemodynamics in critically ill patients. Adequate monitoring techniques are required to detect rapid changes in blood flow like those seen during recruitment manoeuvres. Lung recruitment can be achieved by a slow, lower pressure manoeuvre with less circulatory side-effects. Electric impedance tomography is a promising technique for bedside monitoring of global and regional lung volume changes.