Method of estimating the thoracic fluid content, based on anthropometric data of the patient and determining the electrical impedance of the chest

Abstract

Background. Determination of the thoracic fluid content in the dynamics is becoming increasingly common in clinical \xadtrials and is a promising method for monitoring patients of intensive care units of various profiles. The most affordable and cost-effective methods for monitoring the amount of fluid in the chest at present are those based on measuring the electrical impedance of the chest when scanning it with high-frequency current. These techniques provide good repeatability of results, and are virtually independent of the operator. The purpose of the work is to develop own original technique for determining the thoracic fluid content. Materials and methods. The electric chest impedance was measured when scanning the chest with an electric current of 32 KHz using two pairs of band electrodes according to V. Kubicek. The circumference of the base of the neck and chest at the site of application of the electrodes was measured carefully. The distance between electrodes was also determined. Chest volume was calculated based on the truncated cone model. The thoracic fluid content was evaluated by the equation: V\xa0=\xa0γν/Z(R – r), where V is the volume of fluid in the chest; γ is the average electrical blood conductivity; ν\xa0is the volume of the thorax, calculated on the model of a truncated cone; Z is the value of the electrical impedance of the chest; R is the radius of the thorax, and r is the radius of the base of the neck. The difference between them in this equation should reduce the error associated with the presence in the thorax of connective tissue that has an electrical conductivity different from the electrical conductivity of the blood. Studies were performed in both apparently healthy volunteers and in polytrauma patients with thoracic injury and signs of acute respiratory failure. Results. Our observations showed that the amount of fluid in the chest, calculated by our method, normally approaches 60\xa0%\xa0— 59\xa0±\xa02\xa0% of the chest volume, calculated on the base of the truncated cone model. In the most severe cases of thoracic injury, the relative fluid content in the chest of victims reached 75–80 %, and the amount of fluid in the chest, expressed in conventional units per 1/KΩ, was at the level of 45–50 conventional 1/КΩ. These events were associated with the presence of a clinical picture of acute respiratory distress syndrome degree 2, and all patients were on mandatory pulmonary mechanical ventilation with the creation of constant positive airway pressure and respiratory plateau at the level of 25–27\xa0cm H2O. The positive dynamics of the process was associated with an increase in the oxygenation index, the ability to transfer patients to sponta\xadneous breathing. At the same time, the relative thoracic fluid content in patients decreased to 60–67 %, and in those who needed continued mechanical ventilation\xa0— to 68–73 %. The thoracic fluid content, expressed in conventional units per 1/KΩ, with rapid improvement and the possibility of discontinuation of ventilation was 37–42 conditional 1/KΩ, and if it was necessary to continue mechanical ventilation\xa0— 43–46 conditional 1/KΩ. The results of determining the thoracic fluid content by the authors’ method better corresponded to the clinical picture of thoracic trauma, the severity of the manifestations of acute respiratory distress syndrome than the noninvasive cardiac output monitoring. Conclusions. The developed method for determining the thoracic fluid content can be applied in researches and clinical practice during intensive care of patients with acute respiratory distress syndrome.