quantium Medical Cardiac Output (qCO) uses impedance cardiography in a simple, continuous, and non-invasive way to estimate the cardiac output (CO) and other hemodynamic parameters such as the stroke volume (SV) and cardiac index (CI). The CO estimated by the qCO monitor is referred to as the "qCO". The impedance plethysmography allows determining changes in volume of the body tissues based on the measurement of the electric impedance at the body surface. The assessment of cardiac output (CO) is important because it reveals the main cardiac function: the supply of blood to tissues. CO reflects the hemodynamic flow and hence the transport of oxygen; its clinical applications by non-invasive continuous hemodynamic monitoring are especially useful for some medical specialties like anaesthesiology, emergency care and cardiology, for example to prevent hypoperfusion and to guide fluid administration. Several authors advocate the high reliability and good correlation of cardiography impedance compared to others techniques more established. Nevertheless, some detractors complain about the sensitivity of the technique to artefacts such as the electromyography or breathing movements.
qCO calculated
Impedance cardiography The Impedance Cardiography (ICG or Ztot) signal represents the changes in the thoracic impedance due to variations in the blood flow. In practice, the raw Ztot signal (in O) is transformed to the –dZ/dt waveform (filtered negative first derivative, in O x s-1) by using the first derivative to remark the inflection points of the raw Ztot signal. The most important characteristic points of the –dZ/dt waveform are B, C, and X points (see figure 2). All these points are associated with distinct physiological events within the systolic part of the cardiac cycle, i.e., located after the QRS complex onset. In that sense, the R wave from the ECG signal can be an important reference for detecting such events.
Detecting characteristic points of the ICG signal: problems and solutions. The ICG signal is extremely sensitive to artifacts due to movements and respiration. Baseline variations can considerably alter the dZ/dtmax point and the dependent hemodynamic parameters. High-frequency interferences also can affect the accuracy in estimating some characteristics points, essentially the B point, affecting the LVET dependent parameters. The qCO's algorithm uses classical and advanced adaptive filtering techniques to minimize such problems with good results. The algorithm applies a band-pass filtering stage, which attenuates both the baseline drifts (due to movement or respiration) and the high-frequency signal. Also, it features Adaptive Neuro-Fuzzy Inference System (ANFIS), for higher accuracy, which is able to obliterate even artifacts such as the electrosurgical noise.
Foundation and features of the different methods. Since its discovery, the use of CO monitoring has been restricted to certain kinds of patients because of the risk of the classical procedures. Lately, new kinds of methods and devices have been developed. Nowadays, in practice, the method election is determined mainly by the features of the patients and the possibilities of the hospital to accede to this technology. Different CO monitor methods can be characterized by their invasiveness or their ability to measure continuously. The most ideal should be reliable, continuous, non-invasive, easy to use, and cost-effective. The invasive methods like the still gold standard method Swan-ganz catheter (pulmonary artery catheter), based on transcardiac thermodilution, are being replaced by its invasiveness; hence these systems will not be inspected in this document. The minimally invasive methods also require catheterization, but less harmful. One of them is the Thermodilution Transpulmonary (TDTP), developed in the late 1990s, which presents risk of iatrogenic effect effects such as pneumothorax, Bleeding, infection, thrombosis or vascular ischemia, inherent to the requirement of both arterial and central venous catheter. As a calibrating method, this system performs a thermodilution curve by measuring the changes in blood temperature due to the injection of cold serum and calculates the CO intermittently through a modified equation of Stewart-Hamilton. Furthermore, this method lets a continuous reading of the CO through the analysis of the arterial Pulse Pressure (PP) wave. Admittedly, these systems have some limitations, related for example to the thermodilution, such as thermic artifacts due to the injection of another serum or during the extracorporeal purification treatment (this therapy is required by approximately the 5% of patients in critical areas). Most of the monitors able to provide a continuous measure of the CO, are based on the analysis of the arterial blood pressure(BP) curve, called pulse contour methods. They are founded by the fact that the pulse pressure is proportional to the ejection volume; however, this is only true when aortic resistance remains constant, hence, these methods have some limitations such as obesity, which affects the arterial distensibility, or aortic regurgitation. Furthermore, the values may be modified in patients with Intra-Aortic Balloon Pumps (IABP) or unstable arterial signal as in cases of severe arrhythmias or ventricular extrasystolia. The signal can be altered also by shock or hypothermia states because of the peripheral vasoconstriction or arterial spasm. Usually, for calculating the stroke volume, the methods based on pulse wave analysis need previous and periodic calibrations that must be done in the same patient, which is a drawback. The main non-invasive methods can be classified into two groups. The first group is formed by different variations of the echocardiography and echo-Doppler techniques and the second group is comprised by the aortic bioimpedance and bioreactance.
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