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qCO (quantium Medical Cardiac Output) 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.
The Impedance Cardiography (ICG or Ztot) signal represents the changes of 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 characteristics points of the –dZ/dt waveform are B, C and X points (see figure 2). All these points are associated to 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.
|Possible causes for increasing CO:||Possible causes for decreasing CO:|
|Increased demand of O2. (I.e. during exercise).||Deficit of ventricular filling.(Can be caused by hypovolemia).|
|States of decreased Systemic Vascular Resistance (SVR). (I.e. systemic inflammation).||Low ventricular emptying.
(Caused by e.g. contractility alteration, valvulopathy).
|Increased sympathetic response. (I.e. pain, fear or anxiety states).||High Systemic Vascular Resistance. (E.g. caused by hypertension, vasoconstriction, and mitral insufficiency).|
|Other states such as pregnancy, liver disease and hyperthyroidism.|
|Energy HF band||(High Frequency) Energy in the 0.15-0.4 Hz frequency bandwidth.||Represents the PNS activity, therefore, the HF components increase when the HR decreases. ? PNS ? ? HR ?? HF|
|RMSSD||(Root Mean Square Successive Deference) Short-term variations of RR intervals.||Observes the influence of the PNS on the cardiovascular system.|
|pNN50||Percentage of RR intervals that differ more than 50ms.||A high value of pNN50 gives us information about the spontaneous variation of the HR.|
|SDSD||Standard deviation of successive differences of the RR intervals.||Provides long term information of the variations.|
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