is transferred, simulating an infusion of approximately 300 ml. figure 3: for maximal reliability, the plr test must be performed following some rules. one possible variation of plr starts from the semi-recumbent position 1. the second step comprises going down the trunk and raise legs maintaining the angle between them using the automatic motion of the bed for avoiding artifacts. finally, the third step goes back to position 1 to ensure that the subject recovers the previous hemodynamic parameters. an increase in the co (of = 10%) and ap within the next 60–90 seconds after leg elevation indicates a positive result, suggesting that the patient is fluid responsive and might benefit from additional fluid. by contrast if the test is negative, the patient should not be treated as it would be harmful. the fluid overload can raise extravascular water causing deleterious side effects in lungs. the frank starling’s curve the basic mechanisms for the hemodynamic regulation are: intrinsically cardiac regulation in the function of the venous return. control of hr by autonomic nervous system (ans). the explanation why the heart is capable of adapting to growing volumes of blood flow is called 'frank-starling's mechanism', which says that the more distended is the muscle during the filling, the more strength of contraction and quantity of blood pumped by the left ventricle to the aorta. when the heart reaches a physiological limitation, then blood pumping cannot increase, although the venous return is further increased. the svv contributes to knowing the state of euvolemia in ventilated patients and designates the point on the frank-starling curve where the patient is. figure 4: the link between the fiber length and myocardial contractile force is illustrated. within the preload dependent zone, the sv raises while the ventricular preload does too, however, in the preload independent zone a higher preload or further fluid boluses will not improve the sv. the current qco utilities can be summarized in in general terms to help diagnose, triage, or choose and anticipate the response to treatment in different cases such as cardiopathies, heart failure, hypertension, trauma, sepsis, burn hypovolemic shock or dyspnoea. guide the gdt, for the administration of fluids or inotropes in order to optimize preload, contractility, and afterload. monitoring high-risk patients undergoing any surgery (as long as it does not affect the thorax morphology). improve the cost-efficiency relationship of the hospital, mainly by improving the outcome which implies a shorter hospital stay, thus, saving highly expensive resources, against the low cost of the device and electrodes (which can be the classic electrodes for monitoring the ekg). other parameters provided by the qco in addition to cardiac function, the qco also evaluates indirectly other related fields as is the parasympathetic nervous system (pns), through the evaluation of the r-r intervals and heart rate variability (hrv or beat-to-beat variation). since 1960 the hrv has been analyzed. nowadays, it has become a widely used tool for study the ans modulation in clinical areas such as cardiology. even though, its utility is not well known yet. the interest lies in its profound relationship with breathing, ans, and cardiac autonomic regulation. the most relevant and validated capacity is to prognosticate the coronary heart disease patients, accordingly, a reduced hrv is related to a high risk of mortality after myocardial infarction. another important point is the fact that inflammation is stimulated by the effect of the parasympathetic nervous system and inhibited by the sympathetic, thereby, could also be measured through the r-r intervals to guide the physician. several reports maintain that the hrv analysis is a highly accurate indicator of the autonomic dysfunction in alcoholic and diabetic subjects. also, it is growing interest in applying the hrv to evaluate a neurologic function, and the best results have been found out in psychiatric patients. regarding the anesthesia and critical care fields, the ans dysfunction should be considered because it is related to high morbidity and mortality in patients undergoing surgery under the general anesthesia effect. naturally, events as the tracheal intubation and inhalation agents can produce quick variations in ans. also, should be regarded in critical patients who may show a worse outcome when it is added to certain illnesses such as sepsis . figure 5: hrv parameters recorded during the recovering of a patient with sepsis are depicted. it is noticeable the growing trend according to the improving. hence, this data could be used to drive a prognosis. the hrv depends on the age, sex, and also other physiological variations such as hormonal components, genetics, exercise, environmental factors, thermoregulation, etc. a controlled recording could explain variations in ans due to physiology, pharmacology or pathology there exist several systems for quantifying the hrv, nevertheless, none of them have shown better result than others. the most widely used methods for the hrv analysis can be grouped under time-domain and frequency-domain, the qco offers the following values: parameterfoundationclinical significance 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. pnn50percentage of rr intervals that differ more than 50 ms.a high value of pnn50 gives us information about the spontaneous variation of the hr. sdsdstandard deviation of successive differences of the rr intervals.provides long-term information oncorr the variations. figure 6: the fast fourier transformation (fft) and spectral methods let quantify components of the hrv, by classifying the frequencies in three regions: high (hf= 0.15-0.4 hz), low (lf= 0.04-0.15 hz) and very-low (vlf= 0.003-0.04 hz) frequency powers. the most valuable zone is the hf because is modulated exclusively by the snp. the lf is regulated by both, the pns and sns (sympathetic nervous system). and the vlf modulation is unclear, but it is thought that could be influenced by the pns and renin-angiotensin-aldosterone system.