in both cases, an independent technique is required to provide calibration of continuous q analysis because arterial pp analysis cannot account for unmeasured variables such as the changing compliance of the vascular bed. recalibration is recommended after changes in patient position, therapy or condition. in picco, transpulmonary thermodilution, which uses the stewart-hamilton principle but measures temperatures changes from central venous line to a central arterial line, i.e., the femoral or axillary arterial line, is used as the calibrating technique. the q value derived from cold-saline thermodilution is used to calibrate the arterial pp contour, which can then provide continuous q monitoring. the picco algorithm is dependent on blood pressure waveform morphology (mathematical analysis of the pp waveform), and it calculates continuous q as described by wesseling and colleagues. transpulmonary thermodilution spans right heart, pulmonary circulation and left heart, allowing further mathematical analysis of the thermodilution curve and giving measurements of cardiac filling volumes (gedv), intrathoracic blood volume and extravascular lung water. transpulmonary thermodilution allows for less invasive q calibration but is less accurate than pa thermodilution and requires a central venous and arterial line with the accompanied infection risks. in lidco, the independent calibration technique is lithium chloride dilution using the stewart-hamilton principle. lithium chloride dilution uses a peripheral vein and a peripheral arterial line. like picco, frequent calibration is recommended when there is a change in q. calibration events are limited in frequency because they involve the injection of lithium chloride and can be subject to errors in the presence of certain muscle relaxants. the pulseco algorithm used by lidco is based on pulse power derivation and is not dependent on waveform morphology. statistical analysis of arterial pressure – flotrac/vigileo cardiac function curve in frank–starling's law, illustrating stroke volume (sv) as a function of preload flotrac/vigileo (edwards lifesciences) is an uncalibrated, haemodynamic monitor based on pulse contour analysis. it estimates cardiac output (q) using a standard arterial catheter with a manometer located in the femoral or radial artery. the device consists of a high-fidelity pressure transducer, which, when used with a supporting monitor (vigileo or ev1000 monitor), derives left-sided cardiac output (q) from a sample of arterial pulsations. the device uses an algorithm based on the frank–starling law of the heart, which states pulse pressure (pp) is proportional to stroke volume (sv). the algorithm calculates the product of the standard deviation of the arterial pressure (ap) wave over a sampled period of 20 seconds and a vascular tone factor (khi, or χ) to generate stroke volume. the equation in simplified form is: s v = s t d ( a p ) ⋅ χ (ap)\cdot \chi } , or, b p ⋅ k ( c o n s t a n t ) } . khi is designed to reflect arterial resistance; compliance is a multivariate polynomial equation that continuously quantifies arterial compliance and vascular resistance. khi does this by analyzing the morphological changes of arterial pressure waveforms on a bit-by-bit basis, based on the principle that changes in compliance or resistance affect the shape of the arterial pressure waveform. by analyzing the shape of said waveforms, the effect of vascular tone is assessed, allowing the calculation of sv. q is then derived using equation (1). only perfused beats that generate an arterial waveform are counted for in hr. this system estimates q using an existing arterial catheter with variable accuracy. these arterial monitors do not require intracardiac catheterisation from a pulmonary artery catheter. they require an arterial line and are therefore invasive. as with other arterial waveform systems, the short set-up and data acquisition times are benefits of this technology. disadvantages include its inability to provide data regarding right-sided heart pressures or mixed venous oxygen saturation. the measurement of stroke volume variation (svv), which predicts volume responsiveness is intrinsic to all arterial waveform technologies. it is used for managing fluid optimisation in high-risk surgical or critically ill patients. a physiologic optimization program based on haemodynamic principles that incorporates the data pairs sv and svv has been published. arterial monitoring systems are unable to predict changes in vascular tone; they estimate changes in vascular compliance. the measurement of pressure in the artery to calculate the flow in the heart is physiologically irrational and of questionable accuracy, and of unproven benefit. arterial pressure monitoring is limited in patients off-ventilation, in atrial fibrillation, in patients on vasopressors, and in those with a dynamic autonomic system such as those with sepsis. uncalibrated, pre-estimated demographic data-free – pram pressure recording analytical method (pram), estimates q from the analysis of the pressure wave profile obtained from an arterial catheter—radial or femoral access. this pp waveform can then be used to determine q. as the waveform is sampled at 1000 hz, the detected pressure curve can be measured to calculate the actual beat-to-beat stroke volume. unlike flotrac, neither constant values of impedance from external calibration, nor form pre-estimated in vivo or in vitro data, are needed. pram has been validated against the considered gold standard methods in stable condition and in various haemodynamic states. it can be used to monitor pediatric and mechanically supported patients. generally monitored haemodynamic values, fluid responsiveness parameters and an exclusive reference are provided by pram: cardiac cycle efficiency (cce). it is expressed by a pure number ranging from 1 (best) to -1 (worst) and it indicates the overall heart-vascular response coupling. the ratio between heart performance and consumed energy, represented as cce "stress index", can be of paramount importance in understanding the patient's present and future courses. impedance cardiography impedance cardiography (often abbreviated as icg, or thoracic electrical bioimpedance (teb)) measures changes in electrical impedance across the thoracic region over the cardiac cycle. lower impedance indicates greater intrathoracic fluid volume and blood flow. by synchronizing fluid volume changes with the heartbeat, the change in impedance can be used to calculate stroke volume, cardiac output and systemic vascular resistance. both invasive and non-invasive approaches are used. the reliability and validity of the non-invasive approach has gained some acceptance, although there is not complete agreement on this point. the clinical use of this approach in the diagnosis, prognosis and therapy of a variety of diseases continues. non-invasive icg equipment includes the bio-z dx, the niccomo, and tebco products by bomed. ultrasound dilution ultrasound dilution (ud) uses body-temperature normal saline (ns) as an indicator introduced into an extracorporeal loop to create an atrioventricular (av) circulation with an ultrasound sensor, which is used to measure the dilution then to calculate cardiac output using a proprietary algorithm. a number of other haemodynamic variables, such as total end-diastole volume (tedv), central blood volume (cbv) and active circulation volume (acvi) can be calculated using this method. the ud method was firstly introduced in 1995. it was extensively used to measure flow and volumes with extracorporeal circuit conditions, such as ecmo and haemodialysis, leading more than 150 peer reviewed publications. ud has now been adapted to intensive care units (icu) as the costatus device. the ud method is based on ultrasound indicator dilution. blood ultrasound velocity (1560–1585 m/s) is a function of total blood protein concentration—sums of proteins in plasma and in red blood red cells—and temperature. injection of body-temperature normal saline (ultrasound velocity of saline is 1533 m/s) into a unique av loop decreases blood ultrasound velocity, and produces dilution curves. ud requires the establishment of an extracorporeal circulation through its unique av loop with two pre-existing arterial and central venous lines in icu patients. when the saline indicator is injected into the av loop, it is detected by the venous clamp-on sensor on the loop before it enters the patient's heart's right atrium. after the indicator traverses the heart and lung, the concentration curve in the arterial line is recorded and displayed on the costatus hcm101 monitor. cardiac output is calculated from the area of the concentration curve using the stewart-hamilton equation. ud is a non-invasive procedure, requiring only a connection to the av loop and two lines from a patient. ud has been specialised for application in pediatric icu patients and has been demonstrated to be relatively safe although invasive and reproducible. electrical cardiometry electrode array that measures thoracic electrical bioimpedance (teb) electrical cardiometry is a non-invasive method similar to impedance cardiography; both methods measure thoracic electrical bioimpedance (teb). the underlying model differs between the two methods; electrical cardiometry attributes the steep increase of teb beat-to-beat to the change in orientation of red blood cells. four standard ecg electrodes are required for measurement of cardiac output. electrical cardiometry is a method trademarked by cardiotronic, inc., and shows promising results in a wide range of patients. it is currently approved in the us for use in adults, children and babies. electrical cardiometry monitors have shown promise in postoperative cardiac surgical patients, in both haemodynamically stable and unstable cases. magnetic resonance imaging velocity-encoded phase contrast magnetic resonance imaging (mri) is the most accurate technique for measuring flow in large vessels in mammals. mri flow measurements have been shown to be highly accurate compared to measurements made with a beaker and timer, and less variable than the fick principle and thermodilution. velocity-encoded mri is based on the detection of changes in the phase of proton precession. these changes are proportional to the velocity of the protons' movement through a magnetic field with a known gradient. when using velocity-encoded mri, the result is two sets of images, one for each time point in the cardiac cycle. one is an anatomical image and the other is an image in which the signal intensity in each pixel is directly proportional to the through-plane velocity. the average velocity in a vessel, i.e., the aorta or the pulmonary artery, is quantified by measuring the average signal intensity of the pixels in the cross-section of the vessel then multiplying by a known constant. the flow is calculated by multiplying the mean velocity by the cross-sectional area of the vessel. this flow data can be used in a flow-versus-time graph. the area under the flow-versus-time curve for one cardiac cycle is the stroke volume. the length of the cardiac cycle is known and determines heart rate; q can be calculated using equation (1). mri is typically used to quantify the flow over one cardiac cycle as the average of several heart beats. it is also possible to quantify the stroke volume in real-time on a beat-for-beat basis. while mri is an important research tool for accurately measuring q, it is currently not clinically used for haemodynamic monitoring in emergency or intensive care settings. as of 2015, cardiac output measurement by mri is routinely used in clinical cardiac mri examinations. dye dilution method the dye dilution method is done by rapidly injecting a dye, indocyanine green, into the right atrium of the heart. the dye flows with the blood into the aorta. a probe is inserted into the aorta to measure the concentration of the dye leaving the heart at equal time intervals until the dye has cleared. let c(t) be the concentration of the dye at time t. by dividing the time intervals from into subintervals δt, the amount of dye that flows past the measuring point during the subinterval from t = t i − 1 to t = t i is: ( c o n c e n t r a t i o n ) ( v o l u m e ) = c ( t i ) ( f δ t ) )(f\delta t)} where f is the rate of flow that is being calculated. the total amount of dye is: ∑ i = 1 n c ( t i ) ( f δ t ) = f ∑ i = 1 n c ( t i ) ( δ t ) ^c(t_)(f\delta t)=f\sum _^c(t_)(\delta t)} and, letting n → ∞ , the amount of dye is: a = f ∫ 0 t c ( t ) d t ^c(t)dt} thus, the cardiac output is given by: f = a ∫ 0 t c ( t ) d t ^c(t)dt}}} where the amount of dye injected a is known, and the integral can be determined using the concentration readings. the dye dilution method is one of the most accurate methods of determining cardiac output during exercise. the error of a single calculation of cardiac output values at rest