Noninvasive Estimation Of Cardiac Output Research Articles

Comparison of cardiac output estimates obtained from the Antares oscillometric pulse wave analysis algorithm and from Doppler transthoracic echocardiography.

In cardiology, cardiac output (CO) is an important parameter for assessing cardiac function. While invasive thermodilution procedures are the gold standard for CO assessment, transthoracic Doppler echocardiography (TTE) has become the established method for routine CO assessment in daily clinical practice. However, a demand persists for non-invasive approaches, including oscillometric pulse wave analysis (PWA), to enhance the accuracy of CO estimation, reduce complications associated with invasive procedures, and facilitate its application in non-intensive care settings. Here, we aimed to compare the TTE and oscillometric PWA algorithm Antares for a non-invasive estimation of CO. Non-invasive CO data obtained by two-dimensional TTE were compared with those from an oscillometric blood pressure device (custo med GmbH, Ottobrunn, Germany) using the integrated algorithm Antares (Redwave Medical GmbH, Jena, Germany). In total, 59 patients undergoing elective cardiac catheterization for clinical reasons (71±10 years old, 76% males) were included. Agreement between both CO measures were assessed by Bland-Altman analysis, Student's t-test, and Pearson correlations. The mean difference in CO was 0.04 ± 1.03 l/min (95% confidence interval for the mean difference: -0.23 to 0.30 l/min) for the overall group, with lower and upper limits of agreement at -1.98 and 2.05 l/min, respectively. There was no statistically significant difference in means between both CO measures (P = 0.785). Statistically significant correlations between TTE and Antares CO were observed in the entire cohort (r = 0.705, P<0.001) as well as in female (r = 0.802, P<0.001) and male patients (r = 0.669, P<0.001). The oscillometric PWA algorithm Antares and established TTE for a non-invasive estimation of CO are highly correlated in male and female patients, with no statistically significant difference between both approaches. Future validation studies of the Antares CO are necessary before a clinical application can be considered.

A novel method for the noninvasive estimation of cardiac output with brachial oscillometric blood pressure measurements through an assessment of arterial compliance.

To propose and validate a new method for estimating cardiac output based on the total arterial compliance (Ct) formula that does not need an arterial waveform and to apply it to brachial oscillometric blood pressure measurements (OBPMs). One hundred subjects with normal heart anatomy and function were included. Reference values for cardiac output were measured with echocardiography, and Ct was calculated with a two-element Windkessel model. Then, a statistical model of arterial compliance (Ce) was used to estimate cardiac output. Finally, the measured and estimated cardiac output values were compared for accuracy and reproducibility. The model was derived from the data of 70 subjects and prospectively tested with the data from the remaining 30 individuals. The mean age of the whole group was 43.4 ± 12.8 years, with 46% women. The average blood pressure (BP) was 107.1/65.0 ± 15.0/9.6 mmHg and the average heart rate was 67.7 ± 11.4 beats/min. The average Ct was 1.39 ± 0.27 mL/mmHg and the average cardiac output was 5.5 ± 1.0 L/min. The mean difference in the cardiac output estimated by the proposed methodology vs. that measured by Doppler echocardiography was 0.022 L/min with an SD of 0.626 L/min. The intraclass correlation coefficient was 0.93, and the percentage error was 19%. Cardiac output could be reliably and noninvasively obtained with brachial OBPMs through a novel method for estimating Ct without the need for an arterial waveform. The new method could identify hemodynamic factors that explain BP values in an ambulatory care setting.

Comparing doppler-echocardiography and thermodilution for cardiac output measurements in resuscitated out-of-hospital cardiac arrest patients undergoing targeted temperature management

Abstract Funding Acknowledgements Type of funding sources: None. Background Cardiovascular dysfunction is common after out-of-hospital cardiac arrest (OHCA). Cardiac output measurements can be used to guide treatment during post-resuscitation care and echocardiography allows noninvasive cardiac output estimation. Purpose The aim of the present study was to compare Doppler echocardiography (doppler_CO) with thermodilution using pulmonary artery catheters (PAC_CO) for cardiac output estimation in a large and consecutively included cohort of comatose OHCA-patients undergoing targeted temperature management (TTM). Methods Single-center substudy of 171 patients included in the TTM-trial randomly assigned to 33 or 36 degrees C for 24 hours after OHCA. We measured PAC_CO and doppler_CO simultaneously shortly after admission and again after 24 hours. Measurements and Main Results We excluded 19 (11%) patients without PAC-measurement and 31 (18%) without doppler-measurements resulting in 120 paired measurements at admission. Patients were 61 (±11) years old, 86% were men and 91% had a witnessed OHCA. At ICU-admission, PAC_CO was 4.81 (±1.81) L/min. and doppler_CO was 3.74 (±1.38) L/min., with a mean bias of 1.07 (±1.65) L/min (with 95% limits of agreement of –2.16 to 4.04) L/min. Examining the Bland-Altman plot, precision fell with higher cardiac output (figure). A statistically significant, but moderate correlation was found between doppler_CO and PAC_CO at admission (r = 0.49), p &amp;lt; 0.0001). After 24 hours, PAC_CO was 4.63 (±1.38) L/min. and doppler_CO was 3.61 (±1.14) L/min, with a mean bias of 0.96 L/min. Assessing the change from admission to 24 hours, PAC_CO decreased averagely -0.12 (±2.22) L/min. and doppler_CO decreased -0.19 (±1.91) L/min. The changes from admission to 24 hours correlated between doppler_CO and PAC_CO (r = 0.55), p &amp;lt; 0.0001) with a mean bias of the changes of 0.07 L/min, with 95% limits of agreement of –3.76 to 3.91 L/min. Conclusions Changes in cardiac output during TTM may be evaluated with Doppler echocardiography with little mean bias compared to changes in CO measured with thermodilution, but relatively large changes are needed in the individual patient before it can be considered as real. Abstract Figure. Comparing Doppler vs. thermodilution

Transthoracic echocardiographic assessment of cardiac output in mechanically ventilated critically ill patients by intensive care unit physicians

Background and objectivesTransthoracic echocardiography may potentially be useful to obtain a prompt, accurate and non-invasive estimation of cardiac output. We evaluated whether non-cardiologist intensivists may obtain accurate and reproducible cardiac output determination in hemodynamically unstable mechanically ventilated patients. MethodsWe studied 25 hemodynamically unstable mechanically ventilated intensive care unit patients with a pulmonary artery catheter in place. Cardiac output was calculated using the pulsed Doppler transthoracic echocardiography technique applied to the left ventricular outflow tract in apical 5 chamber view by two intensive care unit physicians who had received a basic Transthoracic Echocardiography training plus a specific training focused on Doppler, left ventricular outflow tract and velocity-time integral determination. ResultsCardiac output assessment by transthoracic echocardiography was feasible in 20 out of 25 enrolled patients (80%) and showed an excellent inter-operator reproducibility (Pearson correlation test r=0.987; Cohen's K=0.840). Overall, the mean bias was 0.03L.min−1, with limits of agreement −0.52 and +0.57L.min−1. The concordance correlation coefficient (ρc) was 0.986 (95% IC 0.966–0.995) and 0.995 (95% IC 0.986–0.998) for physician 1 and 2, respectively. The value of accuracy (Cb) of COTTE measurement was 0.999 for both observers. The value of precision (ρ) of COTTE measurement was 0.986 and 0.995 for observer 1 and 2, respectively. ConclusionsA specific training focused on Doppler and VTI determination added to the standard basic transthoracic echocardiography training allowed non-cardiologist intensive care unit physicians to achieve a quick, reproducible and accurate snapshot cardiac output assessment in the majority of mechanically ventilated intensive care unit patients.

Assessment of fluid responsiveness: recent advances.

In the field of prediction of fluid responsiveness, the most recent studies have focused on validating new tests, on clarifying the limitations of older ones, and better defining their modalities. The limitations of pulse pressure/stroke volume variations are numerous, but recent efforts have been made to overcome these limitations, like in case of low tidal volume ventilation. Following pulse pressure/stroke volume variations, new tests have emerged which assess preload responsiveness by challenging cardiac preload through heart-lung interactions, like during recruitment manoeuvres and end-expiratory/inspiratory occlusions. Given the risk of fluid overload that is inherent to the 'classical' fluid challenge, a 'mini' fluid challenge, made of 100 ml of fluid only, has been developed and investigated in recent studies. The reliability of the passive leg raising test is now well established and the newest publications have mainly aimed at defining several noninvasive estimates of cardiac output that can be monitored to assess its effects. Research in this field is still very active, such that several indices and tests of fluid responsiveness are now available. They may contribute to reduce excessive fluid balance by avoiding unnecessary fluid administration and, also, by ensuring safe fluid removal.

Three-dimensional transoesophageal echocardiography for cardiac output in critically ill patients: A pilot study of ultrasound versus the thermodilution method

Three-dimensional transoesophageal echocardiography (3D-TOE) is a new noninvasive tool for quantitative assessment of left ventricular (LV) volumes and ejection fraction. The objective of this pilot study was to evaluate the feasibility and accuracy of 3D-TOE for the estimation of cardiac output (CO), using transpulmonary thermodilution with the Pulse index Contour Continuous Cardiac Output (PiCCO) system as the reference method, in intensive care unit (ICU) patients. Fifteen ICU patients on mechanical ventilation prospectively underwent PiCCO catheter implantation and 3D-TOE. 3D-TOE LV end-diastolic and end-systolic volumes were determined using semi-automated software. CO was calculated as the product of LV stroke volume (end-diastolic volume-end-systolic volume) multiplied by heart rate. CO was also determined invasively by transpulmonary thermodilution as the reference method. Among 30 haemodynamic evaluations, 29 (97%) LV 3D-TOE datasets were suitable for CO calculation. The mean 3D-TOE image acquisition and post-processing times were 46 and 155seconds, respectively. There was a correlation (r=0.78; P<0.0001) between PiCCO and 3D-TOE CO. Compared with PiCCO, the 3D-TOE CO mean bias was 0.38L/min, with limits of agreement of -1.97 to 2.74L/min. Noninvasive estimation of CO by 3D-TOE is feasible in ICU patients. This new semi-automated modality is an additional promising tool for noninvasive haemodynamic assessment of ICU patients. However, the wide limits of agreement with thermodilution observed in this pilot study require further investigation in larger cohorts of patients.

Velocity Time Integral and Computed Tomography Measured Pulmonary Artery Diameter for the Non-Invasive Estimation of Cardiac Output: A Pilot Study

Velocity Time Integral and Computed Tomography Measured Pulmonary Artery Diameter for the Non-Invasive Estimation of Cardiac Output: A Pilot Study

Noninvasive Cardiac Output Estimation by Inert Gas Rebreathing in Mechanically Ventilated Pediatric Patients

To assess the feasibility and accuracy of inert gas rebreathing (IGR) pulmonary blood flow (Qp) estimation in mechanically ventilated pediatric patients, potentially providing real-time noninvasive estimates of cardiac output. In mechanically ventilated patients in the pediatric catheterization laboratory, we compared IGR Qp with Qp estimates based upon the Fick equation using measured oxygen consumption (VO2) (FickTrue); for context, we compared FickTrue with a standard clinical short-cut, replacing measured with assumed VO2 in the Fick equation (FickLaFarge, FickLundell, FickSeckeler). IGR Qp and breath-by-breath VO2 were measured using the Innocor device. Sampled pulmonary arterial and venous saturations and hemoglobin concentration were used for Fick calculations. Qp estimates were compared using Bland-Altman agreement and Spearman correlation. The final analysis included 18 patients aged 4-23 years with weight >15 kg. Compared with the reference FickTrue, IGR Qp estimates correlated best and had the least systematic bias and narrowest 95% limits of agreement (results presented as mean bias ±95% limits of agreement): IGR -0.2 ± 1.1 L/min, r = 0.90; FickLaFarge +0.7 ± 2.2 L/min, r = 0.80; FickLundell +1.6 ± 2.9 L/min, r = 0.83; FickSeckeler +0.8 ± 2.5 L/min, r = 0.83. IGR estimation of Qp is feasible in mechanically ventilated patients weighing >15 kg, and agreement with FickTrue Qp estimates is better for IGR than for other Fick Qp estimates commonly used in pediatric catheterization. IGR is an attractive option for bedside monitoring of Qp in mechanically ventilated children.

Improved Accuracy of Automated Estimation of Cardiac Output Using Circulation Time in Patients with Heart Failure

BackgroundLung-to-finger circulation time of oxygenated blood during nocturnal periodic breathing in heart failure patients measured using polysomnography correlates negatively with cardiac function but possesses limited accuracy for cardiac output (CO) estimation. Methods and ResultsCO was recalculated from lung-to-finger circulation time using a multivariable linear model with information on age and average overnight heart rate in 25 patients who underwent evaluation of heart failure. The multivariable model decreased the percentage error to 22.3% relative to invasive CO measured during cardiac catheterization. ConclusionsThis improved automated noninvasive CO estimation using multiple variables meets a recently proposed performance criterion for clinical acceptability of noninvasive CO estimation, and compares very favorably with other available methods.

Noninvasive cardiac output estimation by inert gas rebreathing in pediatric and congenital heart disease

Inert gas rebreathing (IGR) techniques provide rapid, reliable estimates of cardiac output in adults with structurally normal hearts. Data on IGR reliability in pediatric and congenital heart disease populations are lacking. Our objective was to validate pulmonary blood flow (Qp) measurement by IGR compared with clinical reference tests, cardiovascular magnetic resonance (CMR), and indirect Fick. Pulmonary blood flow was measured by IGR and CMR or indirect Fick in 80 patients grouped by presence and type of shunt lesion. Inert gas rebreathing precision was assessed using Bland-Altman analysis, repeatability coefficient, intraclass correlation, and coefficient of error. Agreement with the reference tests was assessed with Bland-Altman plots. For comparison, agreement between the 2 reference tests, CMR and indirect Fick, was assessed in 34 contemporary patients. Subjects were aged 7-78 years and had a wide range of cardiac diagnoses. Inert gas rebreathing Qp showed good repeatability (95% limits of agreement for 2 trials = ±22%, repeatability coefficient = 1.2 L/min, intraclass correlation = 0.92, and coefficient of error = 5%). In the absence of left-to-right shunting (n = 67), IGR Qp estimates agreed with CMR and indirect Fick Qp estimates, and the reference tests agreed with each other, with mean bias ≤10% (≤0.5 L/min) and 95% limits of agreement ±33%-38%. Conversely, IGR was unreliable in patients with left-to-right shunt (n = 14), with large bias (-58%, -4.0 L/min) and wide limits of agreement (±76%). Inert gas rebreathing reliably estimates Qp in children and adults with congenital heart disease in the absence of left-to-right shunting, with agreement comparable to that seen between CMR and indirect Fick estimates.

Measuring the cardiac output in acute emergency admissions: use of the non-invasive ultrasonic cardiac output monitor (USCOM) with determination of the learning curve and inter-rater reliability.

Traditionally, assessment of the cardiac output has been limited to theatre or the intensive care unit. However, non-invasive cardiac output estimation is now readily available, and its application may have wider benefit in the emergency setting. The non-invasive ultrasonic cardiac output monitor (USCOM) was investigated to determine its learning curve and inter-rater reliability. Four trainee operators each performed stroke volume measurements on 25 volunteers, compared to an experienced operator pre- and post-passive leg raise. Inter-rater reliability was then assessed on 24 acute emergency in-patients. Mean percentage difference in stroke volume decreased from 19% (95% confidence intervals 14-23) across volunteers 1-5, to 6% (4-8) for the last 5 volunteers scanned. Consequently, on acute emergency in-patients, excellent inter-rater reliability (Lin's concordance correlation coefficient (ρc) 0.96 (0.92-0.98)) and agreement of a change ≥10% in stroke volume following passive leg raise on 23/24 cases were found. Following a training period of less than 5 h, USCOM stroke volume measurements demonstrated excellent inter-rater reliability.

Noninvasive photoacoustic measurement of the composite indicator dilution curve for cardiac output estimation.

Recently, the measurement of indicator dilution curves using a photoacoustic (PA) technology was reported, which showed promising results on the noninvasive estimation of cardiac output (CO) that is an important hemodynamic parameter useful in various clinical situations. However, in clinical practice, measuring PA indicator dilution curves from an arterial blood vessel requires an ultrasound transducer array capable of focusing on the targeted artery. This causes several challenges on the clinical translation of the PA indicator dilution method, such as high sensor cost and complexity. In this paper, we theoretically derived that a composite PA indicator dilution curve simultaneously measured from both arterial and venous blood vessels can be used to estimate CO correctly. The ex-vivo and in-vivo experimental results with a flat ultrasound transducer verified the developed theory. We believe this new concept would overcome the main challenges on the clinical translation of the noninvasive PA indicator dilution technology.

The significance of non invasive cardiac output monitoring system on evaluating circulatory and respiratory function in pig with acute respiratory distress syndrome

To investigate the effect of non invasive cardiac output monitoring (NICO) system in pig model with acute respiratory distress syndrome (ARDS), and to provide experimental basis for clinical application. Eleven anaesthetized and ventilated ARDS male pig models were induced by intravenously infusing 0.2 mL/kg oleic acid. Lung recruitment was condocted by pressure control ventilation on pigs with ARDS. The optimal positive end-expiratory pressure (PEEP) was determined by optimal dead space fraction [the ratio of dead space to tidal volume (VD/VT)]. Cardiac output (CO) was determined by NICO, the respiratory function was monitored, and the VD/VT, dynamic compliance (Cdyn), oxygenation index (PaO₂/FiO₂), the volume of alveolar ventilation (Valv) and arterial blood oxygen saturation (SaO₂) were recorded before infusing oleic acid, after stabilization of ARDS model and at optimal PEEP level, and the intrapulmonary shunt fraction (Qs/Qt) was calculated. CO was also determined by application of pulse indicated continuous cardiac output (PiCCO), and the linear regression analysis between CO determined by NICO and CO determined by PiCCO was conducted. Seven experimental ARDS pigs model were successfully established. The optimal PEEP identified by the lowest VD/VT method was (15.71 ± 1.80) cmH₂O (1 cmH₂O = 0.098 kPa). Compared with before infusing oleic acid, VD/VT and Qs/Qt after stabilization of ARDS model were significantly increased [VD/VT: (72.29 ± 8.58)% vs. (56.00 ± 11.06)%, Qs/Qt: (21.04 ± 15.05)% vs. (2.00 ± 1.32)%, both P<0.05], and SaO₂and Valv were significantly decreased [SaO₂: 0.888 ± 0.108 vs. 0.999 ± 0.053, Valv (mL): 92.06 ± 35.22 vs. 146.11 ± 45.43, both P<0.05]. VD/VT, Qs/Qt, SaO₂and Cdyn at optimal PEEP level were improved to the levels before infusing oleic acid [(61.07 ± 9.30)%, (3.21 ± 6.10)%, 0.989 ± 0.025, (117.14 ± 41.14)mL]. Cdyn and PaO₂/FiO₂after stabilization of ARDS model were significantly lowered compared with those before infusing oleic acid [Cdyn (mL/cmH₂O): 14.43 ± 5.50 vs. 38.14 ± 6.72, PaO₂/FiO₂(mmHg, 1 mmHg = 0.133 kPa): 78.71 ± 23.22 vs. 564.37 ± 158.85, both P < 0.05]. Cdyn and PaO₂/FiO₂at optimal PEEP level [(19.71 ± 4.86)%, (375.49 ± 141.30) mmHg] were elevated compared with the levels after stabilization of ARDS model (both P <0 .05), but still lower than those before infusing oleic acid (both P < 0.05). Compared with the levels after stabilization of ARDS model, CO at optimal PEEP level showed obvious decrease from (4.18 ± 2.46) L/min to (3.95 ± 2.69) L/min without significant difference (P > 0.05). There was linear correlation between CO determined by NICO and CO determined by PiCCO (r²=0.925, P < 0.001). NICO technique provides a useful and accurate non invasive estimation of CO and respiratory function.VD/VT provided by NICO can titrate the optimal PEEP in patients with ARDS.

First in vivo application and evaluation of a novel method for non-invasive estimation of cardiac output

Surgical or critically ill patients often require continuous assessment of cardiac output (CO) for diagnostic purposes or for guiding therapeutic interventions. A new method of non-invasive CO estimation has been recently developed, which is based on pressure wave analysis. However, its validity has been examined only in silico. Aim of this study was to evaluate in vivo the reproducibility and accuracy of the “systolic volume balance” method (SVB). Twenty two subjects underwent 2-D transthoracic echocardiography for CO measurement (reference value of CO). The application of SVB method required aortic pressure wave analysis and estimation of total arterial compliance. Aortic pulses were derived by mathematical transformation of radial pressure waves recorded by applanation tonometry. Total compliance was estimated by the “pulse pressure” method. The agreement, association, variability, bias and precision between Doppler and SVB measures of CO were evaluated by intraclass correlation coefficient (ICC), mean difference, SD of differences, percentage error (PR) and Bland–Altman analysis. SVB yielded very reproducible CO estimates (ICC=0.84, mean difference 0.27±0.73L/min, PR=16.7%). SVB-derived CO was comparable with Doppler measurements, indicating a good agreement and accuracy (ICC=0.74, mean difference=−0.22±0.364L/min, PR≈15). The basic mathematical and physical principles of the SVB method provide highly reproducible and accurate estimates of CO compared with echocardiography.

Blood pressure, vascular resistance, and +Gz tolerance during repeated +Gz exposures.

Cardiovascular reflexes that regulate blood pressure (BP) adapt during repeated exposure to +Gz acceleration separated by short (< 20 s) breaks, but whether this effect is preserved with longer intervals remains unknown. There were 17 subjects who completed 5 repeated gradual onset (0.1 G x s(-1)) runs (GOR1-5) to 60 degrees peripheral light loss (PLL) on a human centrifuge, separated by 2 min. Heart rate (HR) and BP were measured before and during each GOR and noninvasive estimation of cardiac output (CO) was used to calculate total peripheral resistance (TPR). Mean resting (+/- SE) systolic BP (138 +/- 4 vs. 128 +/- 3 mmHg) and TPR (13.9 +/- 1.2 vs. 12.7 +/- 1.0 mmHg x L(-1) x min(-1)) were elevated after GOR1 and remained elevated thereafter. Compared with that before GOR1, resting HR was decreased (5-6 bpm) before GOR2-5. Resting CO decreased from 8.3 +/- 0.6 L x min(-1) before GOR1 to a nadir of 7.2 +/- 0.5 L x min(-1) before GOR4 and GOR5. The change in HR under increased +Gz decreased from +39 +/- 3 bpm during GOR1 to +31 +/- 3 bpm during GOR4 and GOR5, but the decrease in eye level BP under +Gz was unaffected. RGT did not change across the five GORs. Increased resting BP and TPR following a single +Gz exposure suggest alterations in the cardiovascular system expected to confer improved tolerance during subsequent exposures. However, these changes were insufficient to improve +Gz tolerance measured during repeated GORs separated by 2 min.

Echocardiographic validation of a novel method for noninvasive estimation of cardiac output based on pulse contour analysis

Echocardiographic validation of a novel method for noninvasive estimation of cardiac output based on pulse contour analysis

Non-invasive estimates of cardiac output: Comparison of a novel pulse waveform analysis method with inert gas re-breathing

Non-invasive estimates of cardiac output: Comparison of a novel pulse waveform analysis method with inert gas re-breathing

The “systolic volume balance” method for the non-invasive estimation of cardiac output based on pressure wave analysis

The “systolic volume balance” method for the non-invasive estimation of cardiac output based on pressure wave analysis

The “systolic volume balance” method for the noninvasive estimation of cardiac output based on pressure wave analysis

Cardiac output (CO) monitoring is essential for the optimal management of critically ill patients. Several mathematical methods have been proposed for CO estimation based on pressure waveform analysis. Most of them depend on invasive recording of blood pressure and require repeated calibrations, and they suffer from decreased accuracy under specific conditions. A new systolic volume balance (SVB) method, including a simpler empirical form (eSVB), was derived from basic physical principles that govern blood flow and, in particular, a volume balance approach for the conservation of mass ejected into and flowed out of the arterial system during systole. The formulas were validated by a one-dimensional model of the systemic arterial tree. Comparisons of CO estimates between the proposed and previous methods were performed in terms of agreement and accuracy using "real" CO values of the model as a reference. Five hundred and seven different hemodynamic cases were simulated by altering cardiac period, arterial compliance, and resistance. CO could be accurately estimated by the SVB method as follows: CO = C × PP(ao)/(T - P(sm) × T(s)/P(m)) and by the eSVB method as follows: CO = k × C × PP(ao)/T, where C is arterial compliance, PP(ao) is aortic pulse pressure, T is cardiac period, P(sm) is mean systolic pressure, T(s) is systolic duration, P(m) is mean pressure, and k is an empirical coefficient. SVB applied on aortic pressure waves did not require calibration or empirical correction for CO estimation. An empirical coefficient was necessary for brachial pressure wave analysis. The difference of SVB-derived CO from model CO (for brachial waves) was 0.042 ± 0.341 l/min, and the limits of agreement were -0.7 to 0.6 l/min, indicating high accuracy. The intraclass correlation coefficient and root mean square error between estimated and "real" CO were 0.861 and 0.041 l/min, respectively, indicating very good accuracy. eSVB also provided accurate estimation of CO. An in vivo validation study of the proposed methods remains to be conducted.

Non-invasive Estimation of Cardiac Output in Mechanically Ventilated Patients: A Prolonged Expiration Method

A non-invasive method for the estimation of cardiac output in mechanically ventilated patients is described. The method is based on prolonged expiration, and relies on measurement of gas concentrations and flow rate. A pneumatic system, with an ad hoc designed orifice resistance, has been made and experimentally characterized to adapt the breathing circuit to this application. Cardiac output is calculated using two algorithms and the results are compared with the ones obtained by thermodilution. To this purpose, we prospectively enrolled twenty mechanically ventilated patients, who had undergone cardiac surgery, and both algorithms show good correlation with thermodilution (R > 0.8). The application of the first algorithm gave mean cardiac output values slightly lower than those obtained by thermodilution (-6%), while the application of the second algorithm gave higher values (+30%). Difference standard deviations between paired measurements is 0.72 L min(-1) for the first algorithm and 1.07 L min(-1) for the second one. Standard deviation obtained by the application of the first algorithm is slightly lower than those relative to other minimally invasive techniques. Through prolonged expiration, and standardization and automation of the procedure on mechanically ventilated patients, the proposed system allows to obtain a non-invasive estimation of cardiac output.