Review article: Non‐invasive assessment of cardiac output with portable continuous‐wave Doppler ultrasound

Cardiac output is considered an important parameter when assessing the cardiovascular status of a critically ill patient. Both non‐invasive (e.g. bioimpedance, echocardiography) and invasive methods (Swan Ganz catheter) have been used to measure cardiac output. The ultrasonic cardiac output monitoring device provides a new method of non‐invasively assessing cardiac output in various clinical settings. The ultrasonic cardiac output monitoring device was introduced clinically in 2001, and appears to be a promising adjunct in the assessment of the cardiovascular state in a variety of patient cohorts. In this short review article, we will introduce this new technique, discuss the required skills and compare it with methods already in use. In particular, a critical comparison with the ‘gold standard’, the invasive measurement of cardiac output with the pulmonary artery catheter, will be given.     

Continuous measurement of cardiac output with the use of stochastic system identification techniques

The limitations of developing a technique to measure cardiac output continuously are given. Logical explanations are provided for the economic, technical, and physiologic benefits of a stochastic system identification technique for measuring cardiac output. Heat is supplied by a cathetermounted filament driven according to a pseudorandom binarsequence. Volumetric fluid flow is derived by a crosscorrelation algorithm written in the C language. In vitro validation is performed with water in a flow bench. The computed flow (y) compared with the in-line-measured flow (x) yields the linear regression y = 1.024x-0.157 (r = 0.99). The average coefficient of variation is less than 2% over a volumetric fluid flow range of 2 to 10 L/min.     

Thermodilution cardiac output measurement with a large left-to-right shunt

Cardiac output was measured by the thermodilution method in a patient with a left-to-right shunt undergoing cardiac catheterization. It appeared that the thermodilution method measured systemic rather than pulmonary blood flow. This occurred because of the slow injection of injectate in the presence of a large left-to-right shunt. Theoretical thermodilution cardiac output curves are provided to illustrate the interaction of these two factors when three different durations of injection and four different shunt sizes are used.     

Evaluation of heavy water for indicator dilution cardiac output measurement

We evaluated deuterium oxide (D₂O) as a tracer for cardiac output measurements. Cardiac output measurements made by thermodilution were compared with those made by indicator dilution with D₂O and indocyanine green as tracers. Five triplicate measurements for each method were made at intervals of 30 minutes in each of 9 anesthetized, mechanically ventilated goats. Cardiac output ranged between 0.68 and 3.79 L/min. The 45 data points yielded a correlation coefficient of 0.948 for the comparison of D₂O indicator dilution cardiac output measurements with thermodilution measurements and a linear regression slope of 1.046. D₂O indicator dilution measurements were biased by –0.11±0.22 L/min compared with thermodilution measurements and had a standard deviation of ±0.12 L/min for triplicate measurements. Hematocrits ranging between 20 and 50 vol% had no effect on optical density for D₂O. D₂O is more stable than indocyanine green and approximately one-tenth the price (40 cents per injection compared with $4). The basic instrumentation cost of approximately $9,000 is an additional initial expense, but provides the ability to perform pulmonary extravascular water measurements with a double-indicator dilution technique. D₂O has potential as a tracer for the clinical determination of indicator dilution cardiac output measurements and pulmonary extravascular water measurements.This study was supported by a U.S. Veterans Administration Merit Review Grant (103). Dr Schreiner is a recipient of an American Society of Anesthesiologists starter grant (1985–86). Dr Leksell is on leave from the Dept of Anesthesia, Karolinska Hospital, Stockholm, Sweden, and is supported by grants from the Swedish Medical Research Council, Karolinska Institute, Sandoz AB, and the Swedish Medical Association.     

Non-invasive cardiac output monitoring by aortic blood flow measurement with the Dynemo 3000

The operating principles and methods for the continuous determination of aortic blood flow (ABF) with the Dynemo 3000 system are described in detail. The system uses a novel transesophageal ultrasonic Echo-Doppler probe simultaneously to measure aortic diameter and blood flow velocity at the same anatomic level, in real-time. Non-invasive ABF measurement is combined with vital sign data from standard monitors to provide a composite hemodynamicprofile including volume, after load and contractility data used by the physician to optimize therapy. A review of the clinical validation and comparison to thermo dilution measurements showing a significant positive correlation over a wide range of clinical flow situations is also briefly presented. This revised version was published online in July 2006 with corrections to the Cover Date.     

Semi-continuous versus injectate cardiac output measurement in intensive care patients after cardiac surgery

Objective Commercially available semi-continuous cardiac output (SCCO) monitoring systems are based on the pulsed warm thermodilution technique. There is evidence that SCCO fails to correlate with standard intermittent bolus cardiac output (ICO) in clinical situations with thermal instability in the pulmonary artery. Furthermore, ventilation may potentially influence thermodilution measurements by enhanced respiratory variations in pulmonary artery blood temperature and by cyclic changes in venous return. Therefore, we evaluated the correlation, accuracy and precision of SCCO versus ICO measurements before and after extubation.Design Prospective cohort study.Setting Intensive care unit (ICU) of a university hospital.Patients and participants 22 cardiac surgical ICU patients.Interventions None.Measurements and results SCCO and ICO data were obtained at nine postoperative time points while the patients were on controlled mechanical ventilation. Further sets of measurements were taken during the weaning phase 20 min before extubation, and 5 min, 20 min and 1 h after extubation. SCCO and ICO measurements yielded 286 data pairs with a range of 1.8–9.9 l/min for SCCO and 1.9–9.8 l/min for ICO. The correlation between SCCO and ICO was highly significant (r=0.92;p<0.01), accompanied by a bias of –0.052 l/min and a precision of 0.56 l/min. Correlation, accuracy and precision were not influenced by the mode of respiration.Conclusions Our results demonstrate excellent correlation, accuracy and precision between SCCO and ICO measurements in postoperative cardiac surgical ICU patients. We conclude that SCCO monitoring offers a reliable clinical method of cardiac ouput monitoring in ICU patients following cardiac surgery.     

Peak cardiac power output in healthy,trained men

Previous investigations into peak cardiac power output (CPO peak) have been limited to clinical populations and healthy, but non‐athletic adults, and normative data on trained individuals would allow a greater understanding of this parameter. Therefore, we recruited eight healthy, well‐trained male cyclists. Peak oxygen consumption ( peak) was assessed using an incremental ergometer test, and following a 40‐min recovery period, peak cardiac output (T peak) was measured during a constant load test that elicited peak (±5%) using the Defares CO₂ rebreathing technique. CPO peak was calculated as described by Cooke et al. (1998) . Mean (±SD) values during the constant load test were: peak, 4·94 ± 0·41 l min^?1; T peak, 36·5 ± 3·7 l min^?1; mean arterial pressure, 123 ± 8 mmHg and CPO peak, 9·9 ± 1·0 W. These results demonstrate CPO peak in a well‐trained population to be approximately twice those observed in healthy, but non‐athletic adults. The current data provide useful information regarding the upper limits and possible ‘trainability’ of cardiac pumping capacity for sedentary and clinically compromised individuals.     

Validation of non‐invasive haemodynamic methods in patients with liver disease: the Finometer and the Task Force Monitor

Patients with advanced cirrhosis often present a hyperdynamic circulation characterized by a decrease in systolic and diastolic blood pressure (SBP and DBP), and an increase in heart rate (HR) and cardiac output (CO). Accurate assessment of the altered circulation can be performed invasively; however, due to the disadvantages of this approach, non‐invasive methods are warranted. The purpose of this study was to compare continuous non‐invasive measurements of haemodynamic variables by the Finometer and the Task Force Monitor with simultaneous invasive measurements. In 25 patients with cirrhosis, SBP, DBP and HR were measured non‐invasively and by femoral artery catheterization. CO was measured non‐invasively and by indicator dilution technique. The non‐invasive pressure monitoring was considered acceptable with a bias (accuracy) and a SD (precision) not exceeding 5 and 8 mmHg, respectively, as recommended by the Association for the Advancement of Medical Instrumentation. The accuracy and precision of the Finometer and the Task Force Monitor were as follows: SBP: ?3·6 ± 17·9 and ?8·9 ± 17·5 mmHg, respectively; DBP: 4·2 ± 9·6 and 1·9 ± 8·6 mmHg, respectively; HR: 2·0 ± 6·9 and 2·2 ± 6·2 bpm, respectively; and CO: 0·1 ± 1·6 and ?1·0 ± 2·0 L min^?1, respectively. The study demonstrates that the overall performances of the Finometer and the Task Force Monitor in estimating absolute values of SBP, DBP, HR and CO in patients with cirrhosis are not equivalent to the gold standard, but may have an acceptable performance with repeated measurements.     

Intraoperative validation of a new system for invasive continuous cardiac output measurement

Objective  Although bolus thermodilution technique for cardiac output (CO) measurement has widespread acceptance, new systems are currently available. We evaluated a continuous CO system (TruCCOMS, Aortech International Inc.) that operates on the thermal conservation principle and we compared it with the reference standard transit time flow measurement (TTFM). Materials and methods  Nine consecutive cardiac surgery patients were evaluated. After general anesthesia and intubation, a TruCCOMS catheter was percutaneously placed in the pulmonary artery (PA). After median sternotomy and pericardiotomy, a TTFM probe was placed around the main PA. Right ventricular (RV) CO measurements were recorded with both TruCCOMS and TTFM at different times: before cardiopulmonary bypass (CPB) (T0), during weaning from CPB (T1), and prior to sternal closure (T2). Data analysis included paired student t test, Pearson correlation test, and Bland–Altman plotting. Results  TruCCOMS CO values were significantly lower at T0 (TruCCOMS 4.0 ± 1.0 vs. TTFM 4.5 ± 1.0 L/min; P < 0.0001) and T1 (TruCCOMS 3.6 ± 0.5 vs. TTFM 4.2 ± 0.7 L/min; P < 0.0001), and comparable at T2 (TruCCOMS 4.5 ± 0.7 vs. TTFM 4.6 ± 0.8 L/min; P = 0.4). Pearson test showed a significant correlation between TruCCOMS and TTFM CO measurements (RT0 = 0.9, RT1 = 0.8, RT2 = 0.6; P < 0.0001). Bland–Altmann plotting showed a bias of −0.53 ± 0.43 L (−12%) at T0, −0.64 ± 0.43 L (−14.5%) at T1, and −0.1 ± 0.66 L (−0.8%) at T2. Conclusion  Although TruCCOMS may significantly underestimate CO, measurement trends correlate with TTFM. For this reason, a negative trend in RV output should trigger more specific diagnostic procedures.     

A meta-analysis of studies using bias and precision statistics to compare cardiac output measurement techniques

Introduction. Bias and precision statistics have succeeded regression analysis when measurement techniques are compared. However, when applied to cardiac output measurements, inconsistencies occur in reporting the results of this form of analysis. Methods. A MEDLINE search was performed, dating from 1986. Studies comparing techniques of cardiac output measurement using bias and precision statistics were surveyed. An error-gram was constructed from the percentage errors in the test and reference methods and was used to determine acceptable limits of agreement between methods. Results. Twenty-five articles were found. Presentation of statistical data varied greatly. Four different statistical parameters were used to describe the agreement between measurements. The overall limits of agreement in studies evaluating bioimpedance (n = 23) was ±37% (15–82%) and in those evaluating Doppler ultrasound (n = 11) ±65% (25–225%). Objective criteria used to assess outcome were given in only 44% of the articles. These were (i) limits of agreement approaching ±15–20%, (ii) limits of agreement of less than 1 L/min, and (iii) more than 75% of bias measurements within ±20% of the mean. Graphically, we showed that limits of agreement of up to ±30% were acceptable. Conclusions. When using bias and precision statistics, cardiac output, bias, limits of agreement, and percentage error should be presented. Using current reference methods, acceptance of a new technique should rely on limits of agreement of up to ±30%.     

Near continuous cardiac output by thermodilution

A new thermodilution method for frequent (near continuous) estimation ofcardiac output, without manual injection of fluid into the blood, was tested.The method utilizes a pulmonary artery catheter equipped with a fluid filledheat exchanger. The technique is based on cyclic cooling of the blood in theright atrium and measurement of the temperature changes in the pulmonaryartery. Using this technique, a new estimate of cardiac output can be obtainedevery 32 s. Cardiac output estimates, obtained for a running mean of threemeasurements with this method, were compared to the mean of three conventionalthermodilution measurements. The measurements were obtained during shortperiods of stable respiration and circulation.In six pigs, we made 46 paired measurements of conventional thermodilution(TD) and near continous (TDc) thermodilution. The cardiac output(CO TD) ranged from 2.4–13.7 l/min (mean 5.4 l/min). Thebest linear fit through the paired data points was CO TDc =–0.57 + 1.01 CO TD. The mean difference between themethods was –0.50 l/min (S.D. = 0.39). The mean coefficient of variationof repeated measurements with the near continuous thermodilution was3.6%.Considering changes of more than 0.25 l/min to be significant, all changes incardiac output measured by conventional thermodilution were followed by therunning mean of three near continuous thermodilution estimates.This study demonstrates the feasibility of the new method to monitorcardiac output, and to detect all changes greater than 0.25 l/min.     

米力农改善心脏手术后心力衰竭所致低心排血量综合征患者的肾功能

目的评价正性肌力药米力农在治疗体外循环(CPB)心脏手术后心力衰竭所致低心排血量综合征时对患者肾脏的影响。方法选取2018年1月至2020年6月间在我院心胸外科治疗的CPB心脏手术患者,在患者心脏停跳后30 min(基线期)和60 min(治疗后)进行肾脏和全身血流动力学检测。CPB术后30 min发生低心排血量综合征患者接受米力农治疗,根据患者CPB心脏手术后是否应用米力农,将患者分为米力农组(n=59)和对照组(n=82)。比较两组患者血流动力学指标和肾血流动力学指标基线期值、治疗后值、治疗后较基线期变化值。比较治疗后两组患者预后指标及并发症发生率。结果治疗后米力农组患者心脏指数变化值[(0.55±0.26) L/(min·m²) vs.(-0.35±0.28) L/(min·m²),t=19.394,P <0.001]、心搏容量指数变化值(t=8.776,P <0.001)、氧释放系数变化值(t=8.143,P <0.001)、混合静脉血氧饱和度变化值(t=9.935,P <0.001)与对照组比较显著提高,周身血管阻力指数变化值(t=10.574,P <0.001)、肺血管阻力指数变化值(t=10.654,P <0.001)与对照组比较显著降低。治疗后米力农组患者肾血流量变化值[(117.30±153.82) m L/min vs.(-63.73±157.64) m L/min,t=6.795,P <0.001]、肾脏供氧量变化值(t=4.248,P <0.001)与对照组比较显著提高,肾小球滤过分数变化值(t=6.382,P <0.001)、肾血管阻力变化值[(-0.06±0.05) mm Hg/(m L·min) vs.(0.03±0.06) mm Hg/(m L·min),t=9.407,P <0.001]和肾氧摄取率变化值(t=7.625,P <0.001)与对照组比较显著降低。结论米力农在心脏手术后早期用于治疗急性心力衰竭所致低心排血量综合征,可以增加患者心输出量和肾血流量,扩张肾血管。米力农可以改善患者易感肾脏的氧合作用,但不会引起肾小球滤过率的显著变化。     

Frequently repeated fick cardiac output measurements during anesthesia

A computer-based system was developed for monitoring cardiac output using the Fick principle during general anesthesia. The variables of the oxygen-consumption Fick equation were measured using the following system: oxygen uptake by an originally developed respiratory gas monitoring system, arteriovenous oxygen saturation difference by pulse and fiberoptic oximetry, and hemoglobin concentration by an in vitro oximeter. Fick cardiac output and systemic vascular resistance were calculated every 30 seconds. Fick cardiac output was compared with thermodilution cardiac output in 11 anesthetized patients. A total of 208 corresponding cardiac output measurements showed a range of 2 to 9 L · min^-1. The correlation coefficient between the thermodilution and Fick cardiac outputs was 0.961, with a regression equation of Fick cardiac output = 1.058 thermodilution cardiac output 0.359. The difference between the thermodilution and Fick cardiac outputs was 0.103 ± 0.395. The Fick cardiac output was significantly lower than the thermodilution cardiac output, especially in the low flow range. We demonstrated that this new monitoring system was clinically feasible and sufficiently accurate, under the limited circumstances of our study. The integration of routinely used equipment has made possible a frequently repcatable method for estimating cardiac output in patients.     

脉搏指示连续心排血量监测在感染性休克早期液体复苏中的应用

目的 探讨脉搏指示连续心排血量(PICCO)监测技术在感染性休克患者液体复苏中的应用价值.方法 2010年1月至2011年12月58例感染性休克患者根据治疗过程中是否应用PICCO监测技术将患者分为PICCO组(28例)和对照组(30例),对比分析两组患者治疗后早期目标导向治疗的液体复苏(EGDT)达标率、乳酸水平、中心静脉压(CVP)、氧合指数,72 h内液体入量、液体平衡、ICU内呼吸机应用时间、ICU住院时间、ICU内72 h后多脏器功能不全(MODS)发生率、28 d病死率.结果 (1)与对照组比较,PICCO组72 h内总的液体入量[(9565±1623) ml与(12 245 ±2253)ml,t=2.673,P=0.021]及正平衡[(3656±1904) ml与(5465±2765) ml,t=2.357,P=0.012]较对照组明显减少.(2)PICCO组72 h氧合指数较对照组明显增高(252.6±87.4与226.8±69.4,P＜0.05),呼吸机应用时间较对照组明显缩短[(134.7±42.8)h与(193.3±92.4)h,t=1.356,P=0.023].(3)两组在相同时间段乳酸水平、CVP值、6 hEGDT达标率、ICU住院时间、72 h后ICU内MODS发生率、28 d病死率方面比较差异均无统计学意义(P均＞0.05).结论 与CVP指导的常规液体复苏相比,PICCO监测技术可以更准确地对感染性休克患者进行容量管理,指导早期液体复苏.     

Partial CO2 rebreathing indirect Fick technique for non-invasive measurement of cardiac output

Objective.Evaluation in animals of a non-invasive and continuous cardiac output monitoring system based on partial carbon-dioxide (CO₂) rebreathing indirect Fick technique. Methods.We have developed a non-invasive cardiac output (NICO) monitoring system, based on the partial rebreathing method. The partial rebreathing technique employs a differential form of the Fick equation for calculating cardiac output (Q_T) using non-invasive measurements. Changes in CO₂ elimination (ΔVCO₂) and partial pressure of end-tidal CO₂ (Δ PETCO₂) in response to a brief period of partial rebreathing are used to measure pulmonary capillary blood flow (Q_PCBF). A non-invasive estimate of anatomic and intrapulmonary shunt fraction (Q_S/Q_T), based on oxygen saturation from pulse oximetry (SpO₂) and inspired oxygen concentration (FIO₂), is added to compute total cardiac output [Q_T=Q_PCBF/(1−Q_S/Q_T)]. The performance of the NICO was compared with iced 5% dextrose bolus thermodilution cardiac output (TDco) measurements in 6 dogs. Cardiac output was varied using dobutamine, and halothane, and by clamping of the inferior vena cava. Two hundred and forty-six (n = 246) paired measurements of TDco and NICO over a range of cardiac outputs (TDco range = 0.60–8.87 l/min) were compared using Bland-Altman analysis and weighted correlation coefficient. Results.The Bland–Altman technique yielded a NICO precision of ± 0.70 l/min (13.8%) with a mean bias of −0.07 l/min (−1.4%) compared to TDco. The weighted correlation coefficient between TDco and NICO values was: r= 0.93 (n= 246). Conclusion.The partial CO₂ rebreathing technique for measurement of cardiac output is non-invasive, automated, and based on the well accepted Fick principle. The limits of agreement between NICO and TDco is within the recommended value for NICO to be a clinically acceptable method for cardiac output measurement. The results of this canine study show that NICO performed as well, and in some cases better, than other currently available non-invasive cardiac output techniques over a wide range of cardiac outputs. This revised version was published online in July 2006 with corrections to the Cover Date.     

Lithium dilution cardiac output measurements using a peripheral injection site comparison with central injection technique and thermodilution

Objective. The lithium dilution technique for the measurement of cardiac output by the central injection of lithium chloride was introduced by Linton et al. in 1993. In the present report, we compare lithium dilution cardiac output measurement (LD) by the peripheral injection of lithium chloride (pLD) and by central venous injection (cLD), cardiac output determined by electromagnetic flowmetry (EM), and conventional thermodilution cardiac output measurement (TD) on ten swine. Methods. The animals were monitored with a pulmonary artery catheter, a femoral artery catheter, and an electromagnetic flowmeter placed around the ascending aorta. cLD, pLD, TD, and EM were determined at the baseline, then in a hyperdynamic state produced by dobutamine administration, at a second baseline, and finally in a hypodynamic state induced by propranolol during deep anesthesia. Data were analyzed by linear regression analysis and the comparison method described by Bland and Altman; bias and precision were calculated using the method of Sheiner and Beal. Results. The correlation coefficient between pLD and EM (0.86) was significantly less than that between cLD and EM (0.96), however it was not significantly different from that between TD and EM (0.85). The precision value of pLD (0.14) was the same as that of TD (0.14). Conclusion. The results of the present study indicate that pLD is a reliable technique.     

Pulsatile flow simulator for comparison of cardiac output measurements by electromagnetic flow meter and thermodilution

This study examined a pulsatile flow simulator for the purpose of evaluating two measurement devices, an extracorporeal flow probe with an electromagnetic flow meter and several thermodilution catheters. We measured the performance of these devices in a range of low to high flows. Using either saline or blood as a perfusate, we obtained different results with these fluids (p < 0.001). Each catheter behaved in a linear manner, although variation occurred among the catheters with both saline (minimum slope 1.090, maximum slope 1.190) and blood (minimum slope 1.107, maximum slope 1.154). An increase in rate and stroke volumes of the simulator did not demonstrate an identifiable trend in error. The thermodilution catheters were most accurate at 5.0 L/min irrespective of rate, stroke volume, or perfusate used. In contrast, the electromagnetic flow meter accurately represented flows across the wide range of outputs examined (2.4 to 10.7 L/min). (Slope with saline 1.091, slope with blood 1.080) Throughout the range of flow, the flow meter gave a calibration line 5% higher with blood than with saline. The results indicate that accurate measurement of pulsatile blood flow can be achieved in vitro with an electromagnetic flow meter using saline as a perfusate, provided a correction factor is determined and applied to convert values for saline to accurate values for blood.