Changes In Aortic Blood Flow Research Articles

Accuracy and precision of the USCOM: does a meta-analysis provide the answer?

Chong and Peyton's recent meta-analysis of bias and precision studies on the clinical performance of the Ultrasonic Cardiac Output Monitor (USCOM Ltd., Sydney, Australia) 1 provides a percentage error for the device against thermodilution cardiac output of 42.7%, which is well outside the current benchmark for clinical acceptability set at 30% 2. However, the authors' discussion of this result is notable because, rather than accepting the obvious conclusion that the USCOM is simply too unreliable to be used clinically, they provide several arguments to excuse their poor result and conclude that further studies to investigate the tracking ability of the USCOM are needed. And yet their meta-analysis data in no way leads to such a conclusion. Perhaps an explanation lies in the first paragraph of their publication where they propose the need for devices that help to optimise haemodynamics, and Bland and Altman analysis may not the best way of investigating this property 3. If one looks more deeply into the precision of the USCOM one finds that there are two main sources of error: (a) that arising from the Doppler measurement of aortic blood flow, the velocity time interval (vti); and (b) that arising from the USCOM estimation of aortic valve cross-sectional area (CSA), which is derived from an algorithm based on patient size and has nothing to do with the use of Doppler. Thus, as no two individuals are similar in shape or size, this estimation of CSA accounts for much of the error found in USCOM cardiac output readings. Although important when absolute values are required, the CSA component remains constant and plays no part in the measurement error when tracking changes in cardiac output. Thus, do we really know how precisely the USCOM measures changes in aortic blood flow from Chong and Peyton's meta-analysis? As Chong and Peyton acknowledge, percentage errors between studies varied enormously. In liver transplant patients, Su et al. found percentage errors of 15%, whilst in cardiac surgery patients, Van den Oever et al. found it to be 62% 4, 5. Furthermore, Chong and Peyton questioned the experience of the person performing the USCOM scan 6. Unlike other minimally invasive cardiac output monitoring technologies, such as bioimpedance and pulse contour analysis, the USCOM is extremely operator dependant and substandard technique can lead to low and inaccurate readings, a point that Chong and Peyton make their paper. Unfortunately, the level of skill needed to obtain reliable USCOM scans appears to be very high and 20 examinations to gain competence as suggested by some authors 7 is, in our opinion, just the starting point on a long learning curve. Furthermore, patient factors such as a low cardiac output and old age also adversely affect one's ability to perform good quality USCOM scans. Our earliest impression of using the USCOM in elderly anaesthetised patients was that good quality scans were usually difficult to obtain. However, in a younger population of obstetric patients with much higher cardiac outputs my colleagues had no such complaints about the ease and quality of scans and have made good use of the USCOM as a clinical research tool 8. Recently, we completed a study that investigated the effect of age on USCOM readings following induction of anaesthesia. The quality of each USCOM scan was assessed using the six-point Fremantle score described by Dey and Sprivulis 7, where the presence of key features in the USCOM scan are scored. A Fremantle score above 4 is considered acceptable. We found that above the age of 60 years, over half of our patients had unacceptable USCOM scans based on Fremantle criteria (Fig. 3). Could the variation between studies found by Chong and Peyton be due to differences in patient populations and varying abilities to use a device that is clearly very operator dependant? We agree with Chong and Peyton's final conclusion that the USCOM needs its ability to track cardiac output over time properly evaluated, as simple comparisons against thermodilution cardiac output using Bland and Altman analysis does not give us the answers that we need.

Noninvasive cardiac output monitoring with bioreactance as an alternative to invasive instrumentation for preclinical drug evaluation in beagles

Introduction Non-invasive measurement of cardiac output (CO) using bioreactance signals (NICOM) was recently validated in swine and human adults. The present study was designed to test the hypothesis that NICOM flow measurements can accurately measure CO and detect acute drug-induced changes in aortic blood flow (AoQ) ≥ 10% in beagles. Methods Data from 5 anesthetized, open chest beagles used for preclinical screening of novel neuromuscular blocking drugs were analyzed for the study. AoQ was measured beat-to-beat with a probe on the aortic root and averaged over 30 s intervals. NICOM CO measurements were simultaneously obtained every 30 s. NICOM precision (random variation) and accuracy relative to AoQ were assessed from individual segments of steady-state data. The ability of NICOM to detect acute alterations in AoQ ≥ 10% was determined from other segments with dynamic change. Results 516 simultaneous CO measurements between 826 and 2436 mL/min were analyzed. Steady-state measurements (20% of the dataset) revealed an average AoQ − NICOM difference (bias) of 63 ± 38 mL/min, a percent error of 6.1%, and a NICOM precision of 6.1%. Within the acute change dataset, 21/23 events reflecting a ≥ 10% change in beat-to-beat AoQ were detected by the NICOM (sensitivity of 0.91). In none of 10 instances where drug or fluid injection altered AoQ < 10% did the NICOM indicate a change (specificity of 1.0). Discussion Continuous, non-invasive measurement of CO by bioreactance provides data that satisfactorily approximates invasive measurement of aortic blood flow in beagles. In addition, despite a 30 s interval between measurements, the NICOM appears to have sufficient fidelity to detect and quantify acute, drug-induced changes in CO. These data suggest that the NICOM may represent an alternative to open chest instrumentation for CO measurement in preclinical drug evaluation studies.

Measuring aortic diameter improves accuracy of esophageal Doppler in assessing fluid responsiveness

Fluid responsiveness requires the accurate measurement of cardiac output that can be approached by aortic blood flow (ABF) as measured by esophageal Doppler monitoring (EDM). EDM devices may either include an echo-determination of aortic diameter or estimate aortic diameter from nomograms and thus consider it as constant. However, it is unclear if measuring aortic diameter increases the accuracy of EDM to identify fluid responsiveness. Aortic diameter varies with arterial pressure such that its measure could be essential for assessing the changes in ABF during acute circulatory failure. We attempted to demonstrate that measuring aortic diameter improved the accuracy of EDM to assess fluid responsiveness. Prospective study. University hospital intensive care unit. Seventy-six patients with acute circulatory failure in whom a fluid challenge was given. Rapid volume expansion (500 mL of NaCl 0.9%). We measured aortic velocity and area by EDM before and after fluid loading and evaluated the effects of fluid challenge on ABF, either measured after fluid infusion (measured ABFafter) or estimated assuming an unchanging aortic area (estimated ABFafter). If measured ABFafter was used for assessing fluid response, it was increased above 15% compared with ABF at baseline in 41 patients (responders). Conversely, estimated ABFafter increased above 15% from ABF at baseline in 27 patients only; that is, the effects of the challenge were underestimated in 14 patients. In these 14 patients, the relative change in mean arterial pressure during volume expansion was of greater magnitude than in patients who were classified as nonresponders by considering measured ABFafter. Monitoring the changes in aortic diameter improves the accuracy of EDM in assessing the hemodynamic effects of a fluid challenge, especially if it induces a large increase in arterial pressure. Estimating rather than measuring the aortic diameter may lead to underestimation of fluid responsiveness.

Passive leg raising predicts fluid responsiveness in the critically ill*

Passive leg raising (PLR) represents a "self-volume challenge" that could predict fluid response and might be useful when the respiratory variation of stroke volume cannot be used for that purpose. We hypothesized that the hemodynamic response to PLR predicts fluid responsiveness in mechanically ventilated patients. Prospective study. Medical intensive care unit of a university hospital. We investigated 71 mechanically ventilated patients considered for volume expansion. Thirty-one patients had spontaneous breathing activity and/or arrhythmias. We assessed hemodynamic status at baseline, after PLR, and after volume expansion (500 mL NaCl 0.9% infusion over 10 mins). We recorded aortic blood flow using esophageal Doppler and arterial pulse pressure. We calculated the respiratory variation of pulse pressure in patients without arrhythmias. In 37 patients (responders), aortic blood flow increased by > or =15% after fluid infusion. A PLR increase of aortic blood flow > or =10% predicted fluid responsiveness with a sensitivity of 97% and a specificity of 94%. A PLR increase of pulse pressure > or =12% predicted volume responsiveness with significantly lower sensitivity (60%) and specificity (85%). In 30 patients without arrhythmias or spontaneous breathing, a respiratory variation in pulse pressure > or =12% was of similar predictive value as was PLR increases in aortic blood flow (sensitivity of 88% and specificity of 93%). In patients with spontaneous breathing activity, the specificity of respiratory variations in pulse pressure was poor (46%). The changes in aortic blood flow induced by PLR predict preload responsiveness in ventilated patients, whereas with arrhythmias and spontaneous breathing activity, respiratory variations of arterial pulse pressure poorly predict preload responsiveness.

Changes in aortic blood flow induced by passive leg raising predict fluid responsiveness in critically ill patients

IntroductionEsophageal Doppler provides a continuous and non-invasive estimate of descending aortic blood flow (ABF) and corrected left ventricular ejection time (LVETc). Considering passive leg raising (PLR) as a reversible volume expansion (VE), we compared the relative abilities of PLR-induced ABF variations, LVETc and respiratory pulsed pressure variations (ΔPP) to predict fluid responsiveness.MethodsWe studied 22 critically ill patients in acute circulatory failure in the supine position, during PLR, back to the supine position and after two consecutive VEs of 250 ml of saline. Responders were defined by an increase in ABF induced by 500 ml VE of more than 15%.ResultsTen patients were responders and 12 were non-responders. In responders, the increase in ABF induced by PLR was similar to that induced by a 250 ml VE (16% versus 20%; p = 0.15). A PLR-induced increase in ABF of more than 8% predicted fluid responsiveness with a sensitivity of 90% and a specificity of 83%. Corresponding positive and negative predictive values (PPV and NPV, respectively) were 82% and 91%, respectively. A ΔPP threshold value of 12% predicted fluid responsiveness with a sensitivity of 70% and a specificity of 92%. Corresponding PPV and NPV were 87% and 78%, respectively. A LVETc of 245 ms or less predicted fluid responsiveness with a sensitivity of 70%, and a specificity of 67%. Corresponding PPV and NPV were 60% and 66%, respectively.ConclusionThe PLR-induced increase in ABF and a ΔPP of more than 12% offer similar predictive values in predicting fluid responsiveness. An isolated basal LVETc value is not a reliable criterion for predicting response to fluid loading.

Changes in Arterial Pressure during Mechanical Ventilation

Mechanical ventilation induces cyclic changes in vena cava blood flow, pulmonary artery blood flow, and aortic blood flow. At the bedside, respiratory changes in aortic blood flow are reflected by "swings" in blood pressure whose magnitude is highly dependent on volume status. During the past few years, many studies have demonstrated that arterial pressure variation is neither an indicator of blood volume nor a marker of cardiac preload but a predictor of fluid responsiveness. That is, these studies have demonstrated the value of this physical sign in answering one of the most common clinical questions, Can we use fluid to improve hemodynamics?, while static indicators of cardiac preload (cardiac filling pressures but also cardiac dimensions) are frequently unable to correctly answer this crucial question. The reliable analysis of respiratory changes in arterial pressure is possible in most patients undergoing surgery and in critically ill patients who are sedated and mechanically ventilated with conventional tidal volumes.

International workshop “effects of static magnetic fields relevant to human health”: Rapporteurs report: dosimetry and volunteer studies

The presentations in this session focussed on effects in humans. Both experimental results and model calculations were presented. Dr Donald W. Chakeres, Ohio State University, Columbus OH, USA, presented the results of exposure of volunteers to strong static fields (Chakeres et al., 2003a, b). One group was exposed to incremental field strengths by introducing them in the magnet using discrete steps. At maximum exposure the head was exposed to approximately 8T, while the maximum field strength in the heart region was approximately 7T. At each exposure level heart rate, blood pressure, blood oxygenation, core temperature, ECG, and respiratory rate were measured. No effects were seen, except for a tendency for blood pressure to increase and an alteration in the ECG. The latter can explained by small changes in aortic blood flow due to coupling in the direction of the aortic arch (but not to the descending part of the aorta). The blood pressure variations were smaller than the differences due to changing from supine to upright position. A second group of healthy volunteers was tested at 8 and 0.05T (outside the magnet) using 12 standardized neuropsychological tests and by comparing auditory reaction times. Small changes were noted for the most demanding tests, but no adverse effects were seen. For 9 out of 12 measures the subjects did better at 8 T. Several subjects complained about metallic taste, dizziness, nystagmus and discomfort due to the use of the head coil. These complaints vanished immediately after removal from the magnet. At follow-up 3 months later no symptoms were reported.

Ventilation techniques to minimize circulatory depression in rabbits with surfactant deficient lungs

Changes in aortic blood flow were measured in rabbits with both normal and surfactant depleted lungs in order to elucidate the effect of different modes of ventilation on the circulation while optimizing arterial oxygenation (PaO2). Conventional mechanical ventilation (CMV), reversed inspiratory to expiratory ratio of CMV (IRV), high frequency positive pressure ventilation (HFV), and high frequency oscillation (HFO) were used. Normocapnia was maintained throughout during different modes of ventilation. In normal lungs the aortic blood flow during IRV was significantly lower with similar levels of PaCO2 compared with CMV, HFV, and HFO. In lavaged lungs, without positive end-expiratory pressure (PEEP), the aortic blood flow during CMV was significantly higher than with other modes of ventilation. When 10 cm H2O of PEEP was applied, the PaO2 increased maximally to normal values at all modes of ventilation, but the aortic blood flow was significantly reduced (P < 0.05) during CMV and IRV compared to HFV and HFO. The aortic blood flows at 5 cm H2O of PEEP were very similar during CMV, HFV, and HFO but significantly reduced during IRV. This study showed that at an optimal arterial oxygenation with higher PEEP levels, maintenance of aortic blood flow was maximal during HFV and HFO.

Can pulsed Doppler technique measure changes in aortic blood flow in conscious rats?

To determine whether the pulsed Doppler (PD) system could be used to measure cardiac output we compared estimates of changes in aortic flow obtained from PD or electromagnetic (EM) probes on the ascending aorta in separate groups of conscious Long-Evans and Brattleboro rats under a variety of conditions. EM probes showed significant increases and decreases in aortic blood flow after administration of nitroprusside or methoxamine, respectively, but 20-MHz PD probes connected to a PD mainframe with a pulse repetition frequency (PRF) of 62.5 kHz showed no systematic changes in Doppler shift, due to "aliasing" of the Doppler signal. Aortic blood flow was significantly increased with intravenous volume expansion or isoprenaline administration, but these changes were not reliably detected by PD probes operating in a system with a PRF of 62.5 kHz. Calculation of expected peak Doppler shift signals from the phasic EM flow signal showed that these exceeded the Nyquist limit (PRF/2, i.e., 31.25 kHz) for the commercially available pulsed Doppler system. However, a modified pulsed Doppler module capable of resolving aliases and operating at a PRF of 125 kHz produced reliable results for changes in thoracic aortic Doppler shift, in good agreement with the EM probe data. In addition, placement of a cuff around the thoracic aorta did not alter cardiac baroreflex sensitivity.

Blood flow in the ascending and descending aorta in term newborn infants

The combination of 2-dimensional real-time and pulsed Doppler echocardiography provides a noninvasive method of measuring blood flow and has been used to study blood flow at the lower thoracic level in the descending aorta of the human fetus. Using these techniques, we evaluated the proportion of cardiac output distributed through the descending aorta in 20 healthy term newborn infants at 1–6 days of age. Blood flow in the ascending aorta was 759 ±135 ml/min (mean ±S.D.) and in the descending aorta 482 ±92 ml/min indicating that 63.8% of the left ventricular output is distributed through the descending aorta. Peak systolic and mean blood flow velocities were comparable at the two sites, whereas the diameter of the descending aorta was 19% smaller than that of the ascending aorta. The changes in aortic blood flow were correlated with the changes in aortic diameter.

152 BLOOD FLOW IN THE ASCENDING AND THE DESCENDING AORTA IN TERM NEWBORN INFANTS

The combination of 2-dimensional real-time and pulsed Doppler echocardiography provides a noninvasive method of measuring blood flow and has been used to study blood flow at the lower thoracic level of the descending aorta (DA) in the human fetus. Using these techniques, we evaluated the proportion of cardiac output distributed through the DA in 13 term newborn infants (mean±SEM BW 3551±95 g, GA 40.1±0.3 w). Blood flow velocity was measured using a 3.5 MHz, range gated, pulsed Doppler velocity meter with the transducer positioned in the suprasternal notch. Blood flow in the ascending aorta (AA) was measured at the aortic root level and descending flow below the isthmus. The internal diameter of the AA was assessed at the level of the aortic valve using M-mode echocardiography and the diameter of the DA in the posterior atrioventricular groove with the use of the 2-D long axis view. Blood flow in the AA was 789±42 ml/min and in the DA 499±26 ml/min indicating that 63.5% of the left ventricular output is distributed through the DA. The mean blood flow velocity was comparable at both sites, whereas the mean diameter of the DA was 19% smaller than the diameter of the AA. In healthy term newborn infants approx. 2/3 of cardiac output is directed to the DA and 1/3 to brachial and carotid arteries. The changes in aortic blood flow are correlated with the changes in aortic diameter.