Department of Anaesthesiology, University of Ulm, Steinhövelstr. 9, D-89075 Ulm, Germany*Corresponding author
Accepted for publication: July 13, 2001
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Abstract |
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Br J Anaesth 2001; 87: 7117
Keywords: monitoring, transoesophageal echocardiography; liver, blood flow; liver, hepatic perfusion; measurement techniques, Doppler echocardiography; heart, right heart dynamics
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Introduction |
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Methods |
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All patients were without known cardiovascular disease and were demonstrated to be in sinus rhythm by an electrocardiogram. According to the ASA criteria, 14 patients were classified as group I, 14 patients as group II and six patients as group III. The mean age was 46.5 and ranged from 23 to 80 yr. After induction of general anaesthesia and endotracheal intubation, patients lungs were mechanically ventilated (FIO2 0.30.4; PEEP 5 cm H2O) with a tidal volume of 810 ml kg1 at a ventilatory frequency of 810 breaths min1 adjusted to maintain end-tidal PCO2 between 35 and 40 mm Hg.
Sonography and Doppler sonography measurements
Transabdominal approach
After induction of general anaesthesia, transabdominal recordings of the right, middle, and left hepatic vein were obtained using a Hewlett Packard echocardiograph (Sonos 5500, Hewlett-Packard Inc., Andover, MA, USA) with a 2.5 MHz transducer. For visualization of the right hepatic vein a lateral thoracic approach was used, for visualization of the middle and left hepatic vein a subxiphoidal or right subcostal view was chosen. Two-dimensional pictures of the hepatic veins were obtained in the B-mode. Subsequently blood-flow velocities in each hepatic vein were acquired by use of pulsed Doppler (PW)-mode. The Doppler sample-volume was placed in the centre of the vessel, 13 cm from its junction with the inferior vena cava. Correction of the angle between the Doppler beam and long axis of the hepatic vein was performed for each Doppler measurement using the software of the echocardiograph. Two-dimensional pictures and Doppler signals were obtained at the end of expiration along with lead II of the ECG and stored on a magneto-optical disc. To verify the end of expiration, an external breathing circuit pressure gauge (Fa. Draeger, Lübeck, Germany) was connected to the echocardiograph to visualize the airway pressure on the screen. Analysis of the echo data was performed off line using the software of the echocardiograph. Two-dimensional images and flow velocity curves were judged adequate for analysis only when clear signals were depicted at the end of expiration.
Transoesophageal approach
After acquisition of the transabdominal views a 5.0 MHz multi-plane transoesophageal probe (Omniplane I, Hewlett-Packard Inc., Andover, MA, USA) was inserted and connected to the same echocardiograph. To visualize the hepatic veins the tip of the probe was advanced into the antrum of the stomach and flexed anteriorly. By rotating the probe clockwise and adjusting the imaging angle of the transducer array to 4080° a saggital view of the inferior vena cava and the right hepatic vein was obtained. By rotating the probe counter-clockwise and increasing the angle of the transducer array to 5090° the middle hepatic vein was visualized, by further counter-clockwise rotation of the probe and increasing the angle of the transducer array to 80130° an image of the left hepatic vein was acquired. When feasible, a two-dimensional picture of each hepatic vein was acquired in the B-mode and a Doppler sonography curve was obtained in the PW-mode. As described for the transabdominal approach the images were recorded along with an ECG-lead at the end of expiration and stored on a magneto-optical disc.
Analysis of the Doppler sonography curve
Hepatic venous flow during a cardiac cycle can be divided into a systolic forward flow, diastolic forward flow, and a diastolic reversed flow induced by atrial contraction.24 Sometimes, also, a small systolic reversed flow can be identified. Peak flow velocities and velocity time integrals of the four phases were measured by manual planimetry using the integrated software of the echocardiograph. The velocity time integral is the calculated area under the Doppler curve over a specified period.5 The velocity time integral represents the distance the blood travels during the specified period. The total velocity time integral during one cardiac cycle was determined by addition (forward flow) and subtraction (reversed flow), respectively, of the four single velocity time integrals.
Calculation of the blood flow in the hepatic veins
Blood flow in a distinct hepatic vein was calculated using the formula: Blood flow = VTIx (D/2)2xHR (VTI: total velocity time integral of one cardiac cycle,
(D/2)2 cross-sectional area of the vessel, HR: heart rate).6 7 The diameter (D) of the vessel was determined at the same part of the vessel, where the PW-Doppler sample volume was placed. Heart rate was derived from the ECG recorded together with the Doppler images.
Statistical analysis
Discrete variables were described as absolute and relative frequencies. All continuous variables were presented as median (range). Differences between transthoracic and transoesophageal data were compared using the MannWhitney Rank Sum Test. Statistical significance was assumed when the P-value was <0.05.
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Results |
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Acquisition of adequate Doppler sonography curves of the main hepatic veins
Using TOE adequate Doppler tracings of the right hepatic vein were obtained in 100% (34/34) and of the middle hepatic vein in 97% (33/34) of the patients. A characteristic Doppler sonography curve of the middle hepatic vein obtained by TOE is depicted in Figure 2. In most patients the angle between the left hepatic vein and the Doppler beam was greater than 60° (see Fig. 1C). Therefore, adequate Doppler tracings of the left hepatic vein could only be acquired in 18% (6/34) of the patients by the transoesophageal approach.
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Comparison of the Doppler curves obtained by the transoesophageal and transabdominal approach
In all patients a diastolic forward flow and in about 90% of the patients a systolic forward flow was observed in the three main hepatic veins using both techniques. As demonstrated in Table 1 a systolic and diastolic reverse flow was detected by TOE in more patients than with the transabdominal approach.
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Discussion |
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Furthermore, good quality Doppler curves of the right and middle hepatic vein can be obtained by TOE. Almost all patients revealed adequate Doppler curves of the right (100% of the patients) and middle hepatic vein (97% of the patients) by the transoesophageal technique. Using the transabdominal approach, adequate Doppler curves of the right and middle hepatic vein could only be obtained in 91% and 50% of the patients. For the middle hepatic vein, this finding is in accordance with the results of Appleton and colleagues3 and Klein and colleagues8 who, using the transabdominal approach, obtained adequate Doppler tracings of the middle hepatic vein in 30% and 46% of healthy subjects, respectively. These groups did not assess hepatic vein flow in the right hepatic vein.
Using TOE assessment of hepatic vein flow in the left hepatic vein is not possible in most patients, as the angle of insonation is greater than 60°. However, the transabdominal technique allowed acquisition of Doppler curves of the left hepatic vein in only 47% of the patients and the majority of the Doppler curves revealed artefacts probably caused by heart motion, which is transmitted to the left side of the liver. Therefore, the left hepatic vein is of limited value for blood flow analysis or calculations.
TOE was superior to TAS for detection and visualization of systolic and diastolic reverse flow. We observed this phenomenon more often using TOE in comparison with the transabdominal approach. This observation suggests that, because of the anatomical proximity, the transoesophageal technique is more sensitive to detect low blood-flow velocities or small changes of blood flow in the hepatic veins. Analysis of reverse flow in the hepatic veins is interesting in the context of different diseases of the right heart. First, the extent of the diastolic reverse flow is associated with the contractility of the right atrium.9 10 In patients with atrial fibrillation or atrial electromechanical dissociation, as seen after cardioversion of atrial fibrillation or cardiopulmonary bypass, a diastolic reverse flow will not be present. Second, analysis of hepatic venous reverse flow allows supplementary quantification of tricuspid regurgitation.9 11 12 In severe tricuspid regurgitation, an appreciable holosystolic reverse flow appears and all forward flow occurs during diastole. Patients with lesser degrees of tricuspid regurgitation have little or no systolic reverse flow. Third, right ventricular diastolic dysfunction is associated with characteristic changes in hepatic venous flow pattern.9 13 14 Patients with a decreased right ventricular compliance reveal an increase in diastolic reverse flow. In analogy to pulmonary venous reverse flow an increase in the diastolic reverse flow might be useful to distinguish pseudonormal right ventricular filling in patients with impaired relaxation from normal right ventricular filling.15 16
In comparison with data recently published by Klein and colleagues8 for the middle hepatic vein, we observed a similar mean value for the diastolic peak velocity (23.3 (12.6) vs 23.0 (8.0) cm s1), but a markedly lower value for the systolic peak velocity (16.8 (9.5) vs 38.0 (11.0) cm s1) by the transabdominal approach. One explanation for this difference is that we used patients who were under general anaesthesia and mechanical ventilation, whereas Klein and co-workers investigated awake spontaneously breathing patients. This is supported by the studies of Pinto and colleagues4 who observed lower peak velocities of the systolic flow under general anaesthesia and mechanical ventilation in comparison with values obtained preoperatively under spontaneous breathing.
Two previous studies did use TOE in an attempt to assess hepatic venous blood flow.17 18 Gardebäck and co-workers, using a biplane TOE probe, were not able to visualize the different hepatic veins in a long axis view and they did not describe which of the three hepatic veins was chosen for blood flow calculations.17 Recently, Sato and colleagues, for the first time, used multi-plane TOE for determination of hepatic blood flow in the middle hepatic vein during laparoscopic surgery.18 The present study is the first attempt to assess blood flow in the different hepatic veins by using multi-plane TOE.
The method of calculating blood flow by Doppler sonography contains several limitations. The formula used for Doppler sonographic calculation of blood flow is based on the mean velocity within the vessel and the cross-sectional area of the vessel.6 Inaccuracy in determining the diameter of the vessel by the use of ultrasound will produce a large error because the errors will be multiplied in determining the area. This method assumes that the velocity profile within the vessel is consistent.6 7 This assumption seems to be accurate for the velocity profile within valve orifices, as several studies have demonstrated an excellent agreement between echocardiographic and invasive determination of cardiac output.19 20 Within small vessels the velocity profile is known to be more parabolic. Measuring the maximal velocity within such a vessel will result in an overestimation of the true blood flow. A specific limitation of the transoesophageal approach is that assessment of total hepatic vein flow is not possible, because blood flow in the left hepatic vein cannot be determined in most patients. Nevertheless, multi-plane TOE might be an interesting tool to monitor changes in hepatic and splanchnic perfusion, in particular during therapeutic interventions in intensive care patients. Recently, we have demonstrated in a pig model that TOE provides a reasonable method to detect changes in hepatic vein flow.21 Experimental studies in humans are necessary to test this diagnostic option of TOE against the standard methods, for example indocyanine green-clearance.22
Hepatic vein flow is determined by a series of different factors. Both elevated intra-thoracic pressure induced by PEEP ventilation and elevated intra-abdominal pressure induced by pneumoperitoneum during laparoscopic surgery may decrease liver and splanchnic perfusion.23 24 Also, an increase in central venous pressure as a consequence of right heart failure may reduce hepatic blood flow.25 In particular, in septic patients different catecholamines have been shown to have different effects on hepatic and splanchnic perfusion.26 27
Accurate assessment of hepatic vein flow patterns is not limited to questions of hepatic and splanchnic perfusion. Analysis of hepatic vein flow patterns may provide interesting insights into right heart dynamics intra-operatively or in critically ill patients.10 28 Our results suggest that the transoesophageal technique seems to be a superior technique in the analysis of right atrial and ventricular function in most patients. Furthermore, determination of hepatic vein flow by Doppler sonography has been demonstrated to be helpful in diagnosing acute rejection after liver transplantation.29 Intra-operative determination of hepatic vein flow during liver resection and transplantation has been shown to be a helpful tool to support the surgical procedure.30 Recently, Nagueh and co-workers have demonstrated, that analysis of hepatic vein flow pattern allows reliable estimation of central venous pressure.31
Although TOE is regarded as a safe method and reports of serious complications are rare, it may cause serious injuries such as hypopharyngeal and oesophageal perforation or oesophageal bleeding as a result of insertion and use of TOE probes.32 34 Especially in patients with portal hypertension and oesophageal varices, the risk of bleeding complications is increased. Therefore, the demonstrated advantages of TOE have to be balanced against the potential for causing inadvertent morbidity, which is significantly higher than with TAS.
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References |
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