Anesth Analg 2008; 106:530-534
© 2008 International Anesthesia Research Society
doi: 10.1213/ane.0b013e31816071b0
CRITICAL CARE AND TRAUMA
A New Method to Estimate Regional Pulmonary Blood Flow Using Transesophageal Echocardiography
Yuichi Yatsu, MD,
Toshihito Tsubo, MD,
Hironori Ishihara, MD,
Hitomi Nakamura, MD, and
Kazuyoshi Hirota, MD, FRCA
From the Department of Anesthesiology, University of Hirosaki School of Medicine, Hirosaki, Aomori-ken, Japan.
Address correspondence and reprint requests to Yuichi Yatsu, MD, Department of Anesthesiology, University of Hirosaki School of Medicine, Hirosaki 036-8562, Japan. Address e-mail to masuika{at}cc.hirosaki-u.ac.jp.
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Abstract
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BACKGROUND: We report a novel method to estimate regional blood flow in the atelectatic lung using transesophageal echocardiography in an experimental dog model. To verify the efficacy of the method, we investigated the ability of this experimental model to detect changes in regional pulmonary blood flow.
METHODS: Fourteen anesthetized and ventilated mongrel dogs were randomized into an isoproterenol group (n = 7) or a dopamine group (n = 7). To produce an atelectatic lesion, 60 mL/kg of saline was infused into the left pleural space. The velocity time integral (VTI) derived from pulse Doppler was evaluated as an index of blood flow in the atelectatic lesion. To investigate the response of the method to changes in blood flow, the VTI and the shunt fraction (Qs/Qt) were measured during systemic administration of isoproterenol 0.05 µg · kg–1 · min–1 (as a pulmonary vasodilator) and dopamine 10 µg · kg–1 · min–1 (as a pulmonary vasoconstrictor).
RESULTS: Both VTI and Qs/Qt were increased significantly by isoproterenol administration. There was a significant correlation between the percentage changes of VTI and Qs/Qt with isoproterenol administration (r2 = 0.50, P < 0.001). Both VTI and Qs/Qt were unchanged during administration of dopamine.
CONCLUSIONS: Transesophageal echocardiography may be useful in detecting changes in regional pulmonary blood flow in an atelectatic lesion.
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Introduction
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Blood flow in the lung has been measured by various methods, including the use of radioactive-labeled microspheres, fluorescent-labeled microspheres, and single photon emission computed tomography.1,2 However, these methods have limitations in their clinical application due to their difficulty of use, the amount of time required, the degree of invasiveness and the need to use radiation.1,2 In an experimental dog model, we report a novel method of estimating regional blood flow in atelectatic lesions using transesophageal echocardiography (TEE). The velocity time integral (VTI) was used as an index of pulmonary blood flow.3,4 As regional blood flow in the atelectatic lesion is shunt flow, this method might enable the detection of changes in an intrapulmonary shunt in patients.
The aim of this study was to verify the accuracy of this new methodology by detecting changes in regional pulmonary blood flow caused by pulmonary vasodilation and constriction using isoproterenol5–7 as a pulmonary vasodilator and dopamine7,8 as a pulmonary vasoconstrictor.
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METHODS
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Animal Preparation
The study was approved by the Animal Experimentation Committee of Hirosaki University. Fourteen adult mongrel dogs of either sex (9-14 kg) were included. Anesthesia was induced with pentobarbital 25 mg/kg IV. After tracheal intubation, mechanical ventilation (Servo 900C, Siemens Elema, Lund, Sweden) was administered with a tidal volume of 10 mL/kg and an Fio2 of 1.0, and the respiratory rate was adjusted to maintain end-tidal CO2 concentration in the range of 4.0% to 5.0%. Anesthesia was maintained with a continuous IV pentobarbital drip of 2 mg · kg–1 · h–1 and muscle paralysis was obtained with vecuronium. The dogs were kept in supine position throughout the experiment. Normal saline was administered IV at a rate of 4 mL · kg–1 · h–1 as a maintenance infusion. A polyethylene catheter was placed in the femoral artery to permit the measurement of systemic arterial blood pressure and to obtain blood for blood gas analysis. A thermodilution Swan-Ganz catheter was advanced into the pulmonary artery to be used for measuring pulmonary arterial pressure and cardiac output (CO) and to obtain samples of mixed venous blood for blood gas analysis. The Swan-Ganz catheter was connected to a thermal-dilution CO computer (Vigilance, Baxter, Irvine, CA). Shunt fraction (Qs/Qt) was calculated using the standard formula.
A balloon-tipped catheter was inserted from the femoral vein into the inferior vena cava and used to control venous return. To ensure that changes in regional blood flow in the atelectatic lesion were not due to a change in CO but to a direct action of the drug on the pulmonary blood vessels, CO was maintained at the level of pre-catecholamine administration ±10% during catecholamine administration by restriction of venous return using the balloon-tipped catheter.
Production of an Atelectatic Lesion
To produce an atelectatic lesion, we chose the pleural effusion (PE) dog model.9,10 A polyethylene catheter was inserted in the left sixth intercostal space. Warmed (37°C) normal saline (60 mL/kg) was infused over a 30-min period into the left pleural space until an effusion was formed. One hour was allowed for the development of atelectasis.
TEE
TEE was performed using a 5-MHz 64-element transesophageal multiplane echoprobe (Hewlett Packard SONOS 1500; Andover, MA) and recorded on 0.5-inch videotape. The lower left lung area was observed via the descending aorta at the low esophageal position. After the four-chamber view of the heart was observed, the probe was rotated about 90 degrees counterclockwise. VTI of blood flow within a second or third generation pulmonary vein generation was detected. The angle of the Doppler sampling direction was estimated by an angle cursor on the sample line, and the view was adjusted so that the angle was <20 degrees. Waveforms that were reproducibly similar over three consecutive cardiac cycles were obtained. Figure 1 shows the ultrasound image of the pulmonary venous flow in the atelectatic lesion. VTI is defined as the area under the curve of the pulse Doppler profile, and has been reported to be one of the good indices of blood flow.3,4
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Figure 1. A transesophageal echocardiography image and Doppler flow profile obtained in a pulmonary vein which was used to calculate the velocity time integral (VTI).
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Measurements
In unpublished preliminary experiments, isoproterenol 0.05 µg · kg–1 · min–1 and dopamine 10 µg · kg–1 · min–1 were found to produce similar increases in CO of about twofold above baseline in dogs. In this experiment, dogs were randomly assigned to one of two treatment groups. The isoproterenol group received isoproterenol at the rate of 0.025 and 0.05 µg · kg–1 · min–1. The dopamine group received dopamine at the rate of 5 and 10 µg · kg–1 · min–1. These catecholamines were administered for 30 min at each dose. The measurements were performed at 1 h after the creation of the PE (Control), 30 min after the infusion of catecholamines was initiated (Dose 1, Dose 2) and 1 h after the cessation of catecholamines (After). CO measurements were recorded in triplicate and the mean was calculated.
Statistical Analysis
All data are presented as the mean ± sd. The inter- and intra-group comparisons were analyzed with repeated measures analysis of variance, with Dunnet’s multiple comparison test as the post hoc test. The correlation between two variables was tested by linear regression analysis. In regression analysis between VTI and Qs/Qt, the percent of change from "control" was determined, which was the value before the administration of catecholamines. A two-sided P < 0.05 was considered to be significant.
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RESULTS
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Blood flow in a second or third generation pulmonary vein within the atelectatic region was detected successfully by TEE in all dogs. Induction of PE impaired gas exchange, with minimal alterations in hemodynamic variables (Table 1) that were not different between the two groups before drug infusion.
Drug administration altered hemodynamic variables; however, CO was successfully controlled at the level of pre-catecholamine administration ±10% during catecholamine administration using the balloon-tipped catheter placed in the vena cava (Table 2).
Pao2 significantly decreased in the isoproterenol group, with a concomitant significant increase in VTI from 8.0 ± 3.0 cm/beat to 10.6 ± 4.6 cm/beat at the lower dose (0.025 µg · kg–1 · min–1) and to 12.3 ± 3.1 cm/beat at the higher dose (0.05 µg · kg–1 · min–1) (P < 0.05). Qs/Qt similarly increased from 26.3% ± 6.8% to 41.8% ± 11.2% and to 49.7% ± 13.7% (P < 0.05) (Table 2). There was a significant linear correlation between percent changes of VTI and Qs/Qt with isoproterenol administration (r2 = 0.50, P < 0.001) (Fig. 2). Pao2, VTI, and Qs/Qt were unchanged by dopamine administration (Table 2).
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Figure 2. The relationship between the percent changes of the velocity time integral (VTI) and those of the shunt fraction (Qs/Qt) during isoproterenol (top) and dopamine (bottom) administration. There was a significant correlation between both values after isoproterenol administration. However, no significant relationship was observed between these factors and dopamine administration.
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DISCUSSION
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Although it is not possible to observe the normal lung using echocardiography because of multiple air-gas interfaces, a lung density or PE can be observed if there is an acoustic window.11–13 We have previously reported that it is possible to observe pulmonary atelectasis in dependent lung lesions in acute respiratory distress syndrome (ARDS) patients via the descending aorta using TEE.11–13 Based on these findings, we developed a new method to estimate blood flow in an atelectatic lesion using TEE in a unilateral PE dog model.
In this experiment, changes in regional pulmonary blood flow in the atelectatic lesion could be observed during isoproterenol administration in the absence of changes in CO. In addition, a significant correlation was observed between VTI and Qs/Qt. These data are consistent with other published work indicating that isoproterenol is a pulmonary vasodilator that impairs oxygenation because of impaired hypoxic pulmonary vasodilation.5,6 In contrast, the expected changes in VTI or Qs/Qt during administration of dopamine were not observed, probably because of the absence of changes in hemodynamic variables.
A limitation of this study is the inability to directly quantify the blood flow measurement observed in this model, as the measured flow is only a part of total blood flow that passes the atelectatic lesion. Hence, echocardiographic evaluation of regional pulmonary blood flow has to be interpreted as relative, not absolute, changes during serial measurement.
We previously reported that TEE can be used to make an accurate diagnosis of lung lesions in intensive care situations.11–13 In our unpublished preliminary observation, we also observed regional pulmonary blood flow using TEE in pulmonary consolidation in ARDS patients (Fig. 3). Considering the results of this animal experiment, as well as the above-mentioned clinical observations, we believe that our proposed method may be a clinically relevant tool for evaluating intrapulmonary shunt flow during the infusion of various vasoactive drugs in patients with respiratory pathologies such as ARDS, since the response to these drugs may vary among patients.
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Figure 3. A transesophageal echocardiography image in a patient with acute respiratory distress syndrome. By means of this color Doppler technique, the red signals, which represent centripetal blood flow, show the pulmonary veins, while blue signals, which represent the centrifugal blood flow, show the pulmonary arteries.
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In conclusion, in this experimental PE dog model, the changes in regional pulmonary blood flow in an atelectatic lesion were detected using TEE. This method may be useful as an experimental model to examine a drug’s effects on the intrapulmonary shunt. In addition, this method might enable clinicians to detect changes in the intrapulmonary shunt in patients with pulmonary lesions such as atelectasis or consolidation. Prior to clinical application, however, further studies will be needed to evaluate the accuracy and reliability of this technique.
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Footnotes
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Accepted for publication October 22, 2007.
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Abstract
Introduction
