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CARDIOVASCULAR ANESTHESIA

Pulmonary Transit of Sonicated Albumin Microbubbles During Controlled Mechanical Ventilation: A Transthoracic Echocardiographic Study

Federico Bilotta, MD^*, Laura Fiorani, MD, Eugenio Lendaro, BS, Sergio Picardo, MD^*, Italia La Rosa, MD^*, Giovanni Rosa, MD^*, and Francesco Fedele, MD

Departments of *Anesthesia, Cardiology, and Biochemistry, University of Rome "La Sapienza," Rome, Italy

Address correspondence and reprint requests to Dr. Federico Bilotta, via dei Tadolini 13, 00196 Rome, Italy.

	Abstract

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Air-filled human serum albumin microspheres are ultrasonic contrast tracers that pass through the right ventricle, traverse the lungs, and effectively opacify the left heart chambers in spontaneously breathing patients. In this clinical study, we assessed whether they also do so in anesthetized patients during and after mechanical ventilation. In 20 anesthetized patients undergoing intermittent positive pressure ventilation (IPPV) for elective peripheral neurosurgical procedures, a sonicated ultrasound contrast drug (0.06 mL/kg) was injected IV before inducing anesthesia in spontaneously breathing patients (baseline), during IPPV, and 5 and 30 min after tracheal extubation. Transthoracic echocardiograms were obtained in the four-chamber apical view and were recorded for off-line analysis. Time to contrast appearance in the right ventricle and pulmonary transit time were measured in cardiac cycles. The peak intensity of right and left ventricular chamber opacification were scored on a scale ranging from 1 (no contrast or traces only) to 5 (attenuation). After each injection, the time for contrast appearance in the right ventricle was similar in all patients. Pulmonary transit time increased significantly during IPPV and was normal 5 min and 30 min after extubation. Right ventricular chamber opacification achieved high-grade intensity and remained constant before, during, and after IPPV. Conversely, although the baseline contrast injection resulted in high-grade left ventricular chamber opacification, the intensity decreased significantly during IPPV, remained low 5 min after extubation, and was normalized 30 min after extubation.

Implications: During intermittent positive pressure ventilation, IV sonicated albumin microbubbles pass through the lungs poorly and inefficiently opacify the left ventricle compared with the effects observed during spontaneous ventilation.

	Introduction

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Because the newer ultrasonic contrast tracers consisting of air-filled human serum albumin microbubbles achieve physiologic transit through the capillary circulation (1), they are highly suitable for studying systemic organ perfusion. Studies in spontaneously breathing patients (2–4) and anesthetized dogs (5) have shown that after an IV injection, these tracers opacify the right ventricle, pass through the lungs, and opacify the left ventricle. Several studies have reported that contrast echocardiography improves detection of left ventricular wall motion abnormalities (2,3). Intraoperative perfusion studies have used contrast tracers administered only by intraarterial and intraaortic injection (6–8). The ability of albumin microbubbles to pass through the lungs depends on many factors, including biochemical interactions with blood gases and intravascular pressure.¹ In contrast echocardiography, pressure is a major determinant of the background scatter of the bubble-generated signal. Because ultrasonic scatter is related to the sixth power of the radius of the bubbles (10), even a small decrease in the size of the bubbles markedly diminishes background scatter and reduces the intensity of ultrasound-reflected signals.

Intermittent positive pressure ventilation (IPPV) alters the physiology of the pulmonary microcirculation (11,12). Positive intrapleural pressure tends to increase pulmonary vascular resistance and to impede the return of blood to the thorax by compressing the pulmonary capillaries, thus reducing cardiac output.

In this study, we wanted to determine whether sonicated albumin microspheres injected IV during IPPV to opacify the right ventricle pass through the lungs and opacify the left ventricle. For this purpose, we measured two variables: the time required for the contrast agent to appear in the right ventricle and traverse the lungs; and the intensity of right and left chamber ventricular opacification (RVCO and LVCO, respectively).

	Methods

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With the approval of our committee on human research, 20 patients undergoing elective peripheral neurosurgical procedures were prospectively recruited. To be eligible, patients had to be conscious, cooperative, and at least 18 yr old. The criteria for exclusion were systemic arterial hypertension (systolic blood pressure >180 mm Hg, diastolic blood pressure >95 mm Hg, or both), systolic pulmonary-artery pressure >40 mm Hg (measured by Doppler), cardiac shunts (including a patent foramen ovale), mechanical prosthetic cardiac valve, mitral stenosis (valve area <2 cm²), aortic stenosis (mean pressure gradient >50 mm Hg), pregnancy (women of child-bearing potential not using oral contraceptives or intrauterine devices were tested), history of blood product hypersensitivity, known protein allergy, manifest heart failure, chronic obstructive pulmonary disease, adult respiratory distress syndrome, and septicemia. All patients received written and oral information regarding the study and gave their written informed consent before being included.

All patients received an IV injection of a 0.06-mL/kg solution of sonicated albumin microspheres before the induction of anesthesia, during spontaneous breathing (fraction of inspired oxygen [FIO₂] .21) (baseline), 5 min after the beginning of IPPV (FIO₂ .40), and 5 min (FIO₂ .50) and 30 min after extubation (FIO₂ .21). For ethical reasons, each patient served as his or her own control.

Patients received intramuscular morphine sulfate 0.1 mg/kg as premedication approximately 30 min before surgery. On the patient’s arrival in the operating room, a 20-gauge catheter was inserted into the right antecubital vein and was connected through a three-way stopcock to an infusion set of isotonic saline solution. Patients were continuously monitored by three-lead electrocardiogram (ECG), noninvasive arterial blood pressures were measured every 2 min, and O₂ saturation was continuously monitored by pulse oximetry. Anesthesia was induced with fentanyl 0.01 mg/kg, diazepam 0.20 mg/kg, and vecuronium 0.08 mg/kg while the patient breathed 50% oxygen. The trachea was intubated, and IPPV was started at a tidal volume of 10 mL/kg of body weight and a respiratory frequency of 12 cycles/min. The lungs were ventilated with air and O₂ to maintain FIO₂ at 0.40. No patient required positive end-expiratory pressure support. After anesthesia had been induced and the trachea intubated, end-tidal CO₂ was monitored by continuous mainstream spectral photometry. Mechanical ventilation was adjusted to maintain end-tidal CO₂ between 30 and 40 mm Hg. Anesthesia was maintained with fentanyl, diazepam, and vecuronium. At the end of the operation, patients were allowed to resume spontaneous breathing, and the neuromuscular blockade was reversed. When patients were orientated and able to respond appropriately, the trachea was extubated, and they received FIO₂ 0.50 via a mask for 10 min. The total duration of IPPV was recorded.

The ultrasonic contrast agent consisted of sonicated air-filled human serum albumin microspheres prepared under sterile conditions at least 1 h before the study. The sonication process consisted of exposing 7 mL of 5% human serum albumin solution to continuous 150 W energy under temperature control. The temperature of the solution was increased up to 58°C, 3 mL of air was added, and the solution was allowed to cavitate for 20 s before sonication was terminated. After this procedure, the solution separated into two layers: an upper layer consisting of large, white bubbles and a lower layer containing a suspension of microspheres (diameter <20 micron; concentration 2–6 x 108 microbubbles/mL). The suspension of microspheres was withdrawn through a three-way stopcock into a sterile syringe.

Patients were studied in left lateral decubitus or in the supine position. Transthoracic echocardiograms in the four-chamber apical view were obtained with a 3.25-MHz probe and were recorded on videocassette tapes for off-line analysis. To provide the best possible image quality, ultrasound settings were optimized for each patient, and these settings were kept constant throughout the study. Imaging began before the contrast agent was injected and continued for 1 min after complete left ventricular contrast clearance.

To resuspend the microspheres, the echo-contrast agent was gently mixed for 1 min before injection. All injections were given manually. During contrast injection, the patient’s right arm was raised to facilitate venous drainage. Each patient received contrast solution drawn from the same batch. Contrast was injected at a rate of 1 mL/s, and the vein was not flushed through with saline. The beginning of IV contrast injection was displayed on imaging recording.

Recorded images were visually evaluated, individually, by two experienced observers who were unaware of the experimental conditions. If the observers’ scores disagreed, the opinion of a third observer was considered discriminant. Interobserver variability was <10%. Time to contrast appearance in the right ventricle after the beginning of the injection was measured in cardiac cycles (CC). Pulmonary transit time was measured in CC from the appearance of the contrast agent in the right ventricle to its appearance in the left ventricle. The peak intensity of RVCO and LVCO was scored on a 5-point scale: 1 = no contrast or traces, 2 = faint opacification (individual target bubbles), 3 = intermediate opacification (mixture of individual target and homogeneous opacification), 4 = full opacification (complete opacification with contrast filling the left ventricular apex), and 5 = attenuation. Attenuation was defined as an excessive concentration of contrast in the cardiac chambers, causing a complete reflection of the ultrasound beam with transient partial or total disappearance of echographic images.

Patients were closely observed for signs of adverse events during the examination and up to 30 min after the last contrast injection. A standard 12-lead ECG was recorded before and at the end of the study. During the test, a three-lead ECG was continuously monitored, arterial blood pressures were measured every 2 min, and O₂ saturation was continuously monitored by pulse oximetry.

Data were entered into a database and checked by double entry and visual inspection. Analysis of variance was used to detect changes in vital signs or laboratory data between baseline and subsequent injections. Bonferroni test was used to correct data for repeated measures. Data are expressed as mean ± SD. P 0.05 were considered to indicate statistical significance.

	Results

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All 20 patients completed the study. Postoperative neurological assessment yielded normal findings, and all patients had an uneventful postoperative course. In all patients, the individual clinical status, ECG, arterial blood pressures, and O₂ saturation remained unchanged before and after the IV injection of sonicated albumin microspheres (data not shown). No significant differences were found for the mean time to contrast appearance in the right ventricle: 5 ± 2 CC after baseline injection in spontaneously breathing patients, 7 ± 3 CC during IPPV, and 6 ± 2 CC at 5 min and 5 ± 3 CC at 30 min after extubation. Pulmonary transit time increased significantly during IPPV (baseline 9 ± 2 CC versus during IPPV 15 ± 4 CC; P < 0.05) and did not differ significantly from baseline 5 and 30 min after extubation (baseline 9 ± 2 CC versus 5 min after extubation 12 ± 3 CC and 30 min after extubation 10 ± 2 CC).

In all patients, each of the four contrast injections achieved high-grade RVCO. The intensity of RVCO remained constant before, during, and after IPPV (Table 1). Conversely, although the baseline contrast injection yielded high-grade LVCO, the intensity decreased significantly during IPPV (baseline LVCO grade 3.9 ± 0.4 versus during IPPV 1.5 ± 0.5, P < 0.005), remained low 5 min after extubation (baseline 3.9 ± 0.4 versus 5 min after extubation 3.2 ± 0.6; P < 0.05), and returned to normal 30 min after extubation (baseline 3.9 ± 0.4 versus 30 min after extubation 3.7 ± 0.5; P = not significant) (Table 2).

	Discussion

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Numerous factors might contribute to hindering the pulmonary transit of sonicated albumin during IPPV. The most likely include increased intrapleural pressure, a gas concentration gradient between the contrast microspheres and the alveoli, enzymatic activity that could dissolve the bubble coatings, and reduced cardiac output. Intravascular pressure correlates inversely with the half-life of albumin microspheres, accelerating concentration decay both in vitro and in vivo (10).¹

The reason that impaired lung transit and LVCO persisted for 5 min but for 30 min after tracheal extubation is unclear. The most plausible explanations include an impaired ventilation/perfusion pattern, the increased FIO₂ (0.50), and pulmonary vasoconstriction.

Our findings of difficult lung transit during IPPV conflict with previous reports in spontaneously breathing patients (2–4) and anesthetized dogs (5). In an earlier study in spontaneously breathing patients, we found a normal time to contrast appearance in the right ventricle, successful RVCO, normal lung transit time, and reproducible LVCO (4). In this earlier study, sonicated albumin microbubbles injected IV also achieved reproducible myocardial contrast enhancement, indicating the degree of myocardial perfusion.

Finding a reliable technique for intraoperative organ perfusion studies during anesthesia would be extremely useful. During surgery, an adequate organ blood flow is critical for normal cellular metabolism and tissue preservation. Because hypoperfusion often precedes functional damage and influences the extent of an insult, assessing tissue perfusion could guide interventions to protect and preserve organ viability. One example illustrating the importance of information on perfusion is that most intraoperative myocardial ischemic events occur without hemodynamic aberrations, which suggests a mechanism of decreased oxygen supply (hypoperfusion) rather than an increased oxygen demand (13). Most methods for the intraoperative detection of myocardial ischemia (ECG and echocardiographic wall motion abnormalities) rely on an imbalance between oxygen supply and demand (14,15).

Vascular contrast echocardiography is an inexpensive technique that uses portable equipment and provides real-time information on organ perfusion. It is therefore ideal for use in the operative setting (8). Previous studies on the safety of the IV injection of hand-agitated and sonicated echo contrast agents have reported a low incidence of complications (2,3). Similarly, intraarterial injections of echo contrast agents have been found to be safe (8). The earliest studies in contrast echocardiography were reported by Gramiak and Shah (16), who demonstrated increased echo signal intensity in the aortic root after intraaortic injections of indocyanine green. Opacifying the left heart chambers after an IV injection proved more difficult because of the large amount of echo enhancer lost during pulmonary transit (17).

Our study protocol leaves unanswered the question of whether impaired pulmonary transit of sonicated albumin microbubbles and weak LVCO during IPPV and early after extubation is related predominantly to the mechanical effect of IPPV or to the composition of inhaled gas.

Current research already proposes a second generation of more stable contrast enhancers, consisting of a bilayer phospholipid shell filled with a mixture of high molecular weight nondiffusible and less soluble gasses (18,19).

ECG equipment designed explicitly for contrast ultrasonography and having direct access to modify radiofrequency may help to avoid image compression and distortion.

In conclusion, once ultrasound contrast enhancers that are able to efficiently cross the pulmonary circulation during IPPV become available, a promising application for the technique of contrast-enhanced echocardiography might be intraoperative real-time assessment of the pulmonary circulation and perfusion of systemic organs (20).

	Footnotes

 
¹ Wiencek JG, Walker R, Gretler D. In vitro and in vivo stability of Albunex contrast agent [abstract]. J Am Coll Cardiol 1992;19:175A.

	References

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a colleague Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via Web of Science (2) Citing Articles via Google Scholar Google Scholar Articles by Bilotta, F. Articles by Fedele, F. Search for Related Content PubMed PubMed Citation Articles by Bilotta, F. Articles by Fedele, F.