Central venous cannulation is frequently performed in hemodynamically unstable patients or those undergoing major operations.1–4 Because of the small size of central veins, this procedure is technically difficult in the pediatric population and carries greater risks for adverse events.5,6
The subclavian vein (SCV) is a favored site for placement of central venous catheters because of better patient comfort with long-term use, lower infection rate, and less collapsibility in case of hemodynamic instability or shock.3–5 However, SCV cannulation presents a higher risk for pneumothorax than internal jugular or femoral routes.2–5
During SCV catheterization, some practitioners disconnect the endotracheal tube from the breathing circuit in anesthetized or mechanically ventilated patients to achieve deflation of the lungs. If this technique causes the lungs to shrink and to move farther away from the SCV, it could decrease the risk for pneumothorax. However, there are no formal studies assessing the influence of lung deflation on the distance between the SCV and the pleura or the size of the vein.
The purpose of this study was to evaluate the effect of lung deflation on the distance from the SCV to the pleura and on the cross-sectional area (CSA) of the SCV in mechanically ventilated pediatric patients.
After obtaining approval from our IRB and written parental consent, we enrolled 50 pediatric patients (25 infants younger than 1 year and 25 children aged 1 to 8 years), ASA physical status I and II, who were scheduled for elective operations under general anesthesia. Patients with cardiovascular or pulmonary diseases, or neurologic disorders, which could increase intracranial pressure, were excluded from the study. Those with structural anomalies of the chest wall, history of lung and chest wall operations, prior central venous catheterization, or upper respiratory tract infection or pneumonia within 2 weeks were also excluded.
All patients were fasted for 8 hours from solids and 2 hours from clear liquids before surgery. Isotonic crystalloid solution was administered to replace maintenance fluid requirements while they remained non per os. Arriving at an operating room, they were monitored with an electrocardiogram, a noninvasive arterial blood pressure monitor, and a pulse oximeter. General anesthesia was induced with thiopental sodium (6 mg/kg) and sevoflurane, and endotracheal intubation was facilitated by using rocuronium 0.6 mg/kg. Mechanical ventilation was initiated (tidal volume of 6–7 mL/kg with no application of positive end-expiratory pressure), and the respiratory rate was adjusted to maintain an end-tidal CO2 between 34 and 38 mm Hg. Anesthesia was maintained with sevoflurane in 100% oxygen.
The patients were placed supine on a horizontal table, with their head in the midline position and the shoulders arched above a rolled sheet, the height of which was 5 cm in infants and 10 cm in children. An 8- to 13-MHz linear probe (12L-RS; GE Healthcare System, Milwaukee, WI) was placed perpendicular to the skin on the infraclavicular area at the midportion of clavicle to obtain images of the SCV and the pleura. Images were recorded during ventilation with the probe held in the same position. The endotracheal tube was then disconnected from the breathing circuit and left open to atmosphere, and images were acquired over the following 2 minutes. The same investigator performed all ultrasound examinations and took care not to alter the position of the probe. If the patient's oxygen saturation as measured by pulse oximetry decreased below 95% during the examination, recording was stopped and ventilation was resumed without taking any further measurement.
The distance from the inferior border of the SCV to the pleura (SCV-pleura distance) and the CSA of the SCV were measured using stored images. The CSA was calculated by preloaded software in the ultrasound machine after the investigator delineated the circumference of the vein using an electronic marker. The measurement was obtained at the end of inflation and 0, 30, 60, 90, and 120 seconds after lung deflation. The sequence of ultrasound images was randomized, and all data were reviewed and interpreted by an independent observer who was blinded to the study protocol. A representative ultrasonographic image of the SCV and pleura in a child is presented in Figure 1.
Analysis was performed according to patient age (infants versus children). All results were expressed as mean ± SD. Differences of the SCV-pleura distances or the SCV CSAs between at end-inflation and just after deflation (at 0 seconds after lung deflation) were evaluated using the paired t test. Changes with time from 0 to 120 seconds were analyzed with the repeated-measures analysis of variance. All statistical calculations were performed with SPSS for Windows version 13 (SPSS, Inc., Chicago, IL), and a P value <0.05 was accepted to indicate statistical significance. Increases of 5% in the SCV-pleura distance and 25% in the SCV CSA were considered clinically relevant.
Patient characteristics are summarized in Table 1. Three infants and 4 children were excluded because of poor image quality, and data from the remaining 22 infants and 21 children were analyzed.
Table 2 shows the SCV-pleura distances. In infants, there was no statistically significant difference in the SCV-pleura distance between the end of inflation (DI) and just after lung deflation (D0). In children, the distances just after lung deflation (D0) were significantly longer than those at end-inflation (DI) (P < 0.05). However, the increases in the distance were <5%, and therefore, not clinically relevant. During deflation, no further increase in SCV-pleura distance was noted over time in either infants or children.
The SCV CSAs are shown in Table 3. There were no statistically significant differences between the CSAs measured at the end of inflation (AI) and those measured just after lung deflation (A0) in either infants or children. After deflation, the CSAs did not change with time in all patients.
Eight infants and 1 child experienced arterial desaturation below 95% during the study. All were assisted by manual ventilation and recovered within 30 seconds. None of the patients had complications related to transient desaturation.
Central venous cannulation has an important role in the management of critically ill or hemodynamically unstable patients.1–4 SCV access has some advantages, such as comfort with long-term use and relatively low risk of infection.3–5 Nonetheless, pneumothorax is more frequent in SCV catheterization than in the internal jugular approach, with an incidence of 0.1% to 6% compared with <0.5%, respectively.2–8
It seems theoretically possible that reduced lung volumes during lung deflation cause the lung to move away from the SCV, decreasing the likelihood of piercing the lung and, consequently, preventing pneumothorax. It has not been validated, however, whether this simple maneuver significantly increases the SCV-pleura distance. Our results indicate that the distances after lung deflation were not different from those at the end of inflation when patients' lungs were ventilated with tidal volumes of 6 to 7 mL/kg.
There are several explanations for the lack of increase in the SCV-pleura distance during deflation. First, changes in this distance in pediatric patients may be too small to be detected by ultrasound. Second, the decrease in lung volume produced by deflation may not be apparent because we applied comparatively small tidal volumes during positive-pressure ventilation. Third, the SCV is contained within a neurovascular bundle traversing a space between the clavicle and the first rib, and therefore the position of the vein remains relatively consistent. Moreover, the pleura is behind the ribs, which have a dense collection of intercostal muscle between them. Consequently, the SCV and the pleura are likely to have consistency in position during respiration.
Our data show that the SCV CSA is not changed with respiratory phase in infants and children. These results are different from those obtained from the internal jugular vein (IJV). The CSA of the right IJV has been reported to increase significantly with an end-inspiratory hold of 20 cm H2O in mechanically ventilated patients.9,10 An elevated intrathoracic pressure can decrease venous return and, consequently, distend the IJV. It is possible that the differences in compliance between the SCV and the IJV partially account for these contradictory findings. The IJV is located superficially and surrounded only by soft tissue. On the contrary, just below the clavicle and above the first rib, the neurovascular bundle and some fibrous tissues in which the SCV is included may decrease the compliance of the SCV. In addition, the relatively small tidal volume and airway pressure used in this study may also explain these contradictory findings.
Our results prove that the SCV-pleura distances measured every 30 seconds after lung deflation remained unchanged. They imply that it is unnecessary to prolong the duration of apnea to achieve further lung deflation in pediatric patients. Waiting for further lung deflation during subclavian catheterization should be avoided in the pediatric population because hypoxemia occurs rapidly, especially in infants. Furthermore, it is important to keep in mind that the patients who need a central venous catheter are often unwell and hence even less likely to tolerate apnea than those in the study group.
Based on some previous articles in which a 20% or 25% change in the CSA was regarded to be clinically significant,11–16 we defined an increase in the SCV CSA of >25% as being clinically relevant. Because the CSA of a circle is directly proportional to the square of its radius, an increase of 5% in the SCV-pleura distance was considered a threshold of clinical relevance. Before beginning the study, we repeatedly measured the length of a known 10-mm line using the electric ruler of the ultrasound machine, and the average measuring error was approximately 0.5 mm. By applying a threshold of a 5% change in the distance, we intended to overcome this measuring error.
This study has some limitations that deserve consideration. First, we were not able to examine the complete needle pathway because the ultrasound machine only provided single plane images. Second, excursions of the SCV and pleural surface occurring during a normal cycle of positive-pressure ventilation, application of positive end-expiratory pressure, or Trendelenburg position have not been measured. These issues are equally important in determining the risk of pneumothorax and need further investigations. Third, the most important outcomes are success in cannulation, time to cannulation, and complications. The current study, however, was not designed to assess the success or complication rates of SCV cannulation, and thus could not clarify the relationship between lung deflation and the success rate of the procedure or the incidence of pneumothorax. The distance measured in this trial is only a surrogate outcome. Finally, all patients we enrolled were neither critically ill nor hypovolemic. We thought the standardization of the data should be a priority. Based on the results of healthy infants and children, advanced investigations aimed at critically ill pediatric patients who may be in more need of a central venous catheter can proceed.
In summary, this study shows that lung deflation does not increase the distance between the SCV and the pleura or the CSA of the SCV in either infants or children. Therefore, we advise that the risks and benefits of this maneuver should be reconsidered, especially in pediatric patients at higher risk for hypoxemia or increased intracranial pressure.
Name: Kyung-Jee Lim, MD.
Contribution: Manuscript preparation, data analysis.
Name: Jin-Tae Kim, MD, PhD.
Contribution: Study design, conduct of study, manuscript preparation, and data analysis.
Name: Hee-Soo Kim, MD, PhD.
Contribution: Study design.
Name: Hyo-Jin Byon, MD.
Contribution: Conduct of study.
Name: Soo-Kyung Lee, MD.
Contribution: Manuscript preparation.
Name: Jung-Man Lee, MD.
Contribution: Data analysis.
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