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Ann Thorac Surg 2001;71:1496-1501
© 2001 The Society of Thoracic Surgeons

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Original article: cardiovascular

Hemodynamics and gas exchange during carbon dioxide insufflation for totally endoscopic coronary artery bypass grafting

^a Department of Anesthesiology, Intensive Care Medicine and Pain Control, J.W. Goethe-University Hospital, Frankfurt, Germany
^b Department of Thoracic and Cardiovascular Surgery, J.W. Goethe-University Hospital, Frankfurt, Germany

Accepted for publication December 18, 2000.

Address reprint requests to Dr Byhahn, Department of Anesthesiology, Intensive Care Medicine und Pain Control, J.W. Goethe-University Hospital, D-60590 Frankfurt, Germany
e-mail: c.byhahn{at}em.uni-frankfurt.de

	Abstract

Top Abstract Introduction Material and methods Results Comment References

 
Background. In addition to single-lung ventilation (SLV), positive-pressure CO₂ insufflation is mandatory for totally endoscopic coronary artery bypass grafting. Studies on the effects of unilateral CO₂ insufflation on hemodynamics produced controversial results, and bilateral insufflation has not been studied to our knowledge. The present study sought to investigate hemodynamics and gas exchange during unilateral and bilateral CO₂ insufflation in patients who underwent totally endoscopic coronary artery bypass grafting.

Methods. Eleven hemodynamic and gas exchange variables were monitored during 22 totally endoscopic coronary artery bypass grafting procedures with unilateral (n = 17) or bilateral (n = 5) CO₂ insufflation at a pressure of 10 to 12 mm Hg. Data were obtained at baseline with double-lung ventilation, after institution of SLV, during insufflation, after cardiopulmonary bypass during SLV, and after return to double-lung ventilation.

Results. Arterial oxygen tension decreased significantly during SLV, whereas the peak inspiratory pressure increased. In addition, central venous pressure and heart rate increased significantly during insufflation, but mean arterial pressure remained unchanged. Although the end-tidal CO₂ pressure did not change, arterial carbon dioxide tension increased progressively to a maximum of 44.6 ± 5.9 mm Hg during unilateral insufflation, and 55.7 ± 14.6 mm Hg during bilateral insufflation (p < 0.05 versus baseline and between groups). Mixed venous oxygen saturation declined during SLV regardless of CO₂ insufflation and recovered to baseline once double-lung ventilation was restarted. Left and right ventricular ejection fractions remained unaltered. No patient required inotropic or vasopressor support.

Conclusions. Carbon dioxide insufflation for totally endoscopic coronary artery bypass grafting with SLV had no adverse effects on hemodynamics. In contrast to a moderate increase of arterial carbon dioxide tension during unilateral insufflation, markedly elevated arterial carbon dioxide tension levels remain a cause of concern during bilateral insufflation.

	Introduction

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Endoscopic techniques have become an established treatment modality for a variety of diagnostic and therapeutic procedures in current surgery. Regardless of the type of endoscopic surgery, CO₂ insufflation is often used to achieve adequate exposure of the intraabdominal or intrathoracic structures and to facilitate the surgical procedure.

Totally endoscopic coronary artery bypass grafting (TECAB) through a transabdominal approach has been described earlier in animals [1], human cadavers [2], and in 2 patients [3]. The computer-enhanced telemanipulation system Da Vinci (Intuitive Surgical, Mountain View, CA) has added a new dimension to the surgical treatment of coronary artery disease, allowing TECAB with transthoracic access [4–6]. To allow adequate exposure of the internal thoracic arteries (ITA) and the heart, single-lung ventilation (SLV) is mandatory during ITA dissection. Furthermore, positive-pressure CO₂ insufflation is required to obtain a collapsed lung, resulting in an artificial tension pneumothorax. Data with regard to potential hemodynamic compromise caused by intrathoracic CO₂ insufflation are still contradictory [7–12].

We therefore prospectively studied a total of 22 patients who underwent TECAB at our institution to assess potential adverse effects of CO₂ insufflation under positive pressure on hemodynamics and gas exchange.

	Material and methods

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After approval by the institution’s ethical review board and obtaining informed written consent, 22 patients with coronary artery disease underwent TECAB procedures with the Da Vinci robot system. In 17 patients (group A, unilateral insufflation), either the left (LITA, n = 16) or right internal thoracic artery (RITA, n = 1) was dissected. Three patients underwent scheduled bilateral insufflation to harvest both LITA and RITA for the treatment of two-vessel disease. In 2 patients, the right pleura was accidentally opened during LITA dissection, which resulted in unplanned bilateral insufflation. These 5 patients formed group B. Internal thoracic artery grafting to the coronary vessels was performed during cardiopulmonary bypass (CPB) with femoral cannulation (Port Access System, Heartport, Redwood City, CA). All patients underwent preoperative transthoracic echocardiography to evaluate the ascending aorta, aortic valve, and left ventricular function. Atherosclerosis of the femoral vessels was ruled out by duplex sonography. In addition, all patients underwent preoperative pulmonary function testing and chest x-ray examination.

Anesthesia and patient positioning
All patients received intramuscular premedication with 0.06 mg fentanyl and 3 mg droperidol one hour before the induction of anesthesia. After induction of anesthesia with sufentanil (25 µg), etomidate (0.2 mg/kg), and succinylcholine (1 mg/kg), the patients were intubated with a left endobronchial 37 to 41F double-lumen tube (Kendall, Neustadt, Germany). Correct position of the tube was verified by both auscultation and fiberoptic bronchoscopy. Intraoperatively, patients were in supine position with the left chest slightly elevated.

Anesthesia was maintained with an end-tidal concentration of 1.1 to 1.4 vol% of enflurane with air in oxygen (fraction of inspired oxygen, 0.5). The respiratory rate was set to 10 breaths per minute, and tidal volume was set to 8 to 10 mL/kg and adjusted by means of repeated arterial blood gas analyses to maintain arterial partial pressure of carbon dioxide and pH within the normal range of 32 to 45 mm Hg and 7.34 to 7.47, respectively. During the entire procedure, 25 µg of sufentanil was administered for analgesia whenever deemed necessary. In addition, all patients received continuous infusions of dopamine (3 µg · kg^-1 · min^-1) and diltiazem (3 mg/h).

Immediately before incision and placement of the ports, the respective lumen of the double-lumen tube was occluded to allow selective ventilation of the right or left lung as required. Respiratory rate, tidal volume, and inspired oxygen concentration were maintained unless arterial oxygen saturation as measured by pulse oximetry dropped below 92%, or if arterial blood gas analysis demonstrated an arterial partial pressure of oxygen less than 100 mm Hg. Similar to double-lung ventilation, respiratory rates and tidal volumes were adjusted to maintain pH as described above and arterial carbon dioxide tension at approximately 40 mm Hg. Single-lung ventilation was terminated when 100% CPB was achieved.

After insertion of a 12-mm Sealing Port into the chest, CO₂ insufflation was initiated and continued until an intrathoracic pressure of 10 to 12 mm Hg was reached.

Measurements
To evaluate hemodynamics and gas exchange during CO₂ insufflation before CPB, arterial blood gas analyses were performed immediately before incision under double-lung ventilation as a baseline value (Baseline), 5 min after institution of SLV (Start SLV), and 30, 90, and 120 min after CO₂ insufflation was started. Two further analyses were performed after weaning from CPB under SLV (SLV after CPB) and 5 min after return to double-lung ventilation (End). In every instance, the samples were analyzed immediately in a laboratory next to the operating room (ABL3, Acid Base Laboratory/Hemoxymeter, Radiometer, Copenhagen, Denmark). End-tidal CO₂ pressure, central venous pressure (CVP), mean arterial blood pressure (MAP), and heart rate (HR) were monitored continuously during the procedure. Left and right ventricular ejection fractions were assessed with transesophageal echocardiography, using a Vingmed System Five TEE (GE Vingmed, Horten, Norway) with a multiplane 5 to 7 MHz probe. A transgastric biventricular view was set at the level of the papillary muscles of the left ventricle. All transesophageal echocardiography data were recorded after discontinuation of mechanical ventilation at end expiration and analyzed off-line by two independent and equally trained echocardiographers, one of which was blinded to all clinical and hemodynamic patient data.

Statistical analysis
All data are presented as mean ± standard deviation. Calculation and data analysis were performed using a statistical package (GraphPad InStat 3.0, GraphPad Software, San Diego, CA). Statistical significance was determined with either Friedman test and Bonferroni adjustment or Wilcoxon-Mann-Whitney test as appropriate. Differences were considered to be statistically significant if p was less than 0.05.

	Results

Top Abstract Introduction Material and methods Results Comment References

 
A total of 22 patients with symptomatic coronary artery disease underwent TECAB. In 2 patients who underwent LITA dissection, the right pleura was accidentally opened during the procedure. These 2 patients were therefore included in group B (bilateral insufflation), leaving 17 patients in group A (unilateral insufflation). In all patients, preoperative pulmonary function testing was not suggestive of coexisting pulmonary disease, nor was the preoperative chest x-ray examination. Patients in group A either had isolated 70% to 99% stenosis of the left anterior descending artery (n = 16) or an 80% stenosis of the right coronary artery (n = 1). In group B, critical stenosis of the left anterior descending artery was present in 2 patients. Three patients had two-vessel disease including 70% to 99% stenosis of both left anterior descending artery and right coronary artery or circumflex artery. No patient had a history of myocardial infarction, and no regional wall motion abnormalities were detected during routine preoperative transthoracic echocardiography. Demographic data, duration of CPB, and aortic cross-clamp time are presented in Table 1.

	Comment

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A number of studies in animal models and humans have sought to answer the question of whether and to what extent the creation of an artificial tension pneumothorax affects hemodynamics [7–9]. Jones and colleagues [7] demonstrated that, under double-lung ventilation, intrathoracic CO₂ insufflation with positive pressure significantly decreased cardiac index, stroke volume, and MAP. Central venous pressure increased thereby. The same research group found similar results in swine when selective lung ventilation was used [8]. Insufflation pressure was 5 to 10 mm Hg. In contrast, human studies are contradictory. Wolfer and colleagues [9] demonstrated that, under SLV and with progressively increased insufflation pressures up to 14 mm Hg, no significant changes of MAP, HR, arterial oxygen saturation, and end-tidal CO₂ pressure occur, but noted a significant increase of CVP. Measurements taken five and 30 minutes after initiation of CO₂ insufflation for endoscopic ITA harvesting showed increased CVP, pulmonary capillary wedge pressure, and mean pulmonary arterial pressure. Mean arterial pressure, HR, cardiac index, and left ventricular ejection fraction were not affected [10]. In contrast, Brock and coworkers [11] demonstrated that cardiac index decreased markedly at insufflation pressures of 10 and 15 mm Hg, but was not altered at 5 mm Hg. These results match those of Raumanns and colleagues [12], who investigated a number of hemodynamic variables at different insufflation pressures during robot-assisted thoracoscopic LITA dissection and showed significant deterioration of cardiac index, RVEF, intrathoracic blood volume index, and pulmonary vascular pressure at insufflation pressures of 10 and 15 mm Hg. Nonetheless, data with regard to the effects of SLV in combination with unilateral insufflation for an extended time or bilateral insufflation on hemodynamics and gas exchange do not exist.

A significant decrease of arterial partial pressure of oxygen is a well-known side effect of SLV [13,14], which also applied to our patients. Arterial oxygen tension decreased in both groups to the same extent and recovered promptly after return to double-lung ventilation at the end of the procedure. In both groups, arterial oxygen tension and oxygen saturation were worst after weaning from CPB under SLV. Likewise, mixed venous oxygen saturation as a marker of tissue oxygen extraction decreased. Once double-lung ventilation was reinstituted, gas exchange variables returned to baseline values. Significantly increased peak inspiratory pressures were noted once SLV had been instituted, maintained during SLV, and returned to baseline after reinstitution of double-lung ventilation. Therefore, increased peak inspiratory pressure predominantly followed SLV [11], although another study found that peak inspiratory pressure to some extent depends on insufflation pressure [12].

End-tidal CO₂ pressures were constant in both groups during the entire period of investigation. Wolfer and colleagues [9] presumed that, unlike laparoscopy, intrathoracic insufflation of CO₂ does not result in considerably increased end-tidal CO₂ pressure, because hypoxic pulmonary vasoconstriction results in less CO₂ absorption in the ipsilateral, collapsed lung. However, arterial carbon dioxide tension was not determined to prove this hypothesis [9]. We were able to demonstrate that there was no relationship between end-tidal and arterial CO₂ pressures during insufflation. Carbon dioxide insufflation resulted in progressively increased arterial carbon dioxide tension and adjustment of tidal volume and respiratory rate allowed us to maintain only end-tidal, but not arterial, CO₂ pressure at baseline level.

The progress of TECAB may allow even complex multivessel revascularization with double ITA grafts in the closed chest. Bilateral CO₂ insufflation will be required for this approach. We could demonstrate markedly higher arterial CO₂ partial pressure increase during bilateral CO₂ insufflation, which could not be compensated by increased tidal volume and respiratory rate. A maximum of arterial carbon dioxide tension of 81.3 mm Hg was noted in a patient 120 minutes after initiation of bilateral insufflation.

Interestingly, the results during bilateral insufflation support the hypothesis of Wolfer and colleagues [9] to some degree. It can be presumed that unilateral, as compared with bilateral CO₂ insufflation leads to considerably less absorption of CO₂ in the collapsed lung because of hypoxic pulmonary vasoconstriction. Owing to hypoxic pulmonary vasoconstriction in the nonventilated lung, the ventilation to perfusion ratio in the ventilated lung decreases, and consequently, CO₂ increases. As a result of disproportionally high absorption, arterial CO₂ pressure was significantly higher as compared with unilateral insufflation. Furthermore, considerable amounts of CO₂ seem be stored in compartments of the body other than blood, and are slowly redistributed and metabolized or exhaled. In contrast to unilateral insufflation, elimination of absorbed CO₂ during CPB was incomplete. Arterial partial pressure of carbon dioxide was 25% beyond baseline after weaning from CPB.

As observed in previous investigations [7–12], we noted CVP to increase similarly in both groups during insufflation with CO₂. Explanations are contradictory. Wolfer and colleagues [9] suggested the increase in the CVP to be caused by increased afterload induced by hypoxic pulmonary vasoconstriction of the nonventilated, ipsilateral lung. In contrast, other studies have shown that a significantly increased CVP occurs during insufflation, regardless of whether selective lung ventilation is used or not [8–10]. In recent patients, we noted a significant increase with the onset of CO₂ insufflation, but not with previous institution of SLV. One might speculate that, in addition to intrathoracic positive pressure, regional wall motion abnormalities that were more pronounced in the right than in the left ventricle during TECAB procedures [15] may also contribute to increased CVP.

Ventricular function as determined by transesophageal echocardiography showed no significant compromise of both left and right ventricular ejection fraction during unilateral insufflation. Under bilateral insufflation, RVEF decreased, but amounted to more than 50% at all times. Because RVEF in particular depends on a number of variables, such as venous filling pressure and preload on one hand, and intrathoracic pressure on the other, interpretation of RVEF data is somewhat difficult. Because RVEF was satisfactory at all times throughout the procedure, and no patient required hemodynamic support or became hemodynamically unstable, decreased right ventricular function during bilateral insufflation was of minor clinical interest.

Regardless of the fact that the majority of the patients received perioperative ß-blockers, and despite continuous intraoperative infusion of diltiazem, a significantly increased HR was noted in both groups during insufflation of CO₂. This is in contrast to other human studies, which demonstrated the HR to be constant [9, 10]. Because our anesthetic regimen is comparable to other studies, our results are somewhat difficult to explain. Because continuous infusion of 3 µg · kg^-1 · min^-1 dopamine was already started after induction of anesthesia and thus at least one hour before baseline values were obtained, chronotropic stimulation caused by dopamine is unlikely. Instead, creation of an artificial tension pneumothorax may compromise venous return and thereby decrease stroke volume. These adverse effects are compensated in full by the increased HR. As a result, MAP is maintained virtually constant. It can be concluded that, regardless of whether unilateral or bilateral insufflation is used, intrathoracic CO₂ insufflation does not provoke adverse hemodynamic effects of clinical significance.

Finally, it should be mentioned that insufflation pressures of 10 to 12 mm Hg for endoscopic cardiac procedures are relatively high. A number of studies have shown that intrathoracic pressures of 5 to 10 mm Hg are adequate for surgical exposure [5, 6, 11, 12]. Regardless of the fact that no adverse hemodynamic effects were observed in the present cohort, insufflation pressure should therefore be kept as low as possible, because significant hemodynamic compromise is most likely to occur when pressure exceeds the 10-mm Hg threshold [11, 12]. However, adverse effects cannot be completely excluded at lower pressure [8].

In conclusion, unilateral insufflation of CO₂ during nonventilation of the ipsilateral lung does not cause adverse effects on hemodynamics and gas exchange to be of clinical relevance. Arterial CO₂ pressure increases within an acceptable physiologic range or just beyond. In contrast, when CO₂ was insufflated bilaterally, the extensive increase of arterial carbon dioxide tension remains a cause of concern. As during unilateral insufflation, no hemodynamic compromise was noted. We therefore believe that unilateral CO₂ insufflation for TECAB procedures can be considered a safe technique in patients with single-vessel disease and good ventricular function. For a definitive evaluation of bilateral insufflation safety, further studies in larger cohorts need to be conducted. In general, inasmuch as no experience has been accumulated in patients with poor ventricular function or considerable comorbidity, TECAB procedures should be conducted with particular caution in such patients until study data become available.

	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 Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to Personal Folders Download to citation manager Author home page(s): Stephan Mierdl Gerhard Wimmer-Greinecker Georg Matheis Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Byhahn, C. Articles by Westphal, K. Search for Related Content PubMed PubMed Citation Articles by Byhahn, C. Articles by Westphal, K. Related Collections Coronary disease Minimally invasive surgery Related Article