Absorption of carbon dioxide during endoscopic vein harvest

Abstract

OBJECTIVES

Carbon dioxide (CO₂) insufflation was used by some devices for endoscopic vein harvest to create a subcutaneous tunnel and facilitate the vein harvest. In the literature, some cases of CO₂ micro- and macro-embolisms or hypercarbia during this procedure are described. The purpose of this study was to evaluate if the use of an open CO₂ system rather than a sealed system might be associated with different CO₂ absorption during the procedure.

METHODS

Patients were randomized into two groups: those patients in the first group were submitted to endoscopic vein harvest using a sealed CO₂ insufflation; in the second group, the harvest was undertaken with an open CO₂ insufflation. Partial pressure of CO₂ in the arterial blood (PaCO₂) and end-tidal CO₂ (ETCO₂) was recorded following anaesthesia induction and before the endoscopic procedure (T0), every 10 min during the endoscopic step (T1) and end after 10 min from the CO₂ insufflation termination (T2).

RESULTS

A total of 60 patients were enrolled. PaCO₂ increased significantly between T0, T1 and T2 in both groups (P = 0.0001) during the endoscopic harvest, but the PaCO₂ level was significantly higher in the group that used the sealed system (44.5 ± 7.9 vs 39.7 ± 7.9 mmHg) at the end of the procedure (P = 0.01). No significant differences between end-tidal CO₂ measured at the same intervals between groups were detected.

CONCLUSIONS

There was a constant increase in the blood gas concentration compared with the basal pre-procedure values. Sealed systems were associated with a significantly higher CO₂ concentration at the end of the procedure compared with the open ones.

INTRODUCTION

Despite the increasing use of arterial conduits, the great saphenous vein (GSV) remains the conduit most often used in coronary artery bypass grafting (CABG) [1]. The GSV is conventionally harvested by an open technique, with a long skin incision.

Endoscopic saphenous vein harvesting (EVH) was introduced into cardiac surgical practice to eliminate the need for long skin incisions, and it was associated with fewer wound complications, shorter hospital stays, less postoperative pain and better patient satisfaction [2].

Carbon dioxide (CO₂) insufflation has been used in endoscopic surgery for a long time, and it has also been used in some EVH systems to create a subcutaneous tunnel facilitating GSV harvesting. Normally, the insufflated CO₂ is absorbed by the blood and must be eliminated by the lungs through increased ventilation [3].

In the literature, some cases of CO₂ embolism during EVH [4–8] with deterioration of patients' haemodynamics resulting in systemic hypotension and raised central venous pressure have been described [9]. The mechanism of CO₂ embolism during endoscopic procedures is supposed to be CO₂ absorption or its direct entry, through an injured vessel, into the blood stream [4]. CO₂ micro- or macro-embolisms or hypercarbia stimulates the sympathetic nervous system, affects blood pressure and heart rate and may be the cause of dysrhythmias [10]. EVH may be performed with an open or sealed CO₂ insufflation system.

The purpose of this study was to evaluate if the use of an open CO₂ system, rather than a sealed one, might be associated with different CO₂ absorption during the procedure.

MATERIALS AND METHODS

Following authorization by the hospital ethical committee, consent was obtained by each patient who was eligible for the study.

Sixty consecutive patients scheduled for isolated CABG using EVH were enrolled in the study. Patients who needed other surgical procedures associated with CABG were excluded from the study. For the EVH, we used two different devices: the VasoView 6 (Maquet, NJ, USA) (VV), which has a sealed CO₂ insufflation system and the VirtuoSaph (Terumo, MI, USA) (VS), which instead uses an open CO₂ insufflation system. Patients were randomly allocated to one of the two groups: patients in which the VV was used (30 patients; VV group) and patients in which the VS was used (30 patients; VS group). The two groups did not differ in any of the considered preoperative variables. The patients' preoperative characteristics are illustrated in Table 1.

Table 1.

Preoperative data

	VS	VV	Pvalue
N	30	30
Age (years)	70 ± 8	71 ± 7	0.6
Weight (kg)	80 ± 14	77 ± 11	0.4
Sex, F	9 (30%)	5 (16.7%)	0.2
BMI	28.3 ± 4.3	26.9 ± 3.4	0.2
Off-pump surgery	2 (6.7%)	1 (3.3%)	1
Cerebrovascular disease	2 (6.7%)	4 (13.3%)	0.4
Diabetes	15 (50%)	12 (40%)	0.4
Hypertension	28 (93.3%)	23 (76.7%)	0.1
COPD	3 (10%)	1 (3.3%)	0.3
Peripheral vascular disease	7 (23.3%)	6 (20%)	0.7
Chronic renal failure	1 (3.3%)	3 (10%)	0.6
Calculated creatinine clearance	62 ± 36	58 ± 33	0.7
Smoking	13 (43.3%)	11 (36.7%)	0.6
Previous myocardial infarction	12 (40%)	15 (50%)	0.4
Log EuroSCORE	4.2 ± 4	4.5 ± 4	0.7

F: female gender; BMI: body mass index; COPD: chronical obstructive pulmonary disease; VS: VirtuoSaph group; VV: VasoView 6 group.

No systemic anticoagulation with heparin was administered to the patients before the beginning of EVH. All patients underwent a median sternotomy, with harvesting of one or both internal thoracic arteries. At the same time, EVH was started, using either VV or VS, by insertion of a conical dissection cannula into a 1–2 cm longitudinal skin incision on the leg, just below the knee, to create a tunnel around the vein. During this procedure, the CO₂ inflation helps vein dissection from connective tissue and allows the formation of the tunnel. Using the VV, a seal was created at the entrance skin incision inflating a trocar port balloon to <25 ml. The tunnel was then filled with CO₂ to a maximal pressure of 12 mmHg and a maximal flow of 3 ml/min. The VS has no sealing trocar port balloon and uses CO₂ insufflation at the same maximal values of pressure and flow. Division of GSV side branches was performed with bipolar electrocautery set at 30 W and with the help of a cutter blade (VV), or with a cut and coagulate system (VS). Proximal GSV ligation was performed through a separate stab incision at the groin. After its removal from the thigh, the GSV was flushed with saline solution and the vein side branches were ligated with 5–0 mersilene. Then the GSV was stored in a saline and heparinized blood solution and flushed again with a saline solution just before implantation. The heparin was administrated only after internal thoracic artery harvest, to achieve an activated clotting time >400 s for on-pump CABG or >300 s for off-pump ones. All EVH were performed by the same experienced surgeon who had already performed over 100 endoscopic procedures.

During EVH, patients were ventilated according to the usual anaesthetic protocol with a mean respiratory rate of 12 (range 10–15), a tidal volume of 10 ml×kg (range 8–12), a positive end-expiratory pressure of 5 cm H₂O (range 3–7) and a fraction of inspired oxygen of 50 (range 40–60). Partial pressure of CO₂ in the arterial blood (PaCO₂) was measured with a emogasanalyzer (GEM 4000; Instrumentation Laboratory, MA, USA) and the end-tidal CO₂ (ETCO₂) was derived from the ventilator monitor (SC 7000; Siemens, MA, USA). Following anaesthesia induction, PaCO₂ and ETCO₂ were recorded before the endoscopic phase of EVH (T0), every 10 minutes during the endoscopic phase (T1) and 10 min after the CO₂ insufflation termination (T2). According to the study's protocol, during this period the anaesthesiologist did not change the respiratory setting.

In a retrospective analysis on 100 patients treated without EVH, we found the PaCO₂ values normally distributed with standard deviation 6. On this basis, if the true mean difference in the two groups is 5 mmHg of PaCO₂, we will need to study 30 experimental subjects and 30 control subjects to be able to reject the null hypothesis that the population means of the groups are equal with probability (power) of 0.9. The Type I error probability associated with two tailed t-test of null hypothesis is 0.05. Data are expressed as mean ± standard deviation or as a percentage. Statistical analysis included paired or unpaired t-test for continuous data and χ² test for categorical data. Differences with a Pvalue <0.05 were considered significant.

RESULTS

Almost all saphenous vein segments were harvested from the right thigh; occasionally the vein was isolated in other locations or multiple harvest sites were performed (Table 2). There were no cases of conversion from EVH to traditional harvest and all harvested grafts were considered suitable for surgical use. The mean length of the harvested GSV was 43 ± 9 cm (range 30–96 cm) and the mean number of tears of the vein's requiring repair with polypropylene stitches were 1 ± 1 per harvest. There was a small difference between the two procedural mean times, VS being faster than VV, although this did not reach significance (19 ± 8 min vs 23 ± 15 min; P = 0.2). Operative EVH differences between groups are reassumed in Table 3.

Table 2.

Sites harvest

	VS	VV
Right thigh	27 (90.3%)	25 (83.3%)
Right and left thighs	0	1 (3.3%)
Right thigh prolonged to leg	1 (3.3%)	0
Right thigh prolonged to leg and left thigh	0	1 (3.3%)
Left thigh	2 (6.7%)	3 (10%)

VS: VirtuoSaph; VV: VasoView 6.

Table 3.

Operative data

	VS	VV	Pvalue
Endoscopic time (min)	19 ± 8	23 ± 15	0.2
Total harvest time (min)	42 ± 14	47 ± 19	0.2
Conversion (n)	0	0
Graft utilized (n)	30	30
Coronary anastomosis/segment (n)	2 ± 1	2 ± 1	0.2
Total segment length (cm)	42 ± 3	44 ± 12	0.3
Tears/segment	0.7 ± 0.8	1.3 ± 1.3	0.05

VS: VirtuoSaph; VV: VasoView 6.

During EVH, a progressive rise in the PaCO₂ occurred in both groups cumulatively: 35.5 ± 5.6 mmHg at preprocedure time (T0), 37.8 ± 6.5 mmHg during endoscopic time (T1) and 42.1 ± 8 mmHg at the end of the endoscopic procedure (T2) (P = 0.0001). PaCO₂ values were 36.5 ± 6.3 mmHg in the VV group and 34.5 ± 4.8 mmHg in the VS group at the preprocedure time (T0, P = ns), 38.8 ± 7.5 and 36.8 ± 5.3 mmHg, respectively during endoscopic time (T1, P = ns) and 44.5 ± 7.9 mmHg in the VV group compared with 39.7 ± 7.3 mmHg at the end of the endoscopic procedure (T2, P = 0.01) (Table 4; Fig. 1).

Table 4.

PaCO₂ and ETCO₂ data

	T0	T1	T2	Pvalue
PaCO2
VV	36.5 ± 6.3	38.8 ± 7.5	44.5 ± 7.9	0.0001
VS	34.5 ± 4.8	36.8 ± 5.3	39.7 ± 7.3	0.0001
p*	0.2	0.3	0.01
ETCO2
VV	23.4 ± 5.8	25.2 ± 5.4	28.9 ± 8	0.0001
VS	22.8 ± 4.3	24.3 ± 5	25.7 ± 6.7	0.0001
p*	0.6	0.5	1

PaCO₂: partial pressure of carbon dioxide in the artery; ETCO₂: end-tidal carbon dioxide; T0: preprocedure time; T1: during endoscopic time; T2: at the end of endoscopic time.

*Between groups.

Figure 1:

Preoperative (T0), intraoperative (T1) and postoperative (T2) PaCO₂ according to the study group (VV, VasoView; VS, VirtuoSaph). Data are expressed as mean and standard errors.

There was one operative death in the VS group due to a low output syndrome. The cause of that was completely unrelated to the EVH. There were no deaths in the VV group.

Data related to patients' postoperative status, just before leaving the operating room, are shown in Table 5. There were no differences in postoperative results between the two groups except for paroxysmal atrial fibrillation, which was more frequent among VV group patients (Table 6).

Table 5.

Patients' postoperative status leaving the operative room

	VS	VV	Pvalue
Inotropic	25 (50%)	25 (50%)	0.6
Antiarrhythmic	1 (3.3%)	0	0.3
Sinus rhythm	27 (90%)	29 (97%)	0.3
Intra-aortic balloon pump	1 (3.3%)	0	0.3
Systolic arterial pressure	110 ± 13	107 ± 14	0.4
Diastolic arterial pressure	55 ± 11	59 ± 11	0.2

VS: VirtuoSaph; VV: VasoView 6.

Table 6.

Postoperative data

	VS	VV	Pvalue
Death	1 (3.3%)	0	0.2
Ventilation >24 h	2 (6.7%)	1 (3.3%)	0.2
Acute renal failure	3 (10%)	1 (3.3%)	0.2
Atrial fibrillation	10 (33.3%)	2 (6.7%)	0.009
LOS (days)	9.3 ± 8.5	7.8 ± 4.6	0.4
Blood loss (cc/12 h)	330 ± 228	365 ± 192	0.5

LOS: hospital stay.

DISCUSSION

In 2005, the International Society for Minimally Invasive Cardiothoracic Surgery published a consensus statement on the use of endoscopic vs open harvesting in CABG [2]. They recommended that EVH was the standard technique to reduce wound-related complications, postoperative pain and length of hospital stay. EVH would also improve patient satisfaction and reduce the need of outpatient wound care [2]. Advantages of EVH are well demonstrated in terms of reduction in wound complications and infection [11].

Although it is reasonable to say that EVH is a valid and safe alternative to the conventional harvesting, several issues related to CO₂ were raised in the literature and should be studied [4–10, 12, 13].

The CO₂ insufflation may cause micro- and macro-embolisms with a mechanism that involves absorption or direct entry of gas through an injured vessel into the circulation. This depends on the gradient between the central venous pressure and the pressure of insufflation [14].

Lin et al. [4] showed that minimal CO₂ embolism occurred frequently during EVH and that there was a risk of massive CO₂ embolism in >0.5% of patients in which blood pressure, ETCO₂, oxygen saturation and cardiac output decrease immediately.

Hypercapnia has been shown to occur following CO₂ pneumoperitoneum: the hypercarbia associated with laparoscopy has been shown to be increased in patients who have severe subcutaneous emphysema, suggesting that subcutaneous absorption of CO₂ occurs and it is an important cause of hypercapnia [13]. Since hypercarbia stimulates the sympathetic nervous system, affects blood pressure and heart rate and may cause dysrhythmias, obviously it should be avoided in patients undergoing cardiac surgery procedures [10].

We examined two of the most diffused EVH systems that uses different CO₂ management: the sealed system (VV) maintains a pressure of gas adequate to keep the tunnel around the vein and avoids gas leaks through the instrument insertion hole with an occluding trocar. However, the trocar also occludes the proximal lumen of the GSV, not allowing the passage of blood flow [15]. The open system (VS) does not occlude CO₂ leaks, but it compensates the CO₂ losses with additional gas insufflation, so it does not require the occluding trocar.

In our experience, despite the more variable CO₂ pressure in the tunnel, optimal GSV visualization was obtained and we did not find EVH to be more technically challenging with an open CO₂ insufflation system, there being no differences in terms of magnitude of the tunnels obtained with the two systems. Moreover, with an open system, a constant blood flow was maintained inside the GSV [15] without increasing the risk of bleeding from GSV side branches or the difficulty of the EVH, completing the endoscopic procedure even faster.

The PaCO₂ at the end of the endoscopic procedure was significantly higher in VV compared with VS (44.5 ± 7.9 vs 39.7 ± 7.3 mmHg, P = 0.01). Probably, this finding can be explained with the continuous loss of CO₂ from the subcutaneous space through the service hole when the open system is used. In fact, the preset maximum gas insufflation pressure of 12 mmHg was rarely reached during VS.

Obviously, these changes are small and did not result in consequences for patients. In fact, we did not find any differences in the early postoperative recoveries between the two groups and on comparing patients with low and high plasmatic CO₂ values. Furthermore, outside the experimental environment, in which ventilatory parameters were maintained constant, the anaesthesiologist can easily counteract them.

A limitation of our study is that it was based on a surrogate end point as we did not use systems to detect CO₂ micro- and macro-embolisms. Moreover, we followed patients until their hospital discharge. As in the literature, findings of clots retained inside the GSV are described, particularly when sealed systems were used [15], we are planning to re-evaluate the patients enrolled at 1-year follow-up with an evaluation of the graft's patency with coronary computer tomography.

The interesting aspect of our findings is the demonstration, in a prospective and randomized study, that a sealed system is associated with major CO₂ absorption compared with the open one tested.

Therefore we recommend paying careful attention to monitoring and limiting the CO₂ flow rate and insufflation pressure while performing EVH, especially with a sealed system, in order to avoid more severe and harmful increases.

FUNDING

The study has no financial funding, but we must note that two authors (Carlo Zingaro and Francesco Massi) are EVH trainers on behalf of both societies producing the two systems which are objects of the study.

Conflict of interest: none declared.