Perioperative factors influencing the mortality of elective abdominal aorta aneurysm repair

Introduction

Abdominal aorta aneurysms diagnosed when the aortic diameter exceeds 3 cm is a recognized cause of death for the European population, approximating 1.3% of deaths among males over 65 years of age [1-3]. Morphologically, the most common type of aneurysm is the fusiform aneurysm, with the risk of rupture being when the diameter exceeds 5 cm, increasing pre-hospital mortality to 50%. Therefore a diameter above 5 cm is an indication for surgery, with the risk of rupture increasing as much as 3% per year. The perioperative mortality rate in major vessel surgery is still high, despite dynamic medical care advancements, a variety of surgical techniques and modes of anaesthesia. Approximately 70% of complications are cardiovascular in origin leading either to death or chronic heart failure preventing this patient population from returning to professional activity [1, 2]. The identification of intra-operative risk factors for perioperative death is of crucial importance for prevention. The appearance of these risk factors might be seen as an early warning sign for the medical team, enabling treatment and monitoring modification, which in turn may lead to a decrease in the rate of life-threatening consequences.

Material and methods

A prospective observational study was conducted in the Vascular Surgery Department of the Pomeranian Medical University in Szczecin, Poland. The study was granted permission from the Bioethical Committee of the Pomeranian Medical University (BN-001/52/08).

The study group consisted of 95 patients, diagnosed with abdominal aorta aneurysm in the sub-renal region scheduled for an operative procedure of straight vascular graft implantation. Patient qualification was fulfilled according to Trans-Atlantic Inter-Society Consensus (TASC) criteria and was based on computed tomography (angio-CT) or magnetic resonance imaging (angio-MRI) of the abdomen. Prior to the procedure the extracranial internal carotid arteries were evaluated with the use of duplex ultrasound. In the case of symptomatic stenoses of 60-99%, as well as asymptomatic stenoses > 70%, carotid artery endarterectomies were performed prior to the abdominal procedure.

During the preoperative visit on the day prior to the procedure and after first obtaining informed consent to participate in the study, a detailed medical and family history was taken. After patient examination, an epidural catheter was placed at the level of Th12-L1 and a triple lumen 7Fr central catheter was inserted into the right internal jugular vein. In the case of contraindications or patient refusal the epidural catheter placement was abandoned.

On the day of the operation patients were premedicated with oral midazolam, 0.25 mg/kg approximately 1 hour prior to the procedure. In the operating room standard cardiovascular system monitoring was applied and consisted of an electrocardiogram, continuous invasive blood pressure monitoring (20G cannula) in the left radial artery (Beton-Dickinson, USA) and central venous pressure monitoring using an Infinity Delta monitor (Draeger G).

Patients qualified for the study underwent general and regional anaesthesia, if the epidural catheter placed during the preoperative visit was in situ. About 1-2 ml/dermatome of 0.5 % Marcaine with 1:200,000 adrenaline was injected into the epidural catheter to obtain sensory block between Th4 and Th12. After achieving the desired level of the epidural anaesthesia, intravenous general anaesthesia induction was started using 0,005 mg/kg of fentanyl, 1.5 mg/kg of propofol and 0.1 mg/kg of vecuronium for muscle relaxation. Maintenance of general anaesthesia was performed with the use of 1 MAC of sevoflurane and fentanyl and vecuronium in fractionated doses, neuromuscular blockade monitoring was used as the train-of-four (TOF) using the peripheral nerve stimulator Infinity Delta (Draeger G). After tracheal intubation intermittent postitve pressure ventilation (IPPV) was supplied by a Fabius anaesthetic machine (Draeger G) with a tidal volume approximating 7 ml/kg, respiratory rate equal to 10/min, and FiO2 of 0.4 (fraction of inspired oxygen). Ventilatory parameters were monitored by an Infinity Delta monitor (Draeger G).

All patients underwent an elective procedure, performed by one of three surgeons — senior consultants in vascular surgery. Using open abdominal laparotomy (midline incision, anterior transperitonital approach) the implantation of a straight vascular prosthesis was performed. A bolus of 30 mg of unfractionated heparin was injected intraoperatively as an intravenous bolus.

After skin closure, sevoflurane was discontinued, neuro-muscular block reversed with 0.03 mg/kg of neostigmine with 0.015 mg/kg of atropine. After extubation the patient was transferred to the postoperative recovery room.

Both colloids and crystalloids were used for intraoperative fluid therapy based on the current fluid balance, with packed red blood cell transfusions when the hemoglobine level was below 10g/dl. Fresh frozen plasma was used when blood loss exceeded 700 mls.

Additionally, intraoperative metabolic monitoring consisted of arterial blood gas analysis and electrolyte levels prior to general anaesthesia initiation and at the 1st and 30th minute after cross-clamp release. The parameters evaluated during the study were: pH, pO2 (partial pressure of oxygen), pCO2 (partial pressure of carbon dioxide), HCO-3 (bicarbonate ion), BE (base excess), lactate, hemoglobine, hematocrit, potassium and sodium ions, all analyses were performed using the GEM 3000 apparatus (USA).

Postoperative analgesia was supplied as patient controlled analgesia (PCA) using 0.1% Marcaine epidural infusion (epidural PCA pump) B. Braun Medical Inc. or intravenous morphine according to local acute pain protocol. Postoperative cardiovascular monitoring was performed using clinical examination, 12-lead electrocardiogram, troponine I, creatinine phosphokinase activity (CPK) and creatinine phosphokinase myocardial isoenzyme (CPK-MB) levels at fixed time intervals (6 and 12 hours post-operatively).

Postoperatively, observation was continued for 28 days, analyzing the influence of preoperative factors, hemodynamic and metabolic parameters regarding the risk of death in the study population.

Statistical analysis was performed using STATISTICA software (StatSoft, Inc. 2009, STATISTICA data analysis software system, version 8.0). Depending on the type of analyzed data the following tests were applied: t-Student test, Mann-Whitney test or c2 test. For the non-parametric U-Mann-Whitney test the results are show as median with the range and/or interquartile spread. Data was statistically significant when p < 0.05.

Results

Demographic data and information about the co-morbidities is summarized in table 1.

Table 1. Co-morbidities of the study population
Tabela 1. Charakterystyka badanej populacji

 

Grupa n = 95 (100%)
Group n = 95 (100%)

Age (years)
Wiek (lata)

70.2 ± 7.6

Male
Mężczyzna

86 (91%)

Ischemic heart disease
Choroba niedokrwienna serca
CCS I 
CCS II



44 (46%)
6 (6%)

Myocardial infarction
Zawał serca

21 (22%)

PTCA

7 (7%)

CABG

7 (7%)

Diabetes mellitus type 2
Cukrzyca typu 2

8 (8%)

Dyslipidemia
Dyslipidemia

44 (46%)

Arterial hypertension
Nadciśnienie tętnicze

95 (100%)

Smoking
Nikotynizm

95 (100%)

Peripheral artery disease
Choroba tętnic obwodowych

22 (23%)

Stroke
Udar mózgu

10 (10%)

Previous CEA
Wcześniejsza CEA

(12%)

Renal failure (creatinine > 2 mg/dl)
Niewydolność nerek

3 (4%)

Preoperative beta-blockers
Terapia ß-blokerami

46 (64%)

Preoperative statins
Terapia statynami

11 (12%)

NYHA II

24 (25%)

NYHA II

8 (8%)

EF (%)

52% ± 8.2

BMI

26.2 ± 3.8

CCS (Canadian Cardiovascular Society Angina Grading Scale) 3/4 skala zaawansowania dławicy piersiowej opracowana przez Kanadyjskie Towarzystwo Sercowo-Naczyniowe; PTCA (percutaneous coronary angioplasty) — przezskórna angioplastyka tętnic wieńcowych; CABG (coronary artery by-pass grafting) — pomostowanie aortalno-wieńcowe; CEA (carotid endarterectomy) — endarterektomia tętnic szyjnych; NYHA (New York Heart Association Functional Classification) — klasyfikacja ciężkości objawów niewydolności krążenia Nowojorskiego Towarzystwa Kardiologicznego; EF (ejection fraction) — frakcja wyrzutowa lewej komory serca; BMI (body mass index) — wskaźnik masy ciała

All patients underwent the procedure of straight aortic prosthesis implantation below the origin of renal arteries. The mean operating time was 155 minutes (140-260) and aorta cross-clamping time equalled 41.8 ± 15.4 minutes (ranges 22-97 minutes). The mean blood loss during the procedure was 569.4 ± 392 ml. Based on invasive blood pressure monitoring, the mean of minimal values for systolic blood pressure was 80 ± 13 mm Hg (range 50-120), and the mean of the maximal values equaled 160 ± 23 mm Hg (range 120-220). The mean value for minimal diastolic blood pressure was 52 ± 10 mm Hg (range 30-80), with the mean for the maximal diastolic blood pressure values equaling 88 ± 9 mm Hg (range 70-110). The chosen perioperative hemodynamic and metabolic parameters are shown in table II.

Table II. Perioperative hemodynamic and metabolic parameters
Tabela II. Zmiany wybranych parametrów hemodynamicznych i metabolicznych

Initial n = 95

1st minute post unclamping
Pierwsza minuta po odklemowaniu aorty

30th minute post unclamping
Trzydziesta minuta po odklemowaniu aorty

P value
Wartość p

pH

7.39 ± 0.05

7.33 ± 0.05*

7.37 ± 0.06*, **

0.048

Lactate
Mleczany

1.0 ± 0.4

1.5 ± 0.6*

1.9 ± 1.9*

0.047

K+ [mmol/l]

3.7 ± 0.4

3.8 ± 0.4

3.9 ± 0.4*

0.048

Hb [g/%]

11.8 ± 2.0

10.0 ± 1.8*

9.5 ± 1.8*

0.046

SBP

124 ± 16

101 ± 18*

104 ± 19*

0.001

DBP

77 ± 8

67 ± 11*

68 ± 13*

0.001

 

Hb (hemoglobin); SBP (systolic blood pressure); DBP (diastolic blood pressure); *versus initial; **versus 1st minute post aorta unclamping

Based on the biochemical results (troponine I values range: 0.01-0.2 mcg/L, CPK-MB: 17-35 IU/L) and 12-lead electrocardiographic studies, 26% of the patients were diagnosed with silent myocardial ischemia. No deaths occurred during the operation or within the first 3 postoperative days. All deaths in the early postoperative period of this study were of vascular etiology, with 6 out of 7 deaths being of direct cardiac origin, whereas one death was a consequence of massive cerebral stroke. In 4 out of 6 cardiac deaths, cardio-pulmonary resuscitation following the cardiac arrests was unsuccessful. The remaining two deaths were myocardial infarctions with severe left ventricular failure. The influence of the chosen parameters on 28-day mortality is presented in table III.

Discussion

Early 28-day perioperative mortality for elective procedures approximates 2.1 to 5.4% as reported in literature, it is 5.5% in the presented study. The main perioperative complications were: unstable angina episodes, heart failure, renal failure, intestinal ischemia and hypovolemic shock. It is estimated that some 40% of postoperative complications are classified as myocardial ischemia and it is the leading cause of early mortality. This can be related to the fact that over 60% of patients have haemodynamically significant coronary stenoses and this study further supports this finding. Given the lack of an ideal non-invasive diagnostic tool that could definitely identify patients at risk of cardiovascular complications, many scales are commonly used for cardiovascular risk stratification (Goldman scale, Glasgow Aneurysm Score, Leiden Risk Model, Revised Cardiac Risk Index) [4-9].

Table III. The influence of the chosen parameters on 28-day mortality
Tabela III. Wpływ wybranych parametrów na śmiertelność do 28. dnia po operacji

 

Non-survivors (n = 7)
Zgon

Survivors (n = 88)
Przeżycie

p

Age (years)
Wiek (lata)

70.6 ± 4.8

69.6 ± 5.4

0.114

Gender (Male/Female)
Płeć (mężczyzna/kobieta)

6 (86%)/1 (14%)

82 (93%)/6 (7%)

0.299

Epidural analgesia (Yes/No)
Znieczulenie zewnątrzoponowe

2 (29%)/5 (71%)

71 (81%)/17 (19%)

0.038

BMI >30/≤ 30

0 (0%)/7 (100%)

7 (9%)/74 (91%)

0.998

IHD (Yes/No)
IHD (Tak/Nie)

4 (57%)/3 (43%)

38 (43%)/50 (57%)

0.620

MI (Yes/No)
MI (Tak/Nie)

2 (29%)/5 (71%)

15 (17%)/73 (83%)

0.851

DM (Yes/No)
DM (Tak/Nie)

0 (0%)/7 (100%)

6 (7%)/82 (93%)

0.899

Cross-clamp time >30/≤ 30 min
Czas odklemowania aorty >30/≤ 30 min

6 (86%)/1 (14%)

72 (82%)/16 (18%)

0.566

Cross-clamp time >40/≤ 40 min
Czas odklemowania aorty >40/≤ 40 min

3 (43%)/4 (57%)

43 (49%)/45 (51%)

0.989

pH*

7.27 ± 0.08

7.34 ± 0.05

0.006

Lactate* [mmol/l]
Mleczany

2.15 ± 1.3

1.48 ± 0.5

0.035

K+* [mmol/l]

4.3 ± 0.6

3.8 ± 0.4

0.021

pH**

7.30 ± 0.08

7.33 ± 0.05

0.512

Lactate** [mmol/l]
Mleczany

1.9 ± 1.0

1.5± 0.5

0.355

K+** [mmol/l]

4.0 ± 0.6

3.8 ± 0.4

0.154

Hb**

9.6 ± 1.8

10.1 ± 1.8

0.250

Hct**

31 ± 6

33 ± 6

0.321

Max HR

118 ± 17

105 ± 19

0.121

Min HR

56 ± 16

49 ± 10

0.402

Max SBP

167 ± 35

161 ± 21

0.315

Min SBP

81 ± 17

78 ± 12

0.402

Max DBP

86 ± 12

88 ± 9

0.450

Min DBP

49 ± 12

52 ± 10

0.654

Diuresis
Diureza

300 (100-1200)

250 (80-1300)

0.321***

Blood loss >1000/≤1000 ml
Utrata krwi >1000/≤1000 ml

1 (14%)/6 (86%)

8 (9%)/80 (91%)

0.421

Blood products — PRBC/FFP (Yes/No)
Preparaty krwiozastępcze — PRBC/FFP (Tak/Nie)

3 (43%)/4 (57%)

15 (17%)/73 (83%)

0.558

BMI (body mass index); IHD (ischaemic heart disease); MI (myocardial infarction), DM (diabetes mellitus); K+ (potassium ions); Hb (hemoglobin), Hct (hematocrit), HR (heart rate); SBP (systolic blood pressure); DBP (diastolic blood pressure); PRBC (packed red blood cells); FFP (fresh frozen plasma); *1st minute after cross-clamp release; **30th minute after cross-clamp release; ***U-Mann-Whitney test

Approximating the risk for cardiac episodes by using the above mentioned scales is an initial step of patient identification, the next level is non-invasive and invasive cardiac evaluation. The aim of a detailed cardiac evaluation is to determine which of the vascular patients might benefit from coronary revascularization prior to abdominal aneurysm operations.

In this study group an increase of lactate, potassium or decrease pH levels during the first minute post cross-clamp release were significant risk factors for perioperative death. Additionally, significantly higher mortality was reported among patients without epidural anaesthesia. The importance of metabolic parameters is a recognized risk factor, however their statistical significance within the first minute post cross-clamp release is in our opinion of crucial importance. It shows definitely that the ischemic disturbances occur during the cross-clamping of a major vessel and that has a major influence on the patient’s postoperative complications. Patients presenting with significant pH decrease, as well as lactate and potassium level increase should receive extra attention in the postoperative period.

The advantages of epidural anaesthesia are widely known and cannot be overemphasized. The results may be explained as secondary to systemic vascular resistance decrease, therefore reducing the myocardial oxygen demand and improving peripheral circulation within the anaesthetized region. Excellent postoperative analgesia protects the patient from the negative consequences of vegetative stimulation, such as tachycardia and hypertension. In the opinion of the authors, epidural anaesthesia combined with general anaesthesia should be a part of an anaesthetic plan, applied to all patients operated on for abdominal aortic aneurysms.

Conclusions

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