Dorothea Jasniak

What Is CABG?

CABG stands for coronary artery bypass grafting. It is one of three treatment options for Myocardial infarction (MI) or unstable angina, resulting from occluded coronary vessels and subsequent ischaemia (oxygen depletion). The other two options are thrombolysis or PCI- percutaneous coronary intervention (16).

CABG is the recommended treatment of choice for patients with the criteria shown above  but also for Diabetics (15).

Why?

1. Diabetics present with above indicted nature of blockages (2).
2. More likely to experience restenosis (renewel of an occlusion) and stent complications due to higher predilection for atherosclerosis and vascular remodelling/narrowing (3). Thus, PCI is more likely to cause complications (15).

The Freedom Trial describes the relevance of CABG with regard to diabetes

Background

For CABG surgery in recent decades outcomes have greatly improved with mortality dropping to as little as 1-2%. But, among the remaining fatal and readmission cases, a disproportionate amount is composed of diabetics. (15)

Diabetes has potential of aggrevating common CABG complications like so:

Strokes:  Diabetes is a known risk factor for post-MI (myocardial infarction) strokes due to its pro-inflammatory pathology, lactate-acidosis by inducing anaerobic metabolism (11), and increased thrombogenesis and platelet activity (18).

Arrhythmias: the most common CABG complication, including all types of abnormal heart rhythms, (1) is increasingly likely to affect diabetics and to result in worse, often fatal, outcomes (7).

Atherosclerosis: the primary cause for CAD but also for CABG readmission due to reocclusion of grafts. Atherosclerosis is the narrowing of vessels by fatty deposits in their walls, called a plaque. Diabetics are more prone for readmission and the risk of death from reoccurent MIs because their vessels are more likely to become reoccluded and their plaques to burst, forming blood clots that occlude the vessel further (3,4,17). 

Acute kidney failure:  Diabetes induces kidney damage via diabetic nephropathy and hypertension (9).    A 2x higher risk was identified (19). 

Impaired wound healing and vascular regeneration:  Hyperglycaemia impairs the immune system and wound healing by reducing collagen deposition (8). Combined with the pro-inflammatory effects of hyperglycaemia (10), diabetics are at higher risk of post-surgical inflammations of their grafted vessels. 

Furthermore,

Diabetes in itself is among the EUROSCORE risk factors.

EUROscore is a logistic regression statistical determination of risk of post- CABG mortality. It highlights conditions, operation-, and patient factors which increase this risk (1).

Besides being a known condition elevating mortality risk (20), diabetics commonly present with comorbidities that are also included in the EUROscore (see picture right). These include:

• Renal impairment ig. raised Creatinine (12, 19)
• Heart Failure (7)
• Hypertension (4)
• Prior MIs (4)
• Vascular complications (7)

Diabetics are a patient group requiring even greater attention and targeted care.                                 

Aim:   As demonstrated above, diabetes has the potential to greatly increase the risk of CABG complications and mortality. Both directly via its pathology and due to its comorbid nature. 

This study aims to investigate to what extent diabetics are at elevated risk of post-CABG mortality.                                                                                                                        In addition, little current literature is availabe on their risk for Arrythmias. So, this study aimed to analyze whether arrhythmias affect Diabetics more frequently as well.

Combined this would raise attention towards providing more care for these patients and help determine if they would increasingly benefit from specialized pre- and postoperative management strategies.  

Methods and Results

1. Literature Search

A systematic search of current literature regarding the effects of diabetes on post-CABG complications and mortality was conducted. The key points of uncertainty that the study focused to explore entailed:

• Whether diabetes increases long and/or short-term mortality? If only long-term, would diabetics benefit more from long-term management strategies rather than only in-hospital measures? 

• • Are there differences among  diabetic groups varying according to their treatment?

• • • Insulin treatment is popularly used as predictor of illness severity. However if this is not true, could this potentially lead to other diabetics being neglected from pre-operative management strategies?

• • • • Are diabetics at higher risk of arrhythmia post-CABG?

2. Data Collection and tailoring according to exclusion criteria 

a) The data was obtained from the Blackpool Victoria hospital (BVH) departmental data base. The longer-term outcome data stems from NHS digital.

b) Exclusion criteria were applied and entailored to only include: patients who underwent isolated CABG within the 20-year period between 1999 and 02.12.2021 

The endpoints were:  30 days, 1-year, 5-year, and 10-year mortality and Arrhythmias

c) Patients were divided into non-diabetic, insulin-dependent, tablet-, and diet-controlled diabetics.         In total 13586 patients analysed.

3. Data analysis using statistical tools

The patients´ EUROscore risk factors were compared using X² tests for the dichotomous variables and ANOVA for the continous variables, namely age. This helped determine whether diabetics truly presented with more comorbidities.

EUROscore factors among the patients

Blue- conditions more prevalent in all diabetics

Red- conditions varying in Insulin-dependent diabetics vs other groups

4. Mortality Analysis

Patients´ mortality incidence was stratified into one of four outcomes: 30 day, 1-, 5-, 10-year mortality. For this, subtraction as used;   the date of death - the date of surgery

To evaluate whether significant differences in mortality and survival exist, two statistical tools were applied:

• X² test

• Kaplan Meier Survival Curve

5. Arrhythmia Analysis:      

The postoperative incidence of arrhythmias was identified by counting: the frequency of new-onset arrhythmias and excluding all patients who presented with any non-sinus (abnormal) rhythms pre-surgery.

The significance was calculated via a X² test.

Conclusion and Limitations

The analysis demonstrated five key findings which can help in reducing risk of post-CABG mortality/complications. 

Findings

1) Insulin-dependent diabetics had a significantly reduced survival and higher frequency of postsurgical deaths than all others. 

This was despite them presenting with less EUROscore risk factors than other diabetics, namely:                                                                     -> less females       ->younger mean age 

2) Differences between Insulin-dependent and non-insulin diabetics, namely diet- and tablet-treated patients, were identified.                                                    As insulin is often used as predictor of illness severity, with other treatments no longer sufficing, this group´s worse pathology could have caused a higher mortality risk.

3) Within mortality differences, significant deviation in survival was only detected from 5 years onward. It is thus long-term survival that is likely to suffer among diabetics. 

4) The lack of elevated frequency of Arrhythmias among diabetics compared to non-diabetics contrasts findings of similar studies (7). A potential reason lies in the short-term follow-up for this outcome, only in-hospital arrhythmias being recorded.

However it also suggests the efficiency of Blackpool Victoria hospital´s presurgical prophylactic measures against arrhythmias. By treating all patients with presurgical antiarrhythmics, differences in arrhythmia risk could have been diluted (14).

5) Despite these measures, Diet-controlled patients still presented with more arrythmias. A group often neglected due to their presumely weaker severity of disease, may potentially have excluded them from specialized antiarrhythmic measures provided to other diabetics. This possibly elevated their incidence of this complication. 

Implications for risk reduction and suggestions for future research.

1+2) Insulin patients being most at risk  and showing differences to both non-diabetics (5, 21) and to other diabetic groups (12), was similarly found by current studies. This underlines how especially insulin-treated diabetics require greater attention to reduce their elevated risk.

Further research could investigate whether insulin treatment in itself may induce the risk of postsurgical complications?

3) More research is needed to evaluate the potential benefit of long-term monitoring/ management strategies to reduce the difference in survival diabetics exhibit in the long term. 

4) It emphasizes the need for long-term follow-ups to evaluate arrhythmia outcomes. 

• Whether the current in-hospital measures suffice or longer term monitoring/management is needed, entails an area requiring further research.  

5) Antiarrythmics and other measures used to reduce arrhythmia risk, should be extended to all diabetic groups, including diet-controlled patients. 

Acknowledgements: I wish to acknowledge the ongoing support and guidance I have received from the team of the cardiac surgery unit at BVH. In particular, I am grateful for the feedback I received from Mr Walker, acknowledging his expertise in cardiothoracic surgery and research as well as his dedication in helping me with this project.

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