What Are the Most Common Surgical Complications?

Roughly one in six patients who undergo surgery in a hospital will experience at least one complication, ranging from a mild wound issue that resolves on its own to a life-threatening organ failure. A large international study of elective surgery across 27 countries found complications in about 17% of patients, with an in-hospital death rate of 0.5%.¹ That one-in-six figure hides enormous variation depending on the type of operation, the patient’s health going in, and where in the world the surgery takes place. Understanding what can go wrong, who is most vulnerable, and what actually prevents complications is worth knowing whether you’re facing surgery yourself or helping someone who is.

How Complications Are Graded

Not all complications are equal, and surgeons use a standardized grading system to distinguish a nuisance from a catastrophe. The most widely adopted version, known as the Clavien-Dindo classification, sorts complications by the treatment they require rather than by what went wrong anatomically. A Grade I complication needs only minor bedside care, like an antiemetic for nausea. Grade II requires medication such as antibiotics or a blood transfusion. Grade III means a return to the operating room or an interventional procedure. Grade IV involves organ failure requiring intensive care. Grade V is death.²

In the original study that validated this system, which followed over 6,300 patients, about 7% had a Grade I complication, 4% had a Grade II, and roughly 5% had Grade III or above. The mortality rate was 1.2%.³ What this tells you is that the majority of complications are minor and manageable, but that a meaningful fraction demand serious intervention. It also gives hospitals a common language: when a surgeon in Tokyo and a surgeon in Toronto both report Grade IIIb complications, they mean the same thing, which makes comparing outcomes across hospitals possible.

The Global Picture

Where surgery happens matters enormously for how safe it is. An international estimate suggests that roughly 50 million patients experience complications after surgery each year worldwide, with more than 1.5 million dying from those complications. When researchers studied cancer surgery across 82 countries, they found that patients in lower-income nations were far more likely to die after a major complication than patients in wealthier ones. For colorectal cancer, 30-day mortality in lower-income countries was over four times higher than in high-income countries after adjusting for patient characteristics.⁴ The gap is not just about surgical skill. It reflects differences in intensive care availability, blood banks, anesthesia staffing, and the ability to rescue patients once something goes wrong.

Surgical Site Infections

Infection at the incision site is one of the complications patients worry about most, and for good reason. It can delay healing, require additional surgery, and in severe cases become life-threatening. One reason surgical site infections are stubborn is that bacteria can form biofilms, dense colonies that stick to tissue or implanted materials and shield themselves from antibiotics and the immune system.⁵ In a study of patients who had persistent infections after surgery, every single one was found to have biofilm on their suture material. Removing that foreign material cured the infection in all cases.⁶

Preventing infection starts before the incision is even made. The skin is cleaned with antiseptic solution, and a long-running debate pits two main options against each other: povidone-iodine and chlorhexidine in alcohol. An earlier trial found chlorhexidine-alcohol cut overall infection rates almost in half compared to povidone-iodine, from about 16% to under 10%.⁷ But a more recent large trial comparing povidone-iodine in alcohol to chlorhexidine in alcohol found no meaningful difference, with infection rates of about 5% in both groups.⁸ The key distinction is the alcohol base. When both antiseptics are dissolved in alcohol, they appear to work about equally well. The earlier result likely reflected the fact that povidone-iodine was tested in a water-based form, which is less effective at penetrating bacterial layers on skin.

Blood Clots and Heart Events

Surgery increases the risk of blood clots forming in the deep veins of the legs, which can break loose and travel to the lungs. This risk is particularly high after joint replacement and cancer operations, where a combination of vein damage, immobility, and substances released from the surgical site ramp up the clotting system.⁹ Prevention involves either blood-thinning medications or mechanical compression devices on the legs. A meta-analysis comparing the two found that compression devices and blood thinners prevented clots at similar rates, but compression devices carried about half the risk of causing postoperative bleeding.¹⁰ There is a catch, though: compression stockings alone are not very effective after knee surgery, so surgeons often use both approaches together for high-risk joint procedures.¹¹

The heart faces its own stresses during and after surgery. Myocardial injury after noncardiac surgery, a condition where the heart muscle is damaged by reduced blood flow during the perioperative period, occurs in roughly 20% of patients undergoing major inpatient operations. Most cases produce no chest pain or obvious symptoms, which is why they go undetected unless cardiac biomarkers are measured with blood tests. Even silent injury is tied to a clear increase in both short-term and long-term death risk.¹² The incidence climbs further when hospitals use newer, more sensitive blood tests, reaching 20 to 30%.¹³ This is one of those areas where the science is ahead of routine practice: routine screening for cardiac injury after major surgery is not yet standard everywhere, even though the evidence suggests it could identify high-risk patients who need closer monitoring.

Lung, Kidney, and Nerve Complications

Lungs can collapse partially after surgery, a condition called atelectasis that is especially common after abdominal and chest operations. Risk factors include being over 50, having a very low or very high body mass index, and needing an incision in the upper abdomen. In one study of emergency abdominal surgery patients, about 28% developed lung complications, and those patients had significantly longer hospital stays and higher death rates.¹⁴ Positive-pressure breathing support, either through a mask or a specialized device, can help re-expand collapsed lung tissue in high-risk patients.¹⁵

The kidneys are vulnerable to drops in blood pressure during surgery. A systematic review found that when mean blood pressure falls below 65 mmHg for more than five minutes, the risk of acute kidney injury rises in a dose-dependent way: the lower the pressure and the longer it stays down, the more likely the kidneys are to be damaged.¹⁶ A separate analysis confirmed that both the depth of low blood pressure and the amount of medication used to bring it back up were independently associated with kidney injury.¹⁷ This highlights a tricky balance for anesthesiologists: keeping blood pressure high enough to protect the kidneys without pushing it so high that it strains the heart.

Peripheral nerve damage is a less discussed but real complication of general anesthesia. When you are unconscious, you cannot feel or respond to pain from an awkward position, so nerves can be compressed or stretched for hours. Stretching a nerve by as little as 5% can begin to choke off its blood supply, and animal studies show that 12% stretch for an hour causes permanent damage.¹⁸ The ulnar nerve at the elbow is the most commonly injured, partly because it sits close to the surface and is less protected in men than in women. Careful padding and arm positioning during surgery is the primary defense.

Ileus and Postoperative Delirium

After abdominal surgery, the gut often temporarily stops working. This condition, called postoperative ileus, leaves patients bloated, nauseated, and unable to eat. Its causes are tangled together: the body’s inflammatory response to being cut open, the stress hormones flooding the system, opioid pain medications that slow the gut, and shifts in electrolytes.¹⁹ The inflammatory component is thought to be central, though much of the detailed timeline has only been mapped in animal models, so the human picture is still being refined.²⁰ Reducing opioid use after surgery, early walking, and chewing gum (which stimulates gut reflexes) are all strategies that have shown some benefit.

For older adults, delirium is among the most feared surgical complications. It presents as sudden confusion, disorientation, or agitation, and it can last days. The aging brain shares certain vulnerabilities with the processes that trigger delirium, including increased susceptibility to inflammation and reduced cognitive reserve.²¹ Delirium is not just an unpleasant episode: it is associated with longer hospital stays, a higher chance of being discharged to a care facility rather than home, and an increased risk of lasting cognitive decline.

Who Faces the Highest Risk

Frailty is one of the strongest predictors of surgical trouble, and its effect is dramatic. A meta-analysis of nearly 700,000 patients across a wide range of surgeries found that frail patients were about 50% more likely to experience any complication, and twice as likely to experience a major one.²² An analysis using the FRAIL scale in older surgical patients found even steeper odds, with frail patients more than three times as likely to develop complications overall and more than two and a half times as likely to develop delirium.²³ Among emergency abdominal surgery patients, the most severely frail had a 30-day mortality rate of 33%, compared to under 4% for the fittest group.²⁴

Blood sugar control is another major factor, and here the science holds a surprise. You would expect patients with diabetes to fare worse, and uncontrolled diabetes does raise risk: patients with higher long-term blood sugar markers and high postoperative blood sugar were over four times as likely to develop major complications.²⁵ But at any given level of high blood sugar after surgery, patients without a prior diabetes diagnosis actually fared worse than those with known diabetes. At the highest blood sugar levels, about 38% of patients without diabetes developed a complication, compared to 24% of patients with diabetes.²⁶ The likely explanation is that the body of someone who has lived with diabetes has adapted somewhat to higher glucose levels, while the same spike in a person whose metabolism is not accustomed to it signals a more extreme stress response. This finding underscores why monitoring blood sugar after surgery matters for all patients, not just those with a diabetes diagnosis.

Does the Hospital Matter

It does, and the evidence is overwhelming. A scoping review found that nearly 87% of studies examining hospital volume and surgical outcomes reported a statistically significant relationship: higher-volume hospitals had better results.²⁷ The effect is especially stark for complex procedures. For pancreatic surgery, patients at high-volume hospitals had roughly 65% lower odds of dying after the operation and modestly lower odds of major complications.²⁸ This is not just a proxy for having a skilled surgeon: high-volume centers tend to have more experienced nursing staff, better intensive care units, faster responses to deteriorating patients, and established protocols for managing complications when they arise. If you have any ability to choose where a complex operation takes place, asking about the hospital’s volume for that specific procedure is one of the most impactful things you can do.

Checklists, Recovery Protocols, and Surgical Approach

One of the simplest interventions in modern surgical safety is a piece of paper. The WHO Surgical Safety Checklist, introduced in 2008, walks the operating team through a series of checks before anesthesia, before the first incision, and before leaving the operating room. In the original global study, complications dropped from 11% to 7%, and the death rate fell from 1.5% to 0.8% after checklist introduction.²⁹ Additional studies have confirmed the effect, with one review reporting relative improvements in mortality of 47% and 62% across two separate outcome studies.³⁰ The gains come not from any single check but from improving communication: forcing the team to pause together, confirm the correct patient and procedure, discuss anticipated blood loss, and verify antibiotic timing.

Enhanced Recovery After Surgery programs, commonly known as ERAS, bundle together a set of evidence-based steps that span the entire surgical journey: eating up until a few hours before surgery instead of fasting overnight, minimizing opioid use, getting patients out of bed and walking within hours of the operation, and removing drains and catheters as early as safely possible. In colorectal surgery, ERAS protocols reduced moderate to severe complications from about 30% to 25%.³¹ In foregut surgery, ERAS patients had 41% lower odds of complications and about a third shorter hospital stays.³² Patients who actually completed all the steps in the protocol did even better than those who deviated from it, which is a reminder that these programs only work when both the team and the patient commit to them.

How the surgery itself is performed also shapes complication risk. A meta-analysis comparing robotic, laparoscopic, and open approaches for cancer surgery found that robotic surgery was associated with fewer complications, fewer blood transfusions, and lower mortality compared to both laparoscopic and open approaches.³³ In a large colorectal surgery comparison, robotic patients had average hospital stays of about 5.6 days versus 7.9 for laparoscopic and 11.2 for open surgery.³⁴ Minimally invasive approaches are not suitable for every patient or every operation, but where they can be used, the smaller incisions translate to less tissue trauma, less pain, and a faster return of gut function.

The Financial Burden

Complications are expensive for everyone involved. An analysis of Medicare claims found that patients with complications stayed in the hospital an average of 12.4 days compared to about 4 days for those without. The estimated cost to the hospital was more than double, and because reimbursement does not increase proportionally, hospitals actually lost money on patients with complications: on average a negative margin of nearly $4,700 per case, compared to a positive margin of about $480 per uncomplicated case.³⁵ A broader study confirmed this pattern, finding that risk-adjusted hospital costs were roughly $20,000 higher for patients with complications and that profit margins dropped from about 6% to near zero.³⁶ This economic reality means hospitals have a financial incentive that aligns with patient interest: preventing complications saves money as well as lives and suffering.

The Toll on Surgeons

Complications do not only affect patients. Surgeons who experience adverse events during or after an operation are sometimes called “second victims,” and the psychological impact can be profound. In one survey, over 90% of surgeons reported experiencing an intraoperative adverse event, and almost every respondent admitted to strong feelings of anxiety, guilt, sadness, or shame afterward.³⁷ A systematic review and meta-analysis confirmed that adverse events commonly trigger sleep disturbance and burnout symptoms in surgeons, and highlighted the need for peer support programs and resilience training within surgical departments.³⁸ The culture of surgery has historically expected stoicism, but a surgeon who is sleep-deprived and emotionally shaken by a complication in one patient is arguably a risk factor for the next. Institutional support for surgeons dealing with bad outcomes is slowly being recognized as a patient-safety issue, not just a wellness initiative.

What You Should Ask Before Surgery

If you are facing an operation, you have a legal right to be told about the risks before consenting. In many jurisdictions, the standard has shifted toward disclosing whatever a reasonable patient would consider important, rather than whatever surgeons typically mention.³⁹ In practice, what you hear varies widely. A study of surgeons performing the same procedure found disturbing variation in which complications each surgeon chose to discuss; some risks were mentioned by nearly everyone while others, including serious ones like bile duct injury during gallbladder removal, were inconsistently disclosed.⁴⁰

This means it is worth being an active participant in the consent conversation rather than treating it as a form to sign. Ask your surgeon what the most common complications are for your specific procedure, how often they see them in their own practice, and what the plan is if one occurs. Ask about the hospital’s volume for the operation and whether an enhanced recovery protocol will be used. If you are over 65 or have chronic conditions, asking about frailty assessment and delirium prevention is reasonable. The consent form is a legal record, but the conversation is where trust is built and where you can gauge whether your surgical team communicates clearly and takes your concerns seriously.⁴¹

1
BJA: British Journal of Anaesthesia. Global patient outcomes after elective surgery: prospective cohort study in 27 low-, middle- and high-income countries ↩︎
2
PubMed. The Clavien-Dindo classification of surgical complications: five-year experience ↩︎
3
PubMed Central. Classification of Surgical Complications: A New Proposal With Evaluation in a Cohort of 6336 Patients and Results of a Survey ↩︎
4
The Lancet. Global variation in postoperative mortality and complications after cancer surgery: a multicentre, prospective cohort study in 82 countries ↩︎
5
PubMed Central. Recent advances in the treatment of biofilms induced surgical site infections ↩︎
6
PubMed Central. Bacterial biofilms on implanted suture material are a cause of surgical site infection ↩︎
7
PubMed. Chlorhexidine-Alcohol versus Povidone-Iodine for Surgical-Site Antisepsis ↩︎
8
JAMA. Povidone Iodine vs Chlorhexidine Gluconate in Alcohol for Preoperative Skin Antisepsis: A Randomized Clinical Trial ↩︎
9
Blood Reviews. Basic mechanisms and pathogenesis of venous thrombosis ↩︎
10
PubMed. Mechanical compression versus subcutaneous heparin therapy in postoperative and posttrauma patients: a systematic review and meta-analysis ↩︎
11
JAMA Internal Medicine. Ardeparin (Low-Molecular-Weight Heparin) vs Graduated Compression Stockings for the Prevention of Venous Thromboembolism ↩︎
12
PubMed. Diagnosis and Management of Patients With Myocardial Injury After Noncardiac Surgery: A Scientific Statement From the American Heart Association ↩︎
13
PubMed Central. Postoperative Myocardial Infarction after Non-Cardiac Surgery: An Update ↩︎
14
PubMed. Risk factors for pulmonary complications after emergency abdominal surgery ↩︎
15
PubMed Central. Perioperative Pulmonary Atelectasis: Part II. Clinical Implications ↩︎
16
The American Journal of Surgery. Intraoperative hypotension and postoperative acute kidney injury: A systematic review ↩︎
17
British Journal of Anaesthesia. Association of intraoperative hypotension and cumulative norepinephrine dose with postoperative acute kidney injury in patients having noncardiac surgery ↩︎
18
Anesthesia & Analgesia. Perioperative Peripheral Nerve Injury After General Anesthesia: A Qualitative Systematic Review ↩︎
19
PubMed. Postoperative ileus: mechanisms and future directions for research ↩︎
20
PubMed. Post-operative ileus: definitions, mechanisms and controversies ↩︎
21
PubMed Central. Postoperative Delirium and the Older Adult: Untangling the Confusion ↩︎
22
The American Journal of Surgery. Impact of frailty on outcomes in surgical patients: A systematic review and meta-analysis ↩︎
23
PubMed Central. Association Between the FRAIL Scale and Postoperative Complications in Older Surgical Patients: A Systematic Review and Meta-Analysis ↩︎
24
BJS Open. Impact of clinical frailty on surgical and non-surgical complications after major emergency abdominal surgery ↩︎
25
Journal of Trauma and Acute Care Surgery. Perioperative glycemic control and postoperative complications in patients undergoing emergency general surgery: What is the role of Plasma Hemoglobin A1c? ↩︎
26
JAMA Surgery. Paradoxical Association of Hyperglycemia and Surgical Complications Among Patients With and Without Diabetes ↩︎
27
PubMed Central. Assessing the hospital volume-outcome relationship in surgery: a scoping review ↩︎
28
PubMed. The relationship of hospital and surgeon volume indicators and post-operative outcomes in pancreatic surgery: a systematic literature review, meta-analysis and guidance for valid outcome assessment ↩︎
29
PubMed. A surgical safety checklist to reduce morbidity and mortality in a global population ↩︎
30
PubMed Central. The effect of the WHO Surgical Safety Checklist on complication rate and communication ↩︎
31
JAMA Surgery. Association Between Use of Enhanced Recovery After Surgery Protocol and Postoperative Complications in Colorectal Surgery ↩︎
32
PubMed. Enhanced recovery after surgery (ERAS) decreases complications and reduces length of stay in foregut surgery patients ↩︎
33
PubMed Central. The COMPARE Study: Comparing Perioperative Outcomes of Oncologic Minimally Invasive Laparoscopic, da Vinci Robotic, and Open Procedures A Systematic Review and Meta-analysis of the Evidence ↩︎
34
PubMed Central. Outcomes Comparing 19,769 Patients Who Underwent Robotic, Laparoscopic, or Open Colorectal Surgery ↩︎
35
PubMed Central. Impact of surgical complications on hospital costs and revenues: retrospective database study of Medicare claims ↩︎
36
JAMA Surgery. Hospital and Payer Costs Associated With Surgical Complications ↩︎
37
Annals of Surgery. When Things Go Wrong: The Surgeon as Second Victim ↩︎
38
PubMed Central. Second victim syndrome in surgeons: systematic review and meta-analysis of the impact of adverse events on surgeons ↩︎
39
Surgical Practice. Risk disclosure and informed consent: Practice caveats in the post Montgomery era ↩︎
40
PubMed Central. Consent and complications: risk disclosure varies widely between individual surgeons ↩︎
41
PubMed Central. Informed consent for surgery: risk discussion and documentation ↩︎