Several studies have already proven that smoking leads to higher complication rates after surgery. A study published in Arthritis Care and Research studied a cohort of nearly 33 thousand patients who underwent either total hip replacement or total knee replacement, of whom 57% were non-smokers, 19% were prior smokers, and 24% were current smokers. This study found a clear statistical relationship that suggested that current smokers were more likely to have complications such as pneumonia, stroke and wound infections. They were also more likely to need a post-operative intensive care and presented higher mortality rates.
For instance, the adjusted odds ratio for Surgical Site Infection (SSI) was 1.41 for current smokers. Smoking also increased the incidence of pneumonia with an adjusted odds ratio of 1.53. The one-year mortality odds ratio for current smokers was 1.63.
Here, it is important to mention that often, a writer or reader that doesn’t fully understand the odds ratio concept tends to over-simplify it. Erroneously someone may interpret an odds ratio of 1.41 as a 41% relative risk. While stating relative risk, it is also expected that the authors provide the absolute change in risk, but that is not routinely practiced.
Why it is important to state all the facts for the benefit of average reader will become clear with this example.
Demystifying odds ratios
Suppose a cohort of 1000 rheumatoid arthritis patients watched for the duration of 5 years had a 4 % stroke events i.e. 40 patients suffered a stroke during the watch period. Another cohort with all other similar average characteristics such as age, race, lifestyle had only a 2% incidence of stroke events i.e. 20 strokes out of 1000.
Odds Ratio will be (40/960) divided by (20/980) = 2.04
Relative Risk for RA Patients to suffer from stroke = (40/1000) divided by (20/1000) = 2
Thus, here we see that because RA-induced stroke is a rare event i.e. Odds ratio, and Relative Risk ratio is in proximity. Such an odds ratio is commonly communicated in a hyperbole with headlines such as “RA patients are twice more likely to suffer a stroke!”.
In the absence of absolute risk data, it can mislead non-suspecting readers to assume the worst. For instance, in this case, only 2 percent more i.e. 20 more patients from the RA cohort are expected to suffer from a stroke in a sample size of 1000 patients. The risk induced by RA is an additional 2 percent, but that would not make for an attention-seeking headline, would it?
More importantly, an odds ratio that is above 1 does indicate a positive relation between the disease and the stroke event and serves its purpose to highlight RA as having an impact on the cardiovascular health.
(Disclaimer: Above sample data is used to demonstrate the difference between odds ratio, relative risk, and the absolute risk, and doesn’t have any medical significance.)
In case you are inclined to learn more about these, here are some useful articles.
How smoking complicates post-surgery recovery
The higher odds ratios indicate a positive association between smoking and a higher incidence of various complications. However, many patients may wonder how exactly smoking causes these complications because the connection between your lungs and a successful knee replacement is not very clear. Here’s how.
Smoking Reduces Oxygen Supply
Oxygen is required by the cells in our body to carry out various functions. However, the molecules called Hemoglobin, that carry Oxygen from the lungs to all parts of our body, do not absorb as much Oxygen when they are exposed to smoke. The need for Oxygen in the tissue present near the surgical site is even higher as the injured tissue tries to regrow and heal. Further, smoking also makes the blood thicker due to the absorption of various tar like chemicals, causing the tiny blood vessels or capillaries to get narrower. This again inhibits a proper supply of Oxygen to the healing tissues. A reduced blood supply can also interfere and limit the growth of musculoskeletal tissues and delay the recovery. Physical therapy is often advised after such surgeries to help muscles regain their strength but the recovery may be delayed if enough Oxygenated blood doesn’t reach the surgical site.
Smoking Weakens Infection Fighting Ability
Smoking also reduces the body’s natural ability to fight off infections. Research suggests that chemicals present in smoke limit the activity of infection-fighting cells called Neutrophils. The primary role of Neutrophils is to attack any foreign body such as bacteria and ward off infection. When the neutrophil activity is diminished, the chances of infection setting in the wound area or the surgical site area increase.
Smoking Increases Blood Pressure
Smoking is known to increase the blood pressure, which can lead to various cardiovascular events, especially when the patient is in recovery mode after surgery. Reports even suggest that smoking can even interfere with respiratory functions during and after anesthesia, thereby increasing the mortality risks to the patient.
Can anything be done to prevent these risks?
Fortunately, research has proven that even if a person who has been a regular smoker can quit smoking at least 6-8 weeks before the surgery, and ceases to smoke after surgery, the rate of complications is much lower than otherwise. There are many other risk factors for surgery that are not always avoidable such as when a person is suffering from chronic diseases like Diabetes, or hypertension, but habits like smoking are avoidable risks. If it is not easy for a patient to give up smoking, counseling can help. A study carried out in Denmark also found that prior smokers who quit smoking were more likely to continue that path even after one year from surgery (22% vs. 3%) compared to smokers who continued with the habit.
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- Smoking as a risk factor for short‐term outcomes following primary total hip and total knee replacement in veterans; Singh, Jasvinder A.; Houston, Thomas K.; Ponce, Brent A.; Maddox, Grady; Bishop, Michael J.; Arthritis Care and Research, Volume 63 (10) – Oct 1, 2011
- Long‐term effects of a preoperative smoking cessation programme; Villebro, Nete; Pedersen, Tom; Møller, Ann M.; Tønnesen, Hanne; The Clinical Respiratory Journal, Volume 2 (3) – Jul 1, 2008
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