New-onset postoperative atrial fibrillation (POAF) is a serious complication of adult cardiac surgery that occurs in 20% to 55% of cases. It should be differentiated from recurrent atrial fibrillation (AF) after surgical ablation for preoperative AF because patients with new-onset POAF after cardiac surgery had no history of ever having AF before the surgery. New-onset POAF is associated with an increase in perioperative stroke, myocardial infarction, ventricular tachycardia or fibrillation (cardiac arrest), need for new pacemakers, intensive care unit days, hospital stay, and 30-day operative mortality. , In addition, long-term survival is less in patients who develop new-onset POAF after coronary artery bypass grafting (CABG) than in those who do not develop new-onset POAF ( Fig. 47.1 ). In 90% of these patients, new-onset POAF manifests within the first 6 days after surgery, coinciding with the peak of the postoperative systemic inflammatory response. This condition imposes a substantial clinical and financial burden in the United States, with annual health care costs exceeding $1 billion, prolonged hospital stays, and increased stress on patients and their families. Thus, there is a clear need for effective preventative strategies and management to mitigate the impact of new-onset POAF.
Scope of postoperative atrial fibrillation (POAF). The development of postoperative atrial fibrillation (POAF) after coronary artery bypass grafting (CABG) is associated with decreased long-term survival compared with patients who do not develop POAF.
(Reproduced from Almassi GH, Schowalter T, Nicolosi AC, et al. Atrial fibrillation after cardiac surgery: a major morbid event? Ann Surg. 1997;226(4):501–513.)
Currently, there is no way to predict which patients will develop new-onset POAF after cardiac surgery, and there is no prophylactic strategy that is universally agreed on to prevent it. Furthermore, it is virtually impossible to assess the true efficacy of any prophylactic regimen designed to prevent new-onset POAF because it is impossible to control for all of the variables in a cardiac surgical procedure that can impact the development of new-onset POAF. Therefore, designing randomized controlled clinical trials to determine the efficacy of prophylactic measures to prevent new-onset POAF after cardiac surgery is difficult at best and usually a futile endeavor.
Preoperative Vulnerability to the Development of New-Onset Postoperative Atrial Fibrillation
Why do some patients develop new-onset POAF after cardiac surgery while others do not? Perhaps the most interesting aspect of new-onset POAF is that it does not develop in roughly two-thirds of cardiac surgery patients regardless of how complex the surgical procedure might be. The reported incidence of new-onset POAF after cardiac surgery varies widely because of a variety of factors, including, among others, the unknown number of patients receiving new-onset POAF prophylaxis in the studies.
New-onset POAF risk factors can be categorized into two groups: modifiable and nonmodifiable. Nonmodifiable factors include age, race, gender, heart failure, and valvular heart disease. Among these, age has been most strongly associated with the development of new-onset POAF after cardiac surgery, likely because of age-related slowing of conduction and an increase in myocardial fibrosis. A large retrospective study involving more than 140,000 patients undergoing noncardiac surgery revealed that the incidence of new-onset POAF varies by both age and sex. Although no significant difference in new-onset POAF risk was observed between sexes in patients younger than 40 years, male patients in older age groups had a significantly higher risk compared with female patients. Additionally, a comparative study demonstrated that men exhibited higher postoperative peak lactate levels and a greater incidence of death during a 4-year follow-up period. Racial disparities have also been reported, with White patients undergoing CABG shown to have a higher incidence of new-onset POAF compared to non-White patients. However, studies assessing the impact of race on new-onset POAF are often limited by a significantly lower representation of non-White patients.
In contrast, modifiable factors, such as obesity and high body mass index (BMI), present opportunities for intervention. Obesity and elevated BMI predispose patients to hypertension and diabetes, which serve as secondary risk factors for new-onset POAF. Another modifiable risk factor is lifestyle choices, which significantly impact the risk of new-onset POAF after cardiac surgery. Maintaining a healthy weight, engaging in regular exercise, limiting alcohol intake, smoking cessation, stress management, adequate sleep, and a balanced diet can help regulate blood pressure, cholesterol levels, and overall heart health, potentially reducing the likelihood of developing new-onset POAF. In this context, patient education is crucial to empower individuals to make informed decisions and adopt heart-healthy habits.
If all cardiac surgery patients, regardless of age, sex, type of surgery, comorbidities, and with no new-onset POAF prophylaxis are included, an average of approximately 35% of them will develop new-onset POAF ( Fig. 47.2 ). Thus if 100 patients undergo cardiac surgery without any prophylaxis, an average of 65 of them will not develop POAF, and 5 of them will develop new-onset POAF even after having the simplest noncardiac surgery. This indicates that these latter 5 patients are far more vulnerable to the development of new-onset POAF than either the 30 patients who develop new-onset POAF only after cardiac surgery or the 65 patients who do not develop new-onset POAF at all. The reason for this variability in the preoperative vulnerability of patients to the development of new-onset POAF after cardiac surgery is unknown.
Scope of postoperative atrial fibrillation (POAF). After adult cardiac surgery of all types, if prophylactic measures against POAF are not instituted preoperatively, an average of approximately 35% will develop POAF. This includes 30% who develop POAF only after undergoing cardiac surgery and 5% who have POAF after any cardiac or noncardiac surgery. The incidence of POAF varies between types of cardiac surgery performed. Interestingly, approximately 65% of patients will not develop POAF no matter how complex the surgical procedure.
All arrhythmias, including new-onset POAF, require two factors to develop: (1) abnormal dispersion of refractoriness (nonuniform recovery) and (2) a trigger. The possible triggers for new-onset POAF are numerous and can include inflammation from postoperative pericarditis, surgical trauma, general anesthesia, elevated circulating catecholamines, autonomic nervous system imbalance, and many other perioperative factors. However, those factors occur with similar frequency in the extremely vulnerable 5% group, the somewhat vulnerable 30% group, and the invulnerable 65% groups described. Therefore, differences in the presence of triggers cannot explain the differences in the vulnerability of the three groups to the development of new-onset POAF. Instead, it is obvious that some abnormality in the pattern of electrical recovery (repolarization) in the atria renders some patients vulnerable and other patients invulnerable to the development of new-onset POAF.
In the 1990s we identified experimentally at least one abnormality to explain why some patients present for cardiac surgery with atria that are more vulnerable than others to developing new-onset POAF, the distribution of local refractory periods in the atria . The construction of “refractory period distribution maps” of local refractory periods is laborious and time consuming and therefore, it is feasible only under experimental conditions. Local refractory period maps require that the local refractory period be determined for each single site in the atrium by repeatedly pacing at that single site with a progressively decreasing pacing interval until failure of capture occurs. The longest cycle length at which the pacing fails to capture is the local refractory period for that particular site but only for that particular site. To construct the global pattern of local refractory period distribution, this same pacing process must be repeated at multiple sites in the atria.
As explained in Chapter 4 , the local refractory periods are shorter in the left atrium (LA) than in the right atrium (RA; see Fig. 4.15 ). The distribution of local refractory periods normally exhibits a smooth transition from the shorter ones in the LA to the longer ones in the RA ( Fig. 47.3 ). If a patient has this distribution of local refractory periods in the atria, it is highly unlikely that any perioperative trigger can induce AF. This is analogous to the inability of premature atrial beats (triggers) to induce AF in an atrium that is incapable of sustaining a macro-reentrant circuit (see Chapter 5 , Fig. 5.33 ). Thus this is the normal pattern of refractory period distribution in the atria that can be expected in the 65% of cardiac surgery patients who do not develop new-onset POAF.
Normal distribution of local refractory periods in the 65% of patients who never have postoperative atrial fibrillation (POAF). The refractory periods at specific local sites in the atria are shorter in the left atrium than in the right atrium and the transition from short refractory periods to longer refractory periods is smooth and sequential. Patients with this normal distribution of local refractory periods are unlikely to develop POAF after cardiac surgery. (See text for further discussion.)
If the dispersion of refractoriness in the atria is such that areas of long refractoriness are occasionally located adjacent to areas of much shorter refractoriness ( Fig. 47.4 ), the patient is more vulnerable to the development of new-onset POAF. We know that an abnormal distribution of atrial refractory periods makes patients vulnerable to the development of AF (see Chapters 4 and 5 ). Therefore, we surmise that this AF vulnerability, coupled with the multiple potential AF triggers that exist after cardiac surgery, results in the 30% of patients who develop new-onset POAF only after cardiac surgery. If the dispersion of atrial refractoriness is even greater so that there are multiple areas of short refractoriness adjacent to areas of long refractoriness ( Fig. 47.5 ), the atrium is extremely vulnerable to the development of AF. These highly vulnerable patients represent the 5% who will develop new-onset POAF after most any type of surgery, cardiac or noncardiac.
Mild dispersion of refractoriness in the 30% of patients who have postoperative atrial fibrillation (POAF) only after cardiac surgery. If the distribution of local refractory periods is not smooth and sequential (“dispersion of refractoriness”), several areas can exist adjacent to one another where there is great disparity between the refractory periods (curved red bars). This abnormality can result in POAF. If the dispersion of refractoriness is mild (i.e., there are not too many of these adjacent areas), the patient may or may not have POAF, and prophylactic measures to prevent it are likely to be beneficial. We believe that the 30% of patients who develop POAF only after a cardiac surgical procedure most likely have this type of mild dispersion of refractoriness.
Severe dispersion of refractoriness in the 5% of patients who always have postoperative atrial fibrillation (POAF). If atrial repolarization is such that the dispersion of refractoriness results in numerable adjacent areas in the atria with widely varying local refractory periods (curved red bars), the patient will be extremely vulnerable to developing POAF. This is the problem that likely exists in the 5% of patients who will develop POAF no matter what surgical procedure they undergo.
The Problem of Prophylaxis for New-Onset Postoperative Atrial Fibrillation
A major problem with studies designed to assess the risk-to-benefit ratio of prophylactic drugs is related to the confounding clinical characteristics of new-onset POAF described in the preceding section. POAF prophylaxis is not needed in approximately two-thirds of patients because regardless of the surgical procedure, they will not develop POAF. In addition, no prophylaxis can prevent POAF from developing in the 5% of patients who will always have POAF no matter how minor the surgical procedure. That leaves only about 30% of cardiac surgery patients who will potentially respond to prophylactic therapy. Unfortunately, 100% of the patients have to be exposed to the side effects of a prophylactic drug ( Fig. 47.6 ). In effect, this triples the potential risk-to-benefit ratio of drug prophylaxis for new-onset POAF and is a major reason why many drugs that are known to reduce new-onset POAF simply cannot be used clinically because their side effects outweigh their benefits.
The risk-to-benefit problem with postoperative atrial fibrillation (POAF) prophylaxis. The unique problem that exists with prophylactic measures, especially drugs, to decrease the incidence of POAF relates to the risk-to-benefit ratio of POAF prophylaxis. Because 65% of patients do not need any prophylaxis and 5% will not respond to it, only 30% of patients have any potential of responding to POAF prophylaxis. However, 100% of them are exposed to the side effects of the prophylactic drug. This effectively triples the risk-to-benefit ratio of prophylactic drugs for POAF.
Is New-Onset Postoperative Atrial Fibrillation the Same as “Naturally Occurring” Atrial Fibrillation?
If one mapped new-onset POAF, the electrophysiologic pattern would be the same as any other AF, but there are several differences between new-onset POAF and AF that occurs spontaneously or that develops secondary to left-heart disorders. As described earlier, numerous triggers occur after cardiac surgery that are not at play in “naturally occurring” AF. In addition, some of these same triggers, such as inflammation and increased catecholamines, not only increase the postoperative triggers but also shorten the refractory periods in the atria and aggravate the dispersion of refractoriness that would not have been “unmasked” except by the surgical procedure. For example, one of the unexpected findings in the original series of Maze patients was that 37% of them had POAF despite having just undergone an operation that we thought rendered the atrium incapable of fibrillating! When this surprising and disappointing occurrence was studied, we found that the most consistent new abnormality after cardiac surgery was a generalized shortening of the atrial refractory periods. For example, we had 9 patients who had preoperative atrial flutter in addition to AF who underwent the Maze procedure. Their preoperative atrial flutter had the typical cycle length of 200 ms, resulting in an atrial rate of 300 beats/min and a ventricular rate of 150 beats/min (see Chapter 5 , Fig. 5.15 ). Postoperatively, atrial flutter could still be induced in these 9 patients but their atrial flutters all had shorter cycle lengths than they had preoperatively. We interpreted this to mean that despite a successful Maze procedure, the atrial refractory periods were temporarily shortened to the extent that smaller macro-reentrant circuits (drivers) could form between the lesions of the Maze procedure, resulting in what amounted to a new-onset POAF even though the patients had a long history of preoperative AF (see Chapter 4 , Fig. 4.14 ). After the atria had sufficient time to heal postoperatively, the atrial refractory periods lengthened back to normal, the new-onset POAF subsided, and the long-term success of the Maze procedure for the “naturally occurring” AF was the same in these patients as in the ones who did not develop new-onset POAF.
Preoperative Identification of Patients Destined to Develop New-Onset Postoperative Atrial Fibrillation
The unfavorable risk-to-benefit ratio of prophylaxis against new-onset POAF could best be solved by having the ability to identify the 30% of cardiac surgery patients preoperatively who are destined to develop new-onset POAF so that prophylactic measures could be applied only in these patients. This could return the risk-to-benefit ratio of prophylactic POAF measures to more practical levels. It is well known that certain patient demographics, cardiac abnormalities, and comorbidities lead to a higher incidence of new-onset POAF, but this information is not particularly helpful in deciding which patients should be subjected to POAF prophylaxis and perhaps more important, which patients should not be subjected to the side effects of unnecessary drugs. For example, all older adult patients with poor cardiac function and preoperative renal failure do not develop new-onset POAF, and the avoidance of POAF prophylaxis in such patients is desirable. However, it is usually these very patients who are most likely to be subjected to POAF prophylaxis that in many cases is unnecessary.
Unfortunately, it is still not possible to identify patients before surgery who are destined to develop new-onset POAF with sufficient specificity to limit POAF prophylaxis to those patients only despite long and continuing efforts to do so. However, conventional echocardiographic studies have demonstrated that hypertension and LA diameter are independent preoperative predictors of POAF. Left atrial volume (LAV) has also been explored, but it is not an ideal predictor of POAF because of variability related to individual body size and composition. For instance, comparing LAV between a 150-lb man and a 200-lb, 6-ft-tall man may lead to inaccurate conclusions because obesity is an independent risk factor for the recurrence of the “natural” types of AF.
In this context, the left atrial volume index (LAVI), which normalizes LA volume to body surface area, offers a more reliable metric than unadjusted left atrial size or volume. LAVI can be determined without advanced speckle-tracking analysis and has been shown to be associated with an increase in AF, including new-onset POAF after cardiac surgery. Osranek and colleagues showed that a LAVI greater than 32 mL/M² was associated with a fivefold increase in the risk of developing POAF independent of age and other clinical risk factors. However, it should be noted that no matched comparisons were performed in the study.
We showed in a matched group study that LAVI was significantly larger in patients who developed new-onset POAF compared with those who did not. Interestingly, however, the absolute value for LAVI in the POAF group was 29.8 mL/M², which is still within the normal range according to current guidelines. This finding suggests that although LAVI can be a helpful echocardiographic parameter, it may not serve as an ideal stand-alone marker for predicting new-onset POAF, highlighting the need for more comprehensive and individualized risk assessment approaches.
Preoperative Left Atrial Strain and New-Onset Postoperative Atrial Fibrillation
Several small observational studies have suggested that preoperative compliance abnormalities of the LA, detected by calculating left atrial strain using speckle-tracking analysis of preoperative echocardiograms, might be able to predict new-onset POAF after cardiac surgery. Left atrial strain measured by speckle tracking echocardiography is a quick, sensitive, and objective parameter that assesses left atrial deformation, providing insights into its reservoir, conduit, and contraction phases ( Fig. 47.7 ). This technique has gained recognition as a valuable predictor of AF recurrence and persistence after various medical and interventional therapies for AF. Moreover, left atrial strain has been identified as a marker of left atrial fibrosis. Previous studies have explored left atrial strain across different clinical scenarios. For instance, peak atrial contraction strain (PACS) was identified as an independent predictor of AF development in patients with cryptogenic stroke. Additionally, both peak atrial longitudinal strain (PALS) and PACS were associated with the occurrence of new-onset POAF in patients undergoing surgical treatment for aortic stenosis.
