Atrial Fibrillation as a Post Operative Complication
Atrial Fibrillation and Heart Surgery
Atrial fibrillation represents a serious post-operative complication in many types of heart surgery. Nearly 40% of all patients undergoing coronary and valve surgery experience this side effect (Saltman, 2004). Post operative atrial fibrillation is an enigma that remains refractory to both pharmacologic and nonpharmacologic attempts at prevention (Saltman, 2004). Reducing the incidence of atrial fibrillation (AF) is a serious concern for heart surgeons in all types of heart surgery. The following will explore the mechanisms that trigger AF, risk factors, monitoring procedures, and new treatments aimed at reducing the occurrence of post operative AF.
Mechanism
One of the key reasons for the ineffectiveness of procedures to minimize AF is a lack of understanding surrounding the mechanism that triggers it (Saltman, 2004). AF is a serious arrhythmia that has recently been classified as a reentrant arrhythmia. In order for a reentrant arrhythmia to occur, two conditions must be met. The first is that there must be some even that initiates the reentrant circuit. There must also be a substrate available that is capable of maintaining the arrhythmia. Understanding these two conditions will help us to develop methods to prevent and control this arrhythmia by short-circuiting either the initiating event or the maintenance event.
First, let us look at the initiation event in the arrhythmia cycle. The initiation event occurs when excitable tissue demonstrates unidirectional conduction block and slow conduction (Saltman, 2004). Unidirectional block occurs when an excitation wave can no longer penetrate tissue in a normal manner. Prolonged refractoriness as is the result of an excitation wave that encounters cell that have not recovered their full excitability potential (Saltman, 2004). Slow conduction occurs when an excitation wave is restricted long enough to permit abnormal tissue to recover excitability. This typically sets the stage for retrograde reentry of the excitation wave (Saltman, 2004).
This is the sequence that triggers normal atrial fibrillation. The only difficulty is determining if this reentry sequence is the culprit behind post operative atrial fibrillation or if some other mechanism is responsible. It is not difficult to imagine how a properly timed impulse may encounter refractory tissue in some areas and normal tissues in others after the trauma of surgery (Saltman, 2004). Atrial fibrillation can be heralded by frequent premature atrial contractions and bursts of atrial tachychardia (Frost et al., 1995). Saltman surmises that these contractions are the likely agents that set up the unidirectional block.
Slow conduction presents a slightly different mechanism. It results from a decrease in cell to cell coupling and a decline in the rapid inward Sodium current. This condition is unusual in normal atrial tissue. Recently, it was discovered that the inflammatory mediator, arachidonic acid can reversibly depress the conduction up to 60% less than the baseline in normal canine and human atrial tissues (Saltman, et al., 2002). This suggests that post-surgical inflammation may alter the normal atrial electrophysiology in such a way as to initiate slow conduction.
However, this effect is only seen in a direction that is transverse to the fiber’s long axis, with no effect apparent parallel to the long axis (Saltman, et al., 2000). This suggests that there is no change in the properties of depolarizing sodium ion currents or repolarizing potassium currents (Saltman, 2004). This led to the application of steroidal and non-steroidal anti-inflammatory drugs for the prevention of fibrillation in sterile talc pericarditis in animals (Saltman, et al., 2000). Research demonstrates that inflammatory markers are particularly increased in patients that exhibit post operative atrial fibrillation (Chung, et al., 2001). These findings would lead us to suspect that an agent in the inflammatory cascade is to blame, but at the current time, which one or the exact mechanism is not known.
Janse, (1997) found that atrial fibrillation is associated when the atrial muscle is enlarged or stretched. This stretching may cause electrophysiological changes that could be attributed to the effects seen in reentry sequence. According to Janse, an acute stretch might cause action potential shortening. The effects of an acute stretch may be different from those experienced in a chronic stretch. Refractory periods were found to be shorter in human atrial tissue that had been stretched. These findings offer a plausible explanation for atrial fibrillation after heart surgery. It was also found that atrial fibrillation begets atrial fibrillation. Once the heart starts fibrillating it tends to develop a vicious cycle that is difficult to stop (Allesse et al., 1998).
These mechanisms explain how AF may be initiated, but it must be sustained to develop into an episode. In order for the AF to achieve continue to perpetuate, the reentrant circuit must be small enough to fit within the confines of the affected tissue (Saltman, 2004). In chronically fibrillating atrial tissue the conduction velocity was found to be normal, yet the refractory period is greatly shortened (Allessie et al., 1998). This same study also found that once the heart returned to normal rhythm the risk of atrial fibrillation increased.
The mechanisms of initiation of atrial fibrillation are only suspected at this point and even less is known about the maintenance mechanism. However, it appears that many agree that postsurgical changes in the atrium are responsible for making it susceptible to both the initiation and maintenance of this arrhythmia (Saltman, 2004).
Risk Factors
Several risk factors have been identified that place a person in the general population at risk for atrial fibrillation. The first factor is that the risk of AF increases with age. More men than women tend to develop AF. Factors such as diabetes, hypertension, congestive heart failure, and valve disease were also found to be major contributors to the risk of atrial fibrillation (Benjamin, et al., 1994). Body mass index and alcohol use were not found to be factors in the development of AF in this study in the original analysis. However, a reexamination of the cohorts used in the study did reveal a correlation between atrial fibrillation and obesity (Iacobellis, 2005).
As we found, stretching and inflammation place a person at greater risk of developing AF. However, cardiovascular diseases including hypertension, coronary disease, and heart failure also place a person at greater risk for AF (Benjamin, et al., 1994). This same study also indicates that interventions meant to treat these conditions also decrease the risk of AF in a patient. The results of this study raise some important questions. The first is whether the patient that develops AF as the result of a surgical procedure may have been at greater risk than the normal population because they already had an underlying condition before the surgery. The second is that the risk of post operative AF may be reduced by eliminating some of the risk factors associated with it.
Koray et al., (2005) found that several preoperative histopathologic factors could be identified that increased the risk of post operative atrial fibrillation. Chronic obstructive pulmonary disease was found to be the only reliable predictor for post operative atrial fibrillation. Those that developed post operative atrial fibrillation were found to have larger sized myolytic valves than those that maintained a normal rhythm. In a study of patients undergoing coronary bypass surgery there was no difference between those that had on-pump and off-pump procedures (Mariscalco, 2006). This study did concur with Koray and associates that pre-existing histopathologic changes in the right atrium increase the chances of post operative AF.
Leung et al. (2004) found that impairment of the left atrium was a significant predictor of post operative atrial fibrillation (also in Osranek and Associates, 2006). Left Atrial Volume Predicts the Risk of Atrial Fibrillation After Cardiac Surgery: A Prospective Study J. Am. Coll. Cardiol., August 15, 2006; 48(4): 779-786.. This study found that similar changes occurred in the elderly population that were seen post operatively in the surgical patients.
The process of the development of AF is not completely understood. However, these structural anomalies associates with increased risk of AF may provide clues to this elusive mechanism. Studies are concentrating on discovering the mechanism that drives AF. Guler et. al., (2007) found that an increase in AngII and the decrease in SubP after may be a factor in postoperative AF.
Monitoring
Better monitoring of patients after heart surgery of any type has led to decreased lengths of stay. Electrocardiogram is the most frequently used form of monitoring. This type of monitoring increases the likelihood that the atrial fibrillation will be detected and managed quickly and effectively. In 1991 the presence of atrial fibrillation increased the stay in intensive care from 1.9 to 3.6 days in intensive care (Loew et al., 1991). Research studies provide the best indication of how monitoring and length of stay have decreased since 15 years ago.
2002 study revealed that the presence of AF after surgery increased the average hospital stay by a total of 5.5 days (Thompson et al., 2002). A meta analysis of studies regarding the effectiveness of post operative treatments for AF revealed that the most common monitoring regime currently being used is ECG or Holter recordings while in the ICU. After ICI discharge daily ECG or predischarge (Jansen et al., 1986; Evrard et al., 2000) Holter monitoring was used to detect arrhythmias. In some cases the monitoring may be continued beyond the hospital stay for 30 (Guarnieri et al., 1999) up to 90 days (Weber et al., 1990). The use of the Holter monitor makes extended monitoring more feasible than when ECG was the only option.
Several anomalies have been found to act as indicators that AF may occur. For instance, prolonged P-wave duration was found to be an indicator of reoccurring AF (Gialiafos, 1999; Steinberg et al., 1993). Patients with a clinical history of AF were found to have a significantly longer intra-atrial and inter-atrial conduction time of sinus impulses (Centurion et al., 2002). The presence of preoperative supraventricular arrhythmias and fluctuations in autonomic balance were identified in some, but not all studies as a risk factor for post operative AF (Jideus et al., 2000).
There are still too many gaps in knowledge about the mechanism of post operative AF to make a prediction about the likelihood of any particular patient outcome. Monitoring techniques have revealed several indicators that the patient may be at an increased risk for developing AF. However, there is still no definitive consensus as to the electrocardiographic characteristics that predict the development of post operative AF (Terranova et al., 2007). Monitoring typically ends when the person leaves the hospital. However, the use of Holter monitoring provides the ability to extend the monitoring period.
Best Practices
The goal of monitoring and study of the mechanisms behind post operative AF is to reduce the risk of patients that undergo any type of heart surgery. It is not enough to identify risk factors. Current monitoring techniques that are intermittent after the release of the patient from ICU may miss several important indicators that AF may occur at some time in the future. The development of practices to eliminate post operative AF are not as advanced as for other post operative complications. The development of better management practices through prevention depends on developing a better understanding through study of the mechanism that drives AF.
The development of better preventative measures depends on the ability to recognize the precursors of an episode. Several types of arrhythmias can occur during percutaneous coronary interventions (PCI). Many of these arrhythmias may result from catheter manipulation, dye injection, reperfuson injury and other disturbances of the heart tissue (Terranova et al., 2007). Any action that disturbs the atrial tissue can cause an AF event. Stretching due to atrial swelling, or stretching in heart failure can also cause AF (Terranova et al., 2007). Management of atrial fibrillation includes prevention of these factors that can cause the initiation of AF. Recent studies are beginning to shed light on the electrophysiological factors that can indicate that AF is about to occur. They are also attempting to spend more time studying the mechanism that drives the initiation of AF and the mechanism that allows it to keep going.
Budeus et al. (2003) studied the incidence of atrial late potentials in patients that also had a proximal stenosis of the right coronary artery. They found that when this condition was treated with percutaneous translunminal coronary angioplasty (PTCA) the atrial later potentials were also gone. In this study atrial fibrillation was associated with stenosis of the right coronary artery. In this case, reduction of pre-existing atrial late potentials may also reduce the incidence of atrial fibrillation later.
Another study found the PTCA on patients with acute anterior wall MI reduced the risk factors of AF by decreasing P-wave durations (Akdemir et al., 2005). Gorenek and associates (2000) found that patients that developed AF during acute MI and underwent primary PCI also returned to normal sinus rhythm. However, they also discovered that those who underwent thrombolytic therapy developed AF within 12 hours of hospitalization. It was found that the most frequent cause of AF was right coronary artery occlusion (Gorenek et al., 2000).
These studies highlight the fact that AF is treated as a secondary condition. It is treated viewed as a complication of another condition rather than a condition of its own merit. In several studies, we found that it resolved as a secondary effect of treatment for another condition. More importantly, it can be brought on by an intervention to treat another condition. AF is a serious condition that can lead to death and needs to be treated as a primary concern in many cases. It is not known why the medical community has chosen to treat AF as a secondary condition rather than a primary one. However, there is clear evidence that this attitude must change. More attention needs to be focused on predicting AF and preventing it. AF is difficult to treat once it gets started. Therefore, the focus needs to be on predicting and preventing this serious condition.
Electrical cardioversion is often used when AF cannot be controlled by other means (Terranova et al., 2007). A beta-blocker is used for rate control. Other drugs such as esmolol, verapamil, or diltiazem may also be used to help bring AF under control. However, these must be used with caution as other conditions, such as pulmonary congestion could make them dangerous. Dofetelide, amiodarone and digoxin are also possibilities to control AF under various circumstances (Terranova et al., 2007). The author notes that in many cases AF tends to revert to normal rhythm spontaneously, but if it does not do so immediately, then treatment of some type should be given. The longer AF continues, the harder it is to stop.
Post operative AF is a common complication of coronary artery bypass surgery (CABG). The incidence of this complication is typically higher during the first week after the operation (Terranova et al., 2007). However, it was found that the incidence of AF as a complication of CABG was greater when Holter monitoring was used as opposed to trials without. This has serious implications for the discovery and treatment of post operative AF. It implies that better monitoring means increased likelihood of catching AF before it becomes a problem. It also implies that infrequent monitoring may miss some of the early warning signs of AF. This would lead to the conclusion that Holter monitoring is the preferred method for monitoring and preventing AF during the postoperative period.
Regardless of improvements in monitoring techniques and post operative patient care incidences of post operative AF are in the increase (Terranova et al., 2007). This can be explained by and increasing elderly population and of patients that have other underlying conditions (Terranova et al., 2007). AF is considered to be non-life threatening in many cases. However, it significantly increases the incidence of subjective symptoms including congestive heart failure, hypotension, and swelling of the tissues (Terranova et al., 2007).
Stroke is a major event that is often associated with AF (Terranova et al., 2007). AF was found to be a major factor predicting stroke after a CABG procedure. AF preceded neurological events in nearly 27% of patients in one study (Terranova et al., 2007). AF is also associated with greater in hospital mortality (Terranova et al., 2007). This included short-term survival and long-term survival of 3-5 years. AF is an important signal for more serious conditions in many cases. AF is a symptom that heralds many more serious events. This would indicate the need for increased continuous monitoring, rather than the intermittent monitoring that is used once patients leave the ICU. Many more complicated conditions could be prevented by watching for the presence of AF, rather than waiting for the big event to occur.
Studies have identified many comorbidities associated with AF. The presence of these conditions should indicate the need for closer monitoring than is typically undertaken. AF indicates an imbalance in the autonomic nervous system, including increased circulation norepinephrine in older patients (Terranova et al., 2007). However, the author notes that thoracic epidural anesthesia was not effective in preventing post operative AF, as would be expected if autonomic nervous system were the only cause of the condition.
AF increases the health care expenditure of many procedures by increasing the amount of time spent in the hospital. It can be a warning sign of many more serious conditions. Although it is typically treated as non-threatening, it can be dangerous in its own right. AF is potentially reversible with a few common sense actions. For instance, the literature suggests a number of prophylactic measures. New post operative antiarhythmic therapies are being developed including sotalol and amiodrone (Terranova et al., 2007). It might be noted that treating AF with these drugs did not reduce the risk of stroke. Instead the treatment caused the patient to be released earlier, when this may not have been the wisest idea. These drug therapies are important in the control of episodes of dangerous AF. However, controlling mild cases of AF may also mask an important warning sign of a more serious condition. The research will have to decide for whether it is in their best interest to treat mild cases of AF, or whether to treat it as any other symptom.
In conclusion, the frequency of AF makes it an important post operative consideration in any type of heart surgery. Yet, there is a lack of understanding of the mechanism that drives AF. This lack of understanding may be one reason for hesitance to treat it in its non-serious form. There has been a rise in attention to the importance of AF, particularly in its relationship to other more serious conditions. Researchers are just beginning to scratch the surface of understanding the AF mechanism. More research in this area will need to be conducted before the complexities of AF can be fully understood.
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