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Ann Thorac Surg 2001;71:128-132
© 2001 The Society of Thoracic Surgeons

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Original article: cardiovascular

Left hemispheric strokes in coronary surgery: implications for end-hole aortic cannulas

^a Division of Cardiothoracic Surgery, Western Pennsylvania Hospital, Pittsburgh, Pennsylvania, USA

Accepted for publication June 6, 2000.

Address reprint requests to Dr Weinstein, 4815 Liberty Ave, Suite 204, Pittsburgh, PA 15224
e-mail: gswmd{at}aol.com

	Abstract

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
Background. Perioperative stroke (POS) is a devastating complication of coronary artery bypass grafting (CABG). Many studies have been published concerning risk factors and possible causes of POS but none have studied which side of the brain is more frequently involved. The finding of a strong preponderance of left-sided strokes calls into question some widely held theories as to the cause of POS and implicates end-hole aortic perfusion catheters as a major factor.

Methods. During a 3-year period (1996 to 1998), prospective data were collected on all 2,217 consecutive CABG patients at one hospital (with surgery by different surgeons in different groups). Strokes were classified as perioperative (within 3 days of surgery) or late (beyond 3 days but during hospitalization).

Results. There were a total of 51 strokes (2.3%): 21 left, 10 right, 7 bilateral, 7 lacunar, 1 brainstem, and 5 indeterminate. There were 18 major territorial perioperative strokes on the left side and 6 on the right side. Thus, 75% (18 of 24) of POS were left-sided. Stroke patients were significantly younger than nonstroke patients (66.3 ± 10.52 versus 71.4 ± 8.47 years, p = 0.009). Other demographic data did not differ significantly.

Conclusions. If aortic clamping, cannulation, or manipulation were responsible for most strokes, then right-sided strokes should predominate, as the innominate artery is closest to the source of such emboli. In contrast, end-hole aortic cannulas direct a high-velocity jet at the left carotid orifice and may be responsible for a large proportion of POS. Side-hole aortic cannulas may reduce the incidence of this complication.

	Introduction

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
Despite advances in coronary artery bypass surgery, perioperative stroke remains a vexing problem, having a devastating effect on patients and their families [1]. Many studies of perioperative stroke have been published but none has identified which side of the brain is most commonly involved. The findings of this study show that most perioperative strokes occur on the left side of the brain, casting doubt on the relative importance many of the currently held theories of causation (such as aortic cannulation and clamping) and implicating end-hole aortic cannulas as a major contributor to perioperative stroke.

	Material and methods

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
Data were prospectively gathered on all 2,217 consecutive patients undergoing isolated coronary artery bypass grafting (CABG) at the Western Pennsylvania Hospital, in Pittsburgh, Pennsylvania, from January 1, 1996 through December 31, 1998. Data collection was done by a trained data collection nurse for internal quality control purposes and for The Society of Thoracic Surgeons Database

The diagnosis of stroke was made by neurology consultants and by brain computed tomography (CT) scans, which were interpreted by neuroradiologists. Strokes were classified by brain location into: left territorial; right territorial; bilateral; lacunar (left, right or bilateral); brainstem; or indeterminate (persistent neurologic defect without definite localization by CT scan). Strokes were further divided into perioperative (within 3 days of surgery) or late. This distinction was made to separate strokes related to intraoperative events from those related to postoperative atrial dysrhythmias or late hypotensive episodes.

Surgery was performed by 18 surgeons in 8 different surgical groups. As might be expected, techniques varied widely from surgeon to surgeon, including perfusion temperature, method of myocardial protection, cannulation, and single aortic cross clamp versus partially occluding clamp techniques.

Cardiopulmonary bypass was carried out with a Baxter-Bentley Spiral Gold membrane oxygenator (Baxter-Bentley, Inc, Irvine, CA); a Baxter-Bentley 40 µm arterial line filter; Duraflo (Baxter-Bentley) coated tubing; a closed, soft, venous reservoir (Baxter-Bentley BMR Gold); Sarns centrifugal pumps (Sarns, Inc, Ann Arbor, MI); a variety of end-hole aortic cannulas and the Sarns Soft-flow side-hole cannula (used by the author since December 1997).

	Results

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
There were 51 strokes in a total of 2,217 patients (2.3%). Patient characteristics are listed in Table 1. Differences between stroke and nonstroke patients were analyzed employing the two-tailed t test for continuous data and Fisher’s exact test for dichotomous data, using the NCSS Statistics Program (NCSS Systems, Kayesville, UT). There were no statistically significant differences in terms of sex, hypertension, congestive heart failure, postoperative atrial fibrillation, peripheral vascular disease, or diabetes mellitus. Surprisingly, the mean age of the 51 patients with stroke was significantly lower than that of the 2,166 patients without stroke: 66.3 ± 10.52 (SD) versus 71.4 ± 8.47 years (p = 0.0091).

View larger version (31K): [in this window] [in a new window]   Fig 1. Location of all 51 strokes (number is shown in parentheses).

View larger version (19K): [in this window] [in a new window]   Fig 2. Location of the 24 major perioperative strokes by hemisphere.

In the author’s personal series, since changing to the Sarns Soft-flow aortic cannula in December 1997, there have been no perioperative strokes in 136 consecutive coronary artery bypass patients. (There was 1 late stroke on postoperative day 5 in a patient with recurrent, intermittent atrial fibrillation.) In the author’s preceding 674 consecutive coronary artery bypass patients at the same institution, there were 10 perioperative strokes (1.5%; p = 0.157 by Fisher’s exact test).

	Comment

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
Perioperative stroke remains one of the most difficult and challenging problems in coronary artery bypass surgery. Despite many advances, the focal stroke rate remains 2% to 4% [1–3].

The etiology of perioperative stroke is certainly multifactorial. The landmark paper by Gardner and associates [3] identified increased age, preexisting cerebrovascular disease, arteriosclerosis of the ascending aorta, protracted cardiopulmonary bypass time, and severe perioperative hypotension as risk factors for stroke. Gardner later identified five patient risk factors: previous stroke, carotid bruit, hypertension, increasing age, and diabetes mellitus. The only intraoperative risk factor he identified was cardiopulmonary bypass time [4]. Other studies have also identified prior stroke, hypertension, renal dysfunction and surgical priority, intraaortic balloon pump usage, and diabetes as risk factors [1, 2].

Mills and Everson [5] stressed the importance of ascending aortic arteriosclerosis as a risk factor. Blauth and associates [6] also stressed the importance of atheroembolism from the ascending aorta. Wareing and colleagues [7] found that detection of ascending aortic arteriosclerosis by palpation was insensitive, detecting severe arteriosclerosis in only 38% of patients, thus establishing the need for intraoperative ultrasonic scanning of the aorta. Bar-El and Goor [8] argued that clamping the atherosclerotic aorta was primarily responsible for perioperative strokes. Subsequent to these studies, many surgeons adopted a "single clamp" technique during which both the proximal and distal anastomoses were performed, to avoid the use of a partially occluding aortic clamp. Nonetheless, perioperative strokes continued to occur.

Many excellent articles have been written about perioperative stroke but none have focused on which side of the brain such strokes occurred. In one of the few studies to focus on location, Barbut and colleagues [9] reviewed strokes in 24 patients, of whom 19 had a stroke within 24 hours of surgery. They found bilateral cerebellar infarcts in 15 (79%), posterior cerebellar artery infarcts in 4 (21%), posterior watershed infarcts in 10 (53%), and middle cerebral artery branch infarcts in 11 (58%). The authors did not indicate which side of the brain was involved in the middle cerebral artery infarcts.

Analysis of the data table in the study by D’Agostino and colleagues [10] shows that there were 17 (70.8%) left hemispheric infarcts versus only 7 on the right. This finding is similar to the 75% left hemispheric incidence in the present study. D’Agostino and colleagues did not make note of the preponderance of left hemispheric events.

Borger and associates [11] found there was a decrease in cerebral microemboli if the distal aortic arch was the site of cannulation. Bilateral transcranial Doppler monitoring of the middle cerebral arteries was done but no statement was made as to whether the left or right middle cerebral artery had more microemboli during either conventional or distal aortic cannulation.

Certainly, severe arteriosclerosis of the ascending aorta is an important cause of perioperative stroke, but Wareing and associates [7] found this to be present in only 13.6% of their patients. Bar-El and Goor [8] found palpable aortic plaques in 20.8% (132 of 632) necessitating minor surgical modifications, but severe aortic arteriosclerosis in only 4.7% ("unclampable" aorta in 2.5%, "untouchable" aorta in 2.2%). The risk of stroke from ascending aortic arteriosclerosis may be ameliorated by the use of intraoperative epiaortic ultrasound scanning to identify patients at risk and to modify operative techniques [7].

Despite the importance of ascending aortic arteriosclerosis in perioperative stroke, the strong preponderance of left hemispheric strokes raises questions as to other factors: If the ascending aorta were the major source of emboli, then one would expect right hemispheric strokes to predominate, as the innominate artery is the first target of such emboli. Katz and coworkers [12] identified protruding atheromas of the aortic arch as a cause of stroke. The atheromas were detected by transesophageal echocardiography. They felt that the aortic arch cannulation or high-pressure jet from the aortic cannula tip might displace and detach the protruding atheromatous material.

End-hole aortic cannulas direct a single high-velocity flow jet at the left carotid and left subclavian arteries (Figs 3 and 4). Side-hole aortic cannulas, such as the Sarns Soft-flow (Fig 5) and the Baxter RMI Dispersion (Fig 6) aortic cannulas produce lower velocity, diffuse flow. According to data from Sarns (Sarns Terumo Cardiovascular Systems Corp., Ann Arbor, MI), the flow velocity at a distance of 1.2 cm from the tip of a conventional end-hole aortic cannula is more than 400 cm/s, as opposed to approximately 100 cm/s from each of the four side holes of the Sarns Soft-flow cannula. The normal peak systolic velocity in the human aorta is approximately 80 cm/s. Baxter (Baxter Research Medical, Inc, Midvale, UT) offers data on the tip jet force of the Baxter RMI dispersion cannula, which cannot be directly compared with velocity measurements but is less than the force from a standard end-hole cannula (0.78 g/mm² versus 0.95 g/mm²).

View larger version (109K): [in this window] [in a new window]   Fig 3. End-hole aortic cannulas direct a high-velocity jet at the orifices of the left carotid and left subclavian arteries, sparing the innominate, which may explain the preponderance of left hemispheric perioperative strokes. (Reprinted with permission of Terumo Cardiovascular Systems Corp.)

View larger version (90K): [in this window] [in a new window]   Fig 4. Typical end-hole aortic cannula produces a single, high-velocity jet at more than 400 cm/s. (Reprinted with permission of Baxter Research Medical, Inc.)

View larger version (88K): [in this window] [in a new window]   Fig 5. The Sarns Soft-flow aortic cannula is plugged at the end and has four side holes which produce four low-velocity streams at approximately 100 cm/s, which is similar to normal aortic systolic velocity of 80 cm/s. (Reprinted with permission of Terumo Cardiovascular Systems Corp.)

View larger version (62K): [in this window] [in a new window]   Fig 6. The Baxter RMI Dispersion aortic cannula produces flow in the shape of a fan, rather than a high-velocity jet. (Reprinted with permission of Baxter Research Medical, Inc.)

Side-hole aortic cannulas, such as the Sarns Soft-flow and the Baxter RMI dispersion cannula eliminate high-velocity jets directed at the arch vessels. In keeping with the hypothesis of Katz and associates [12], the elimination of high-velocity jets directed at the arch vessels should decrease the incidence of perioperative stroke.

In addition, microemboli have been thought to be a cause of postoperative cerebral dysfunction. Many such emboli may represent microparticulate debris sheared off the lumen of an atherosclerotic aorta as a result of high-velocity jets from the aortic cannula [13]. Gaseous microemboli may also be produced by the effect of turbulent flow on dissolved gases [14]. The elimination of high-velocity jets in the aorta may decrease both perioperative stroke and postoperative neurologic dysfunction.

In the author’s personal series, the Sarns Soft-flow aortic cannula was used in the most recent 136 consecutive coronary artery bypasses with no perioperative strokes, as opposed to 10 strokes in the previous 674 patients. Although there seemed to be a trend toward a reduction in the stroke rate, the difference did not reach statistical significance (p = 0.157). This may have been the result of inadequate statistical power: Detection of a clinically meaningful reduction in the stroke rate from the observed prevailing rate of 2.3% to, say, 0.3%, would require 584 patients per group ( = 0.05, power = 0.80).

Weaknesses of the study
This is an observational study, so the conclusions drawn are inferential. Although a randomized prospective study of end-hole versus side-hole aortic cannulas would be desirable, it would take more than 1,000 patients to achieve adequate statistical power to detect a clinically important difference in the stroke rate.

Data extracted from the study by D’Agostino and colleagues [10] appear to confirm the preponderance of left hemispheric strokes found in the present study but confirmatory data from other published studies are not available. Transcranial Doppler studies of both carotid arteries during cardiopulmonary bypass may help to confirm a tendency for emboli to occur more often on the left side, but published studies do not include such data.

Conclusions
There is a strong preponderance of left hemispheric strokes in coronary artery bypass surgery by a ratio of 3:1. Ironically, since the left hemisphere is usually dominant, left hemispheric strokes are more devastating to patients. Although embolization from cannulation or manipulation of an atherosclerotic ascending aorta is undoubtedly responsible for some perioperative strokes, end-hole aortic cannulas may play a more important role. Use of side-hole aortic cannulas may significantly reduce the perioperative stroke rate.

	Acknowledgments

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
The author thanks Janine Janosky, PhD, University of Pittsburgh School of Medicine, Department of Family Medicine and Epidemiology, Division of Biostatistics, for the statistical power calculations.

	References

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 

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