A dual-vent left heart deairing technique markedly reduces carotid artery microemboli

A prospective observational study of emboli exposure in open versus closed chamber cardiac surgery

OBJECTIVE

Exposure to cerebral emboli is ubiquitous and may be harmful in cardiac surgery utilizing cardiopulmonary bypass. This was a prospective observational study aiming to compare emboli exposure in closed-chamber with open-chamber cardiac surgery, distinguish particulate from gaseous emboli and examine cerebral laterality in distribution.

METHODS

Forty patients underwent either closed-chamber procedures (n = 20) or open-chamber procedures (n = 20). Emboli (gaseous and solid) were detected using transcranial Doppler in both middle cerebral arteries in two monitoring phases: 1, initiation of bypass to the removal of the aortic cross-clamp; and 2, removal of aortic cross-clamp to 20 minutes after venous decannulation.

RESULTS

Total (median (interquartile range)) emboli counts (both phases) were 898 (499-1366) and 2617 (1007-5847) in closed-chamber and open-chamber surgeries, respectively. The vast majority were gaseous; median 794 (closed-chamber surgery) and 2240 (open-chamber surgery). When normalized for duration, there was no difference between emboli exposures in closed-chamber and open-chamber surgery in phase 1: 6.8 (3.6-15.2) versus 6.4 (2.0-18.1) emboli per minute, respectively. In phase 2, closed-chamber surgery cases were exposed to markedly fewer emboli than open-chamber surgery cases: 9.6 (5.1-14.9) versus 43.3 (19.7-60.3) emboli per minute, respectively. More emboli (total) passed into the right cerebral circulation: 985 (397-2422) right versus 376 (198-769) left.

CONCLUSIONS

Patients undergoing open-chamber surgery are exposed to considerably higher numbers of cerebral arterial emboli after removal of the aortic cross-clamp than those undergoing closed-chamber surgery, and more emboli enter the right middle cerebral artery than the left. These results may help inform the evaluation of the pathophysiological impact of emboli exposure.

Bubbles and bypass: an update

Bubbles in the bloodstream are not a normal condition -yet they remain a fact of cardiopulmonary bypass (CPB), having been extensively studied and documented since its inception some 50 years ago. While detectable levels of gaseous microemboli (GME) have decreased significantly in recent years and gross air embolism has been nearly eliminated due to increased awareness of etiologies and technological advances, methods of use of current perfusion systems continue to elicit concerns over how best to totally eliminate GME during open-heart procedures. A few studies have correlated adverse neurocognitive manifestations associated with excessive quantities of GME.

Newer techniques currently in vogue, such as vacuum-assisted venous drainage, low-prime perfusion circuits, and carbon dioxide flooding of the operative field, have, in some instances, exacerbated the problem of gas embolism or engendered secondary complications in the safe conduct of CPB. Doppler monitoring (circuit or transcranial) primarily remains a research tool to detect GME emanating from the circuit or passing into the patients’ cerebral vasculature. Newer developments not yet widely available, such as multiple-frequency harmonics, may finally provide a tool to distinguish particulate microemboli from GME and further delineate the clinical significance of GME.

Cognitive Dysfunction After Cardiac Surgery: Revisiting Etiology

Cognitive dysfunction remains a frequent complication of cardiac surgery. Despite many years of research, few preventive strategies and no definitive therapeutic options exist for the management of this troublesome clinical problem. This shortcoming may be secondary to an incomplete understanding of the pathophysiology and etiology of cognitive loss after cardiac surgery; a better understanding of the etiology is essential to finding new therapies. The etiology of cognitive dysfunction after cardiac surgery is multifactorial and includes cerebral microembolization, global cerebral hypoperfusion, systemic and cerebral inflammation, cerebral temperature perturbations, cerebral edema, and possible blood-brain barrier dysfunction, all superimposed on genetic differences in patients that may make them more susceptible to injury or unable to repair from injury once it has occurred. This review expands on these potential etiologies in detailing the evidence for their existence.

Selective vulnerability of the inner ear to decompression sickness in divers with right-to-left shunt: the role of tissue gas supersaturation

Inner ear decompression sickness has been strongly associated with the presence of right-to-left shunts. The implied involvement of intravascular bubbles shunted from venous to arterial circulations is inconsistent with the frequent absence of cerebral symptoms in these cases. If arterial bubbles reach the labyrinthine artery, they must also be distributing widely in the brain. This discrepancy could be explained by slower inert gas washout from the inner ear after diving and the consequent tendency for arterial bubbles entering this supersaturated territory to grow because of inward diffusion of gas. Published models for inner ear and brain inert gas kinetics were used to predict tissue gas tensions after an air dive to 4 atm absolute for 25 min. The models predict half-times for nitrogen washout of 8.8 min and 1.2 min for the inner ear and brain, respectively. The inner ear remains supersaturated with nitrogen for longer after diving than the brain, and in the simulated dive, for a period that corresponds with the latency of typical cases. It is therefore plausible that prolonged inner ear inert gas supersaturation contributes to the selective vulnerability of the inner ear to short latency decompression sickness in divers with right-to-left shunt.

Cerebral ischemic lesions on diffusion-weighted imaging are associated with neurocognitive decline after cardiac surgery

BACKGROUND AND PURPOSE

Improvements in cardiac surgery mortality and morbidity have focused interest on the neurological injury such as stroke and cognitive decline that may accompany an otherwise successful operation. We aimed to investigate (1) the rate of stroke, new ischemic change on MRI, and cognitive impairment after cardiac valve surgery; and (2) the controversial relationship between perioperative cerebral ischemia and cognitive decline.

METHODS

Forty patients (26 men; mean [SD] age 62.1 [13.7] years) undergoing intracardiac surgery (7 also with coronary artery bypass grafting) were studied. Neurological, neuropsychological, and MRI examinations were performed 24 hours before surgery and 5 days (MRI and neurology) and 6 weeks (neuropsychology and neurology) after surgery. Cognitive decline from baseline was determined using the Reliable Change Index.

RESULTS

Two of 40 (5%) patients had perioperative strokes and 22 of 35 (63%) tested had cognitive decline in at least one measure (range, 1 to 4). Sixteen of 37 participants (43%) with postoperative imaging had new ischemic lesions (range, 1 to 17 lesions) with appearances consistent with cerebral embolization. Cognitive decline was seen in all patients with, and 35% of those without, postoperative ischemic lesions (P<0.001), and there was an association between the number of abnormal cognitive tests and ischemic burden (P<0.001).

CONCLUSIONS

We have provided a reliable estimate of the rate of stroke, postoperative ischemia, and cognitive impairment at 6 weeks after cardiac valve surgery. Cognitive impairment is associated with perioperative ischemia and is more severe with greater ischemic load.

Occurrence and clinical impact of microembolic signals during or after cardiosurgical procedures

BACKGROUND AND PURPOSE

Microembolic signals (MESs) are detectable within the transcranial Doppler frequency spectrum downstream from vascular atherothrombotic or cardiothrombotic lesions. A frequent occurrence of MESs has also been shown during bypass surgery or after mechanical valve implantation. We sought to compile the knowledge on MES prevalence, the clinical impact of these cardiogenic MESs, and microemboli composition.

SUMMARY OF REVIEW

We performed a systematic MEDLINE search and summarized the currently available literature about MESs during or after cardiosurgical procedures for this state-of-the-art report.

CONCLUSIONS

The nature of cardiogenic MESs is heterogeneous, and their prevalence is highly variable, reflecting their different origin from a broad spectrum of cardiosurgical conditions. The occurrence and number of MESs during cardiac catheterization and percutaneous coronary angioplasty seem to have a clinical impact but need to be explored further. In patients with prosthetic heart valves, in those with left ventricular assist devices, and during cardiac surgery, the occurrence of MESs has an important clinical impact, and MES monitoring has proven its reliability. Although the data encourage intensifying MES detection in cardiac disorders, their heterogeneous nature does not yet allow the use of MESs as a general surrogate parameter for neuronal damage or cardial thromboembolic risk.

Best Practice for Cardiopulmonary Bypass in the High-Risk Elderly Patient

The management of cardiopulmonary bypass has evolved over the last 50 years resulting in a largely consistent approach to both adult and pediatric perfusion. Very little has been written or prospectively researched on the best practice for cardiopulmonary bypass in the high-risk elderly patient, despite the challenge this patient cohort presents compared to the general adult population and the rapidly increasing number of such patients undergoing cardiac surgery. We propose a framework for perfusion strategies for the high-risk elderly patient from our current understanding of cardiopulmonary bypass. It should stimulate discussion for a consensus on perfusion strategies for the elderly and encourage further research into perfusion variables as they relate to the outcome of patients of advanced age.

Physical and Pharmacological Neuroprotection in Cardiac Surgery

Stroke and neurocognitive deficits may follow cardiac surgery and have been linked to perioperative cerebral embolism. Alteration of cardiopulmonary bypass (CPB) or surgical technique to reduce embolism is, therefore, a rational neuroprotective strategy. Pharmacological cerebral protection has been advocated as an ideal "back-stop" to such "physical" interventions. A series of relevant studies conducted at Green Lane Hospital, Auckland, New Zeatand is described. Doppler ultra sound was used to monitor cerebral embolism during left heart valve surgery. Subsequently, salvaged CPB circuits were used to investigate several unexpected sources of emboli. The efficacy of a novel left heart deairing technique was audited using the Doppler de vice. Finally, a randomized double-blind trial of lidocaine in cerebral protection during cardiac surgery was con ducted. Most cerebral emboli were recorded after aortic declamping. However, cerebral emboli counts increased during stable CPB when the hard shell venous reservoir (HSVR) was operated at lower blood volumes and when air was seen in the venous return line. In vitro 2 HSVRs were found to generate bubbles when operated at blood volumes in excess of the manufacturer's recom mended minimum. Air in the venous return line was found to readily transit the CPB circuit and vacuum- assisted venous drainage markedly exacerbated this phenomenon. The novel deairing technique was mark edly superiorto conventional methods. Lidocaine admin istered in a standard antiarrhythmic dose for 48 hours from induction of anesthesia reduced the incidence of neuropsychological deficits at 10 days and 10 weeks postoperatively.

Venous air in the bypass circuit: a source of arterial line emboli exacerbated by vacuum-assisted drainage

BACKGROUND

Arterial emboli cause neurocognitive deficits in cardiac surgical patients. Carotid artery emboli, detected ultrasonically, have been observed after venous air entrainment into the cardiopulmonary bypass circuit. We investigated in vitro the extent to which venous air affected emboli detected in the arterial line downstream from a 40-microm filter.

METHODS

Using salvaged clinical cardiopulmonary bypass circuits, fixed volumes of air were introduced into the venous return line at unrestricted rates and at fixed rates using gravity venous drainage and vacuum-assisted venous drainage. Emboli counts were recorded distal to the arterial line filter using a 2-MHz pulsed-wave Doppler monitor. Emboli counts were similarly recorded after the introduction of carbon dioxide into the venous return line instead of air.

RESULTS

The number of emboli rose with increasing volumes of entrained venous air (p < 0.001), and there was an almost tenfold increase with vacuum-assisted venous drainage (p < 0.0001) compared with gravity venous drainage. Venous air was entrained at a significantly faster rate under vacuum-assisted venous drainage (p < 0.0001). When the entrainment rate of venous air was fixed, the difference in emboli numbers recorded for gravity and assisted venous drainage was not significant. There was a significant reduction in arterial line emboli when carbon dioxide rather than air was entrained under both vacuum-assisted and gravity drainage (p < 0.001).

CONCLUSIONS

Entrained venous air during cardiopulmonary bypass is a potential hazard, particularly during vacuum-assisted venous drainage. Every effort should be made to avoid venous air entrainment.