The influence of baseline hemoglobin concentration on tolerance of anemia in cardiac surgery

Urinary interleukin-18 and urinary neutrophil gelatinase-associated lipocalin predict acute kidney injury following pulmonary valve replacement prior to serum creatinine

BACKGROUND

It is becoming increasingly recognized that manifestations of congenital heart disease (CHD) extend beyond the cardiovascular system. The factors contributing to renal dysfunction in patients with CHD are multifactorial, with acute kidney injury (AKI) at time of cardiac surgery playing a major role. AKI is often diagnosed based on changes in serum creatinine and estimated glomerular filtration rate (eGFR). Such measurements are often late and imprecise. Recent data indicate that urinary biomarkers interleukin-18 (IL-18) and neutrophil gelatinase-associated lipocalin (NGAL) are earlier markers of AKI. We sought to determine the efficacy of urinary IL-18 and NGAL for detecting early AKI in patients undergoing surgical pulmonary valve replacement (PVR).

METHODS

Twenty patients presenting for surgical PVR with a history of previous repair of a conotruncal anomaly were enrolled. Preoperative clinical data were measured and urine samples and serum creatinine were collected at 6, 12, 24, and 72 hours post bypass. Urine was evaluated for NGAL and IL-18. AKI was determined using the Risk, Injury, Failure, Loss and End Stage Renal Disease (RIFLE) classification system.

RESULTS

Using the RIFLE classification system, seven patients (35%) were found to have AKI defined as a drop in the eGFR or an increase in serum creatinine. All seven patients with AKI had marked increase from preoperative baseline in urine IL-18 (sixfold) and NGAL (26-fold). Using NGAL and IL-18, AKI was detected at 6 hours postoperatively, resulting in AKI being identified 12-36 hours prior to detection by conventional methods. No preoperative predictors for AKI were identified.

CONCLUSION

Both NGAL and IL-18 are early predictive biomarkers of AKI, and both increase in tandem after surgical PVR. Importantly, both rise before an increase in creatinine or a decrease in eGFR is present. Monitoring both biomarkers may allow for earlier detection and subsequent interventions to prevent AKI at time of surgery for CHD.

NIR spectroscopic imaging to map hemoglobin + myoglobin oxygenation, their concentration and optical pathlength across a beating pig heart during surgery

The purpose of this paper is to demonstrate that near-infrared (NIR) spectroscopic imaging can provide spatial distribution (maps) of the absolute concentration of hemoglobin + myoglobin, oxygen saturation parameter and optical pathlength, reporting on the biochemico-physiological status of a beating heart in vivo. The method is based on processing the NIR spectroscopic images employing a first-derivative approach. Blood-pressure-controlled gating compensated the effect of heart motion on the imaging. All the maps are available simultaneously and noninvasively at a spatial resolution in the submillimeter range and can be obtained in a couple of minutes. The equipment has no mechanical contact with the tissue, thereby leaving the heart unaffected during the measurement.

Preventing acute kidney injury after cardiac surgery

PURPOSE OF REVIEW

Acute kidney injury (AKI) is a serious complication that significantly increases morbidity, mortality and cost of care after cardiac surgery. In this review we identify the current literature that addresses strategies for renal protection and the prevention of AKI after cardiac surgery.

RECENT FINDINGS

Even with aggressive medical care and renal replacement therapy (RRT) the morbidity, mortality and cost of postoperative AKI after cardiac surgery is substantial. An emphasis on preventive strategies would therefore appear to be the most cost-effective approach. Recent literature offers hope that as our understanding of the pathogenesis of AKI after cardiac surgery continues to improve, new directions for the prevention and amelioration of AKI will emerge. Approaches to the prevention of postoperative AKI include careful risk stratification of patients, allowing adequate recovery following a prior AKI, consideration of less extensive surgical procedures, avoidance of cardiopulmonary bypass, minimizing injury from radiocontrast dyes or other nephrotoxic agents, and optimizing cardiovascular function and oxygen delivery. Early identification of AKI and prompt, judicious application of RRT may also improve outcomes. Interest in pharmacologic renoprotection is currently directed toward statins and sodium bicarbonate.

SUMMARY

Postoperative AKI is a serious complication after cardiac surgery. Therapeutic interventions and RRT have limited influence on the outcome of AKI, and a preventive strategy remains the mainstay to attenuate its impact.

Intraoperative hemoglobin levels and transfusion independently predict stroke after cardiac operations

BACKGROUND

Anemia has been associated with adverse cerebrovascular outcomes, particularly after cardiac operations. This study was conducted to determine if hemoglobin levels during and after cardiopulmonary bypass (CPB) predict postoperative stroke in cardiac surgical patients, independent of transfusion requirements.

METHODS

Individuals who had sustained a clinical postoperative stroke (cases) were matched 1:2 with controls by age, sex, surgical procedure, and year of operation. In 617 patients, conditional logistic regression was performed to analyze associations between hemoglobin levels before and after CPB, and stroke.

RESULTS

After adjustment for potentially confounding vascular risk factors, lower hemoglobin after CPB was associated with a higher risk of stroke, even after adjustment for the amount of packed red blood cells transfused (adjusted odds ratio, 1.28; 95% confidence interval, 1.00 to 1.64, per point of lower hemoglobin level; adjusted odds ratio for stroke per higher quartile of packed red blood cells transfused in this model, 1.37; 95% confidence interval, 1.02 to 1.83). Similar associations were not found for hemoglobin concentrations preoperatively nor change in hemoglobin from before to after CPB. A post-CPB hemoglobin level below the median was associated with 37% increased odds of a postoperative stroke occurring (p = 0.02).

CONCLUSIONS

Lower postoperative hemoglobin levels and higher intraoperative transfusion requirements are each independently associated with a higher risk of stroke. Reduced cerebral oxygen delivery due to anemia may contribute to cerebral injury after coronary artery bypass grafting.

Cardiac surgery and acute kidney injury: emerging concepts

PURPOSE OF REVIEW

To review recent advances in understanding of the occurrence, pathophysiology, prophylaxis and treatment of cardiac surgery-associated acute kidney injury (AKI).

RECENT FINDINGS

New definitions and prompt diagnostic tools, enhanced risk stratification strategies, avoidance of nephrotoxins and procedure selection are all areas in which recent contributions to the literature have improved perioperative care for cardiac surgery patients. Although evidence continues to confirm the adverse effects of extreme hemodilution and transfusion as part of cardiac surgery, the previously believed advantages of tight perioperative glucose control for the kidney are being questioned in recent studies. Although very little evidence from randomized trials in cardiac surgery populations supports specific interventions to protect or prevent AKI, recent reports indicate reduced AKI associated with some procedural innovations, and the hope that promising findings for agents such as sodium bicarbonate that require further study may yield effective therapies.

SUMMARY

The vexing problem of AKI following cardiac surgery is common and unsolved. Clinical strategies that stress avoidance rather than treatment remain the mainstay of effective management of patients at high renal risk.

The year in cardiothoracic and vascular anesthesia: selected highlights from 2008

The 2008 highlights in cardiovascular and thoracic anesthesia include the ultimate departure of aprotinin from clinical practice. However, a new antihypertensive drug, clevidipine, was approved for perioperative control of hypertension. There were also advances in pharmacologic myocardial conditioning with agents such as cyclosporine, sodium nitroprusside, and levosimendan. Furthermore, ischemic preconditioning appears ready for testing in large clinical trials designed to improve ischemic outcomes after cardiac surgery. With regard to transfusion, a landmark study suggests that transfused red blood cells stored for >2 weeks may significantly worsen major outcome after cardiac surgery. Furthermore, a second study suggests that relative rather than absolute hemoglobin reduction significantly determines adverse outcomes after cardiac surgery. These studies may greatly affect future transfusion guidelines. Left-sided valve replacement has been revolutionized by transcatheter technology, which progressed significantly in 2008. Important advances in percutaneous coronary intervention included drug-eluting bioabsorbable stents and further insights into the clinical consequences of platelet resistance. These 2008 themes represent a sampling of the total highlights for the year. Many of the advances not covered have been reviewed and discussed in the literature review sections of the Journal in 2008.

Effect of oxygen affinity on systemic perfusion and brain tissue oxygen tension after extreme hemodilution with hemoglobin-starch conjugates in rats

PURPOSE

To determine the oxygen affinity for optimal tissue oxygen delivery with a hemoglobin-hydroxyethyl starch conjugate (HRC 101).

METHODS

Anesthetized rats were hemodiluted (180 ml kg(-1)) with low (P(50) approximately 70 mmHg) or high affinity (P(50) approximately 14 mmHg) HRC 101 at hemoglobin (Hb) concentrations near 100 or 70 g l(-1) (n = 6-8). Hippocampal tissue oxygen tension (P(Br)O(2)), blood flow, arterial blood gases, Hb, hematocrit (Hct) and lactate were measured. Data (mean +/- SD) were analyzed by two-way ANOVA.

RESULTS

Hemodilution reduced the hematocrit to 1 +/- 1% in all groups. P(Br)O(2) was best maintained after hemodilution with low affinity HRC 101 at Hb 100 and 70 g l(-1) (25.2 +/- 7.6 and 16.6 +/- 8.3 torr, respectively). P(Br)O(2) decreased (9.5 +/- 9.3 torr, P < 0.05) and serum lactate levels increased (5.0 +/- 1.7 mmol l(-1), P < 0.05) following hemodilution with the high affinity HRC 101 (Hb 100 g l(-1)).

CONCLUSIONS

HRC 101 with a lower oxygen affinity favored tissue perfusion and maintained P(Br)O(2) after near complete blood volume exchange in rats.

Anemia and red blood cell transfusion in neurocritical care

INTRODUCTION

Anemia is one of the most common medical complications to be encountered in critically ill patients. Based on the results of clinical trials, transfusion practices across the world have generally become more restrictive. However, because reduced oxygen delivery contributes to 'secondary' cerebral injury, anemia may not be as well tolerated among neurocritical care patients.

METHODS

The first portion of this paper is a narrative review of the physiologic implications of anemia, hemodilution, and transfusion in the setting of brain-injury and stroke. The second portion is a systematic review to identify studies assessing the association between anemia or the use of red blood cell transfusions and relevant clinical outcomes in various neurocritical care populations.

RESULTS

There have been no randomized controlled trials that have adequately assessed optimal transfusion thresholds specifically among brain-injured patients. The importance of ischemia and the implications of anemia are not necessarily the same for all neurocritical care conditions. Nevertheless, there exists an extensive body of experimental work, as well as human observational and physiologic studies, which have advanced knowledge in this area and provide some guidance to clinicians. Lower hemoglobin concentrations are consistently associated with worse physiologic parameters and clinical outcomes; however, this relationship may not be altered by more aggressive use of red blood cell transfusions.

CONCLUSIONS

Although hemoglobin concentrations as low as 7 g/dl are well tolerated in most critical care patients, such a severe degree of anemia could be harmful in brain-injured patients. Randomized controlled trials of different transfusion thresholds, specifically in neurocritical care settings, are required. The impact of the duration of blood storage on the neurologic implications of transfusion also requires further investigation.

Aprotinin and renal dysfunction

Aprotinin is a polypeptide serine protease inhibitor used to prevent bleeding and need for transfusions in patients having heart surgery. A recent analysis of an observational study data set suggested the use of aprotinin was associated with an increased risk of developing renal failure. The present article reviews the data from basic science studies in tissues, animals and man together with the data from observational studies and randomised controlled trials. The interpretation of the data is hampered owing to the use of different endpoints to describe mild/moderate renal impairment. Nonetheless, the evidence points to aprotinin use being associated with a transient small rise in plasma creatinine concentration in certain patients. There is no evidence for an increased risk of developing new renal failure requiring dialysis/renal replacement therapy.

Neurological complications of cardiac surgery

Neurological injury resulting from cardiac surgery has a range of manifestations from focal neurological deficit to encephalopathy or coma. As the safety of drug-eluting stents comes into question, more patients will likely undergo coronary artery bypass graft surgery. These projections, along with the growing proportions of elderly patients and those with comorbidities, portend the potential for rising rates of perioperative neurological complications. The risk for neurological injury may be determined by the type of procedure, by patient-specific characteristics, and by the extent of cerebral embolization and hypoperfusion during and after surgery. Changes in surgical techniques, including the use of off-pump surgery, have not decreased rates of brain injury from cardiac surgery. When appropriate, modern neuroimaging techniques should be used in postoperative patients to confirm diagnosis, to provide information on potential etiology, to direct appropriate therapy, and to help in prognostication. Management of postoperative medications and early use of rehabilitation services is a recommended strategy to optimize the recovery for individuals with neurological injury after cardiac surgery.

Anemia and cerebral outcomes: many questions, fewer answers

A number of clinical studies have associated acute anemia with cerebral injury in perioperative patients. Evidence of such injury has been observed near the currently accepted transfusion threshold (hemoglobin [Hb] concentration, 7-8 g/dL), and well above the threshold for cerebral tissue hypoxia (Hb 3-4 g/dL). However, hypoxic and nonhypoxic mechanisms of anemia-induced cerebral injury have not been clearly elucidated. In addition, protective mechanisms which may minimize cerebral injury during acute anemia have not been well defined. Vasodilatory mechanisms, including nitric oxide (NO), may help to maintain cerebral oxygen delivery during anemia as all three NO synthase (NOS) isoforms (neuronal, endothelial, and inducible NOS) have been shown to be up-regulated in different experimental models of acute hemodilutional anemia. Recent experimental evidence has also demonstrated an increase in an important transcription factor, hypoxia inducible factor (HIF)-1alpha, in the cerebral cortex of anemic rodents at clinically relevant Hb concentrations (Hb 6-7 g/dL). This suggests that cerebral oxygen homeostasis may be in jeopardy during acute anemia. Under hypoxic conditions, cytoplasmic HIF-1alpha degradation is inhibited, thereby allowing it to accumulate, dimerize, and translocate into the nucleus to promote transcription of a number of hypoxic molecules. Many of these molecules, including erythropoietin, vascular endothelial growth factor, and inducible NOS have also been shown to be up-regulated in the anemic brain. In addition, HIF-1alpha transcription can be increased by nonhypoxic mediators including cytokines and vascular hormones. Furthermore, NOS-derived NO may also stabilize HIF-1alpha in the absence of tissue hypoxia. Thus, during anemia, HIF-1alpha has the potential to regulate cerebral cellular responses under both hypoxic and normoxic conditions. Experimental studies have demonstrated that HIF-1alpha may have either neuroprotective or neurotoxic capacity depending on the cell type in which it is up-regulated. In the current review, we characterize these cellular processes to promote a clearer understanding of anemia-induced cerebral injury and protection. Potential mechanisms of anemia-induced injury include cerebral emboli, tissue hypoxia, inflammation, reactive oxygen species generation, and excitotoxicity. Potential mechanisms of cerebral protection include NOS/NO-dependent optimization of cerebral oxygen delivery and cytoprotective mechanisms including HIF-1alpha, erythropoietin, and vascular endothelial growth factor. The overall balance of these activated cellular mechanisms may dictate whether or not their up-regulation leads to cytoprotection or cellular injury during anemia. A clearer understanding of these mechanisms may help us target therapies that will minimize anemia-induced cerebral injury in perioperative patients.