Advanced neurologic monitoring for cardiac surgery

Role of Transcranial Doppler in Cardiac Surgery Patients

Abstract

Purpose of Review

This review discusses applications of transcranial Doppler (TCD) in cardiac surgery, its efficacy in preventing adverse events such as postoperative cognitive decline and stroke, and its impact on clinical outcomes in these patients.

Recent Findings

TCD alone and in combination with other neuromonitoring modalities has attracted attention as a potential monitoring tool in cardiac surgery patients. TCD allows not only the detection of microemboli and measurement of cerebral blood flow velocity in cerebral arteries but also the assessment of cerebral autoregulation.

Summary

Neuromonitoring is critically important in cardiac surgery as surgical and anesthetic interventions as well as several other factors may increase the risk of cerebral embolization (gaseous and particulate) and cerebral perfusion anomalies, which may lead to adverse neurological events. As an experimental tool, TCD has revealed a possible association of poor neurological outcome with intraoperative cerebral emboli and impaired cerebral perfusion. However, to date, there is no evidence that routine use of transcranial Doppler can improve neurological outcome after cardiac surgery.

Haemoglobin oxygen saturation as a biomarker: the problem and a solution

Near-infrared spectroscopy measures of haemoglobin oxygen saturation are often used as an indicator of sufficient oxygen delivery to assess injury susceptibility and tissue damage. They have also often been used as a surrogate measure of oxygen metabolism. Unfortunately, these measures have generally failed to provide robust indicators of injury and metabolism. In this paper, we first review when haemoglobin oxygen saturation does work as a robust indicator, and then detail when and why it fails for assessing brain injury and breast cancer. Finally, we discuss the solution to obtain more robust measures of tissue injury and cancer by combining oxygen saturation measurements with measures of blood flow and volume to more accurately estimate oxygen metabolism.

Antiplatelet therapy in patients receiving aortic bioprostheses: A report of clinical and instrumental safety

OBJECTIVE

The main advantage of bioprostheses, avoidance of anticoagulant therapy, is compromised during the early postoperative period; in fact, warfarin is often administered during the first 3 postoperative months.

METHODS

We analyzed 250 patients undergoing tissue aortic valve replacement between January 2002 and December 2005. The patients received either aspirin (group 1) or oral anticoagulation (group 2) during the first 3 months. In a subgroup of these patients, we investigated the possible presence of clinically silent microembolization by means of transcranial Doppler for microembolic signal detection.

RESULTS

Thirty-day mortality was 0%. No major neurologic events occurred. Two episodes of bleeding were observed in both groups. Follow-up time was 24 +/- 14 months. Overall late mortality rate was 0.8% in group 1 versus 12% (mainly cancer related) in group 2. In group 2, 2 deaths were due to major ischemic neurologic events; overall, 3 major neurologic episodes occurred (international normalized ratio was within therapeutic range). There were no neurologic events in group 1 (P = .12). Stroke-free survival did not reach statistical significance between the 2 groups. Transcranial Doppler was performed after a mean interval of 55 +/- 19 days, with no detection of microembolic signals in patients receiving either aspirin or warfarin. There were no episodes of bleeding or neurologic events.

CONCLUSIONS

Aspirin therapy appears to be the appropriate response to both cardiac surgeons' and patients' needs in the early postoperative course after aortic valve replacement with tissue valves, demonstrating adequate antithromboembolic efficacy with no added risk for bleeding as well as ease of administration.

Correlation of transcutaneous hepatic near-infrared spectroscopy readings with liver surface readings and perfusion parameters in a piglet endotoxemic shock model

PURPOSE

To determine whether transcutaneous liver near-infrared spectrophotometry (NIRS) measurements correlate with NIRS measurements taken directly from the liver surface, and invasive blood flow measurements.

PROCEDURE

Laparotomy was performed in 12 Yorkshire piglets, and ultrasound blood flow probes were placed on the hepatic artery and portal vein. Intravascular catheters were inserted into the hepatic and portal veins for intermittent blood sampling, and a pulmonary artery catheter was inserted via the jugular vein for cardiac output measurements. NIRS optodes were placed on skin overlying the liver and directly across the right hepatic lobe. Endotoxemic shock was induced by continuous infusion of Escherichia coli lipopolysaccharide O55:B5. Pearson's correlations were calculated between the NIRS readings and the perfusion parameters.

FINDINGS

After endotoxemic shock induction, liver blood flow, and oxygen delivery decreased significantly. There were statistically significant correlations between the transcutaneous and liver-surface NIRS readings for oxyhemoglobin, deoxyhemoglobin, and cytochrome c oxidase concentrations. There were similar significant correlations of the transcutaneous oxyhemoglobin with both the mixed venous and hepatic vein saturation, and mixed venous and hepatic vein lactate.

CONCLUSIONS

Transcutaneous NIRS readings of the liver, in an endotoxemic shock model, correlate with NIRS readings taking directly from the liver surface, as well as with global and specific organ-perfusion parameters.