Clarifying Complex Relationships to Improve Outcomes After Cardiac Surgery

Harrington et al developed a study to analyze the relationship between preoperative cognitive status and hospital discharge outcomes. Their tool was called Clock-in-the-Box (CIB), a technique that “emphasized working memory and executive function by eliminating verbal cues, and requiring the clock to be drawn in a specific location.” The CIB has been shown to have high inter-rater reliability and is highly correlated with other cognitive measures. The authors hypothesized that patients with impaired preoperative cognitive status would be less able to engage with the postoperative environment, and therefore might have a prolonged postoperative course and be more likely to be discharged to a non-home environment.

This was a retrospective study, and the primary outcomes were postoperative length of stay and primary postoperative discharge status (home or not). One might imagine a future study with a prospective design, but it was surprising to see a center that still admits patients preoperatively for evaluation (generally a day prior) when “same-day admit” has become the national norm for most elective surgeries.

The directions for the tool are of particular interest. The patient is handed instructions as follows: “[I]n the blue box on the next page, draw a picture of a clock; put in all the numbers; set the time to ten past eleven.” Patients then are handed the response sheet: each quadrant contains a colored box, with the blue box in the lower right-hand corner. The response is graded from 0 to 8, with lower scores indicating lower cognitive status; the results are graded based on location in the correct box; how well the drawn object resembles a clock; number inclusion, order, and spacing; correct time; and appropriate size of the hands and circle.

At first glance, the results are somewhat difficult to interpret. Nevertheless, further evaluation offers helpful findings. Using multiple analytical approaches, the authors were able to demonstrate that patients with a lower cognitive ability as measured by the CIB were less likely to return home after heart surgery; that those who didn’t return home had a statistically lower CIB score; and that, using a multivariate analysis, an increasing CIB score was associated with an increase in the likelihood that the patient would return home after surgery. However, they were unable to correlate CIB score with postoperative length of stay.

Readers can take home these crucial points: (1) that a significant portion of patients undergoing cardiac surgery have decreased cognitive abilities as measured by the CIB, (2) that the validated CIB tool can be administered by a cardiac surgery nurse practitioner with relative rapidity and facility, and (3) that patients with decreased cognitive abilities as measured by the CIB were less likely to return home after surgery. Given the aging population and the relationship between aging and dementia, there is significant clinical applicability and relevance to these findings from Harrington and colleagues.

The study conducted by Sanders et al²  focused on another important area in cardiac surgery: non-invasive techniques to monitor tissue oxygenation and to help provide appropriate endpoints of resuscitation. As part of current routine postoperative care of patients who have undergone cardiac surgery, repeated measurements are frequently made of serum lactate as well as mixed venous oxygen saturation as markers of global hypoperfusion. The downsides of these markers are that they require repeated blood samples and, in the example of a mixed venous oxygen saturation value, a pulmonary artery catheter. Near-infrared spectroscopy (see Figure) is a completely noninvasive technique that uses electromagnetic waves (680–800 nm) to measure the percentage of hemoglobin that is oxygenated in peripheral tissues (in this study, the thenar eminence). Unlike pulse oxymetry, this technique does not measure oxygen saturation in arterial blood. Tissue oxygen saturation (StO₂) is the primary variable measured, with values ranging from 80 to 93% in healthy individuals (see Figure). The literature behind this technology has emerged fairly recently,¹³  and its use has been documented for patients with traumatic and septic shock,^14,15  but it is limited in patients after cardiac surgery.¹⁶  The upside of this technology is the lack of any invasive component; the primary downsides are the potential for abnormal values to represent local rather than global ischemia and the paucity of true data definitively associating its use with improved clinical outcomes.

This study of 74 adult patients undergoing cardiac surgery was predominantly exploratory in nature. The authors found that StO₂ rose after induction of anesthesia, then fell during cardiopulmonary bypass. The measure also fell during the first 2 hours after surgery and rose by 6 hours after surgery, remaining stable. The authors also noted a correlation between a smaller rise in St O₂ during the first 5 minutes of anesthesia and lower StO₂ during the first 20 minutes of ICU monitoring with the presence of prespecified postoperative morbidities on day 3 and 15 after surgery.

Investigators replicating this study might provide additional information about the ICU course as well as careful measurement and reporting of hemodynamic values (MAP, CVP, PCWP), serum lactate levels, and mixed venous oxygen saturation, correlating those values with StO₂. In addition, more detailed data about the interventions performed while the patients were in the ICU would add to the literature,^14,16  as would information about whether StO₂ responded in concert with changes in the normally measured variables to interventions such as providing the patient with fluid or adjusting inotropes and vasopressors. If StO₂ technology is to be optimized in the critical care environment, such data are vital.

It is also important to note that several studies have documented that StO₂ values in patients with septic shock may be similar to those of healthy volunteers. This is why it has been recently recommended that StO₂ measurements be made in a dynamic fashion during brief vascular occlusion (venous or arterial) rather than a static fashion as done in this study.¹³  Dynamic measurements are especially important given the vasoplegic state that can occur after cardiac surgery: the shock state that can be present after cardiac surgery is often not that of pure “pump failure.”

Careful selection of who should undergo cardiac surgery and how they should be properly managed after surgery remains an enormously complex, important, and fundamentally multidisciplinary area in patient care. Both of these articles have direct relevance to the practicing ICU clinician. With respect to the study by Harrington et al, we can all go back to our hospitals and ask, “Are we measuring preoperative cognitive abilities in patients who will undergo cardiac surgery?” And we might follow up with the more important question, “Should we?”

With the growing population of geriatric patients who will inevitably be candidates for cardiac surgery, it is crucial to ensure that the ICU team knows which patients may be at risk for not being discharged home. It is especially important that future studies carefully explore and tease apart age to integrate geriatric issues into selection for cardiac surgery while preventing “age bias.” In terms of the Sanders study, careful, thoughtful, incisive, and scientifically sound evaluations of new technological monitoring modalities remain high on the list of goals for the critical care research community.

If postoperative care can be provided that is equal to and perhaps better than current practice, but in a completely noninvasive (and hopefully cost-effective) fashion, we owe it to our patients to get this technology to the bedside as soon as possible. But studies such as the one by Sanders et al are absolutely crucial to prevent situations from arising that occurred with the pulmonary artery catheter: more bedside data for the sake of more bedside data does not necessarily correlate with better outcomes. Once the science has proved that a new technology is associated with improved outcomes, only then should the technology make its way into routine practice at the bedside.