Description

Postoperative mortality rates in cardiac surgery remain elevated when compared with other types of surgery.1 2 In particular, postoperative complications can involve several organs and may have a major impact on the length of hospital stay and quality of life following surgery.2 Current clinical risk prediction models often contain a large number of variables, are time-consuming to calculate and do not perform well on individual bases.3 4 Herein, we present and discuss a novel non-invasive microcirculatory imaging method as a risk stratification tool for cardiac surgery patients.

A 71-year-old woman with a history of hypertension was admitted to our institute with suspected cardiac asthma, presenting as an non-ST-elevation myocardial infarction (NSTEMI) with mitral insufficiency grades 3–4. Following coronary angiogram examination, the patient was diagnosed with three-vessel disease and scheduled for coronary bypass surgery with mitral valve replacement. Preoperative blood analysis showed acceptable levels of serum creatinine 79 µmol/L, C reactive protien 12.8 mg/L and white cell count 8.7×10⁹/L. Thereby clinicians deemed the patient suitable for surgery. Retrospective risk assessment of acute renal failure using the Thakar et al score5 predicted a 2.06% risk of requiring postoperative dialysis.

Non-invasive sublingual microcirculation assessment was performed using incident dark field imaging (IDFI) (Cytocam-IDFI, Braedius, Huizen, Amsterdam, The Netherlands),6 immediately following anaesthesia induction. The Cytocam emits green light at 348 nm wavelength which corresponds to the isosbestic absorption range of haemoglobin, thereby enabling real-time visualisation of red blood cell behaviour within the microvascular bed. The camera was placed into the sublingual cave immediately adjacent to the lingual frenulum, 1–2 cm from the base of the tongue. Image sequences consisting of 150 frames were captured, which revealed substantial microcirculatory leucocytosis with areas of heterogeneous microvascular perfusion (video 1). Leucocytes expressed as bright white globules could be seen sticking and rolling on the wall of the endothelium within the capillary to postcapillary venule unit (figure 1). The direct visualisation of leucocyte adhesion to the endothelial surface at the baseline incurs the presence of compromised endothelial barrier and endothelial hyperpermeability.7 The degradation of the glycocalyx must occur for the expression of adhesion molecules which are required for leucocyte-endothelial adhesion and subsequent diapedesis.7

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Figure 1

(A) Screenshot of the Cytocam-incident dark field captured video clip of the sublingual microcirculation immediately following anaesthesia induction. The red outline indicates a capillary to postcapillary venule (C-PCV) unit. Observation of the C-PCV unit is important for studying leucocyte behaviour within the microcirculation, as leucocyte velocity will change progressing from the capillary into the PCV. Microcirculatory parameters were determined using Automated Vascular Analysis V. 3.2 (Microvision Medical, Amsterdam, The Netherlands), which revealed: total vessel density 26.25 mm/mm², functional capillary density 20.14 mm/mm², portion of perfused vessel 76.75% and mean flow index 1.75. (B) Blue arrows indicated leucocytes, expressed as bright white globes, adhered to the endothelial surface. This creates characteristic ‘bite mark’ appearances on the endothelial wall.

During surgery, low dose norepinephrine was administered to maintain adequate blood pressure and heart rate. The patient required a total extracorporeal circulation time of 237 min. Overall, the surgery was uncomplicated. An attempt at microcirculatory monitoring was made at the end of surgery during skin closure. However, due to excessive bleeding within the oral cavity (from heparin administration), image sequences captured were not of sufficient quality for analysis.6 During cardiac surgery, blood present within the oral cavity alongside pressure artefacts induced within the sublingual cave from the transthoracic echo and mechanical ventilation presents a major limitation of this technique.

Following Intensive Care Unit admission, the patient was diagnosed with acute kidney injury (AKI) based on a 55 µmol/L increase in serum creatinine levels and low urine volume output. Within 48 hours after surgery, the patient developed anuria and was placed on dialysis. From which the patient did not recover and died in the ICU from multiple organ failure within 5 days of admission. Impairment of endothelial barrier detected at the baseline with further exposure to the non-physiological conditions of the cardiopulmonary bypass, fluid shift and inflammation may further contribute to endothelial damage.8 9

This is the first report demonstrating the presence of endothelial barrier dysfunction preceding the development of serious postoperative complications following cardiac surgery in a non-invasive manner. Sublingual microcirculation monitoring as a risk stratification tool may be useful for predicting outcomes at the patient level. The device is portable, easy to set up and cost-effective to run. Thereby, routine bedside evaluations may provide clinicians with crucial information on the integrity of the microcirculation. These simple correctable dysfunctions may be acted on (ie, off-pump surgery) to reduce the risk of postoperative complications. Future investigations should examine the hypothesis between baseline microcirculation derangements and complications following cardiac surgery in a bigger cohort study.