Association of Lactate-to-Bicarbonate Ratio with Adverse Intensive Care Unit Outcomes After Coronary Artery Bypass Grafting: A Retrospective Cohort Study Based on the MIMIC-IV and eICU-CRD Databases

Every year, thousands of people undergo a major heart surgery known as coronary artery bypass grafting. In this procedure, surgeons create new pathways for blood to flow around blocked arteries, giving the heart a fresh supply of oxygen. While the operation is a triumph of modern medicine, the journey does not end when the patient leaves the operating room. They move to an intensive care unit, a place where the body is under constant watch as it begins the difficult work of recovery. For some, this recovery is smooth and swift. For others, it is a slow, complicated struggle marked by lingering weakness, kidney trouble, or even death. Doctors have long searched for early signs that can tell them which patients are in danger before the trouble becomes severe, hoping to adjust care and prevent bad outcomes.

Two chemical signals in the blood have long been used to gauge how well a patient is recovering. The first is lactate, a substance that builds up when tissues do not get enough oxygen or when the body is under extreme stress. High levels of lactate often suggest that the body is struggling to keep up with its energy needs. The second signal is bicarbonate, which acts as a buffer, helping to neutralize acid and keep the body's internal environment stable. When bicarbonate levels drop, it often means the body is losing its ability to handle that acid load. While doctors have looked at these two numbers separately, a new study suggests that looking at the relationship between them might tell a clearer story about a patient's risk.

Researchers set out to test this idea by examining the records of nearly 3,250 adults who had undergone heart bypass surgery and were admitted to intensive care units in the United States. They used data from two massive, publicly available databases that contain detailed medical records from hundreds of thousands of patients. The team focused on the first 24 hours after a patient arrived in the intensive care unit, a critical window where the body's initial reaction to surgery plays out. They calculated a specific ratio for each patient by taking the highest level of lactate recorded during that day and dividing it by the lowest level of bicarbonate recorded in the same period. This method captured the worst moment of metabolic stress rather than just a single snapshot in time.

The study divided the patients into three groups based on this ratio: those with a low ratio, those with a middle ratio, and those with a high ratio. The results showed a clear pattern. Patients in the high ratio group faced a significantly greater risk of adverse outcomes compared to those in the low ratio group. In the larger group of patients, those with the highest ratio were nearly twice as likely to experience a bad outcome, which the researchers defined as either staying in the intensive care unit for an unusually long time or dying before leaving the hospital. This finding held true even after the researchers accounted for other factors that could influence recovery, such as the patient's age, sex, body weight, and pre-existing conditions like diabetes or high blood pressure.

To ensure their results were not a fluke specific to one hospital or one set of records, the researchers tested their findings on a second, independent group of patients from a different database. The results were consistent. In this second group, patients with the highest ratio also faced a much higher risk of prolonged hospital stays or death. The data suggested that the risk did not rise in a simple, straight line; instead, the danger seemed to increase more sharply once the ratio passed a certain threshold. This implies that there is a tipping point where the combination of high stress and low buffering capacity becomes particularly dangerous for the heart and lungs.

The researchers also looked at specific complications to see which ones were most linked to this high ratio. They found that patients with high ratios were much more likely to need mechanical ventilation to help them breathe for more than a day and were more likely to have a prolonged stay in the intensive care unit. While the high ratio was also associated with a higher chance of death, the number of deaths in the study was relatively small, making it harder to draw a definitive conclusion about mortality alone. Similarly, the ratio did not show a strong, independent link to acute kidney injury, suggesting that its primary signal relates more to the overall stress on the body's systems and the ability to recover from the shock of surgery.

One of the most important aspects of this work is how the researchers measured the ratio. They did not just take the first blood test done when a patient arrived. Instead, they looked for the highest lactate and lowest bicarbonate within the first 24 hours. This approach recognized that a patient's condition can change rapidly after surgery. A single test might miss a moment of severe stress that occurs an hour or two later. By capturing the worst moment, the ratio provided a more accurate picture of the body's struggle. The study also noted that patients who did not have these specific blood tests recorded were actually at even higher risk, suggesting that the absence of these measurements itself might be a sign of a very sick patient whose care was focused on immediate survival rather than routine monitoring.

Despite these strong associations, the researchers were careful not to claim that this ratio is a perfect crystal ball. When they tested how well the ratio could predict bad outcomes on its own, it performed better than looking at lactate or bicarbonate alone, but it was not a flawless predictor. It was not as powerful as a comprehensive scoring system that doctors already use to judge the severity of a patient's illness. This means the ratio should not replace the judgment of a doctor or the full picture of a patient's health. Instead, it serves as a useful, inexpensive tool that can add another layer of information. It acts as an early warning light, alerting the medical team that a patient is under significant metabolic stress and may need closer observation or different support strategies.

The study concludes that this simple calculation, derived from two common blood tests, offers a valuable way to identify heart surgery patients who are at higher risk of a difficult recovery. It highlights that the balance between the body's stress signals and its ability to neutralize them is a critical factor in survival. While the findings are promising, the researchers emphasize that this is a retrospective study, meaning it looks back at past data rather than testing a new treatment in real time. Future studies will be needed to see if using this ratio to guide treatment decisions can actually improve outcomes for patients. For now, it stands as a clear signal that in the complex aftermath of heart surgery, the relationship between stress and stability is a key indicator of what lies ahead.