Which of the following is an alarming sign during dialysis that requires immediate action?

Bright red blood returning from the dialyzer is an alarming sign during dialysis that necessitates immediate action because it may indicate a possible complication, such as a hemorrhage or clotting issue within the dialysis circuit. During hemodialysis, the blood is filtered through a dialyzer, and the blood return should generally be a darker red, reflecting the oxygen content. Bright red blood suggests that the blood may be fully oxygenated, which could occur if there is a leak in the dialyzer or if there's a serious issue with the patient’s vascular access. Rapid recognition and intervention are essential to prevent potential morbidity or mortality.

In contrast, patient feedback on comfort is important for overall care but does not usually signal an immediate medical emergency. An increase in heart rate could be a normal response to various factors, including anxiety or fluid shifts, and requires monitoring rather than immediate action unless it falls into a concerning range. Stable blood pressure is a positive sign indicating that the patient is likely tolerating the dialysis session well, providing no immediate cause for alarm.