In this study, renal function tended to improve with a decrease in the Cr level and increase in the GFR after surgery. As shown in Table 2, the GFR of Group 2 increased immediately after surgery compared to that of Group 1, in accordance with longer-duration low blood pressure. Table 3 shows that longer-duration low blood pressure was associated with a higher Cr post/pre immediately after surgery. Taken together, these observations indicated that renal function improved after shoulder arthroscopy in the beach chair position, but the degree of improvement appeared to be lower in patients experiencing periods of hypotension, i.e., a MAP < 50 mmHg.
General anesthesia inhibits the lower extremity muscle contraction necessary for maintenance of venous return to the heart. Venous congestion caused by the beach chair position during shoulder surgery under general anesthesia decreases preload and promotes abnormal cardiac contractions due to catecholamine hypersecretion [6]. The surgeon should ask the anesthesiologist to lower the patient’s blood pressure to improve arthroscopic vision. Blood pressure naturally drops after general anesthesia in the beach chair posture. Therefore, if the operation is performed under general anesthesia, large amounts of vasopressors are needed, and if the operation is performed only with interscalene block and sedation, large amounts of blood pressure-lowering agents, such as nicardipine, are needed [11]. These observations raise questions regarding the appropriate blood pressure for shoulder arthroscopy when the patient is in the beach chair posture. In a retrospective study of 5,127 patients undergoing noncardiac surgery, an increased risk of postoperative AKI (defined as a > 50% or > 0.3 mg/dL increase in serum Cr concentration) was found in cases with intraoperative MAP < 60 mmHg for > 20 minutes and < 55 mmHg for > 10 minutes [4]. In the present study, MAP50 was significantly correlated with Cr post/pre, while no such correlation was observed for MAP60. Therefore, taking postoperative renal function into consideration, it is preferable to maintain the MAP above 60 mmHg during shoulder arthroscopic surgery with the patient in the beach chair position.
In a study of high risk patients undergoing cardiac surgery, postoperative renal function deterioration was defined as an increase in the serum Cr level during the first postoperative week of ≥ 25% compared to the preoperative level [12]. GFR, defined as the amount of blood the kidney cleans in 1 minute, is the most important measure of kidney function and normal values are in the range 90–120 mL/min. That is, the kidneys in subjects with normal kidney function can clean 90–120 mL of blood in 1 minute, which translates to about 120–180 L of blood filtered throughout the day. Kidney function is usually assessed using the Modification of Diet in Renal Disease (MDRD) equation, which calculates the GFR.
The MDRD equation has been studied in thousands of white and black kidney disease patients in the USA, making it easy to assess the GFR based on age, gender, race, and serum Cr concentration. This method has the advantage of easy calculation of GFR without the need for complicated procedures such as urine collection or nuclear medicine tests. However, as the formula is based on patients with kidney disease, it tends to slightly underestimate the values in people with early kidney disease, and in healthy people with GFR ≥ 60 mL/min. In addition, as these studies were performed in white and black subjects, there is some controversy about the applicability to Asian races, such as Koreans. Therefore, the Cr post/pre was set as the primary outcome in this study. As shown in Table 2, follow-up results more than 1 day after the operation were not available in many patients, and all such indicators were therefore excluded from the correlation analysis. There was a significant difference in GFR between Groups 1 and 2 (109.72 ± 25.44 vs. 116.88 ± 37.34, respectively, P = 0.034) immediately after surgery. However, as the demographic data of the two groups were very different (Table 1), the validity of the comparison is not clear.
In fact, there have been many reports of complications related to brain function associated with surgery in the beach chair position. Significant hypotension can lead to cerebral hypoperfusion and neurological complications, such as cerebral ischemia, pituitary apoplexy, and cardiovascular collapse [7–9]. To prevent these complications, some studies used sequential compression devices [13] or attempted intravenous bolus administration of arginine vasopressin [14]. In the present study, when blood pressure was low, 50 ∝g of phenylephrine or 5 mg of ephedrine was injected as a bolus. At the same time, 200–300 cc of plasmalyte was loaded so that the medication could exert its effects quickly and correct the volume depletion. If the vasopressor requirement is high, sustained phenylephrine infusion (1–2 g/h) should be started. However, because most patients had an intravenous 18-gauge cannula in the arm opposite the surgical shoulder, NIBP was difficult to acquire every 5 minutes. Although anti-reflux valves were used in all patients, reflux or blockage nevertheless occurs frequently. Therefore, patients with well-maintained blood pressure may receive only low levels of fluid. This is thought to account for why only 250 cc of fluid was administered to the patient in whom the Cr level more than doubled. One study showed that patients undergoing shoulder surgery in the beach chair position are more likely to show impaired autoregulation, as demonstrated by the higher cerebral oximetry index and lower regional cerebral oxygen saturation (rScO2) than patients undergoing surgery in the lateral decubitus position [15]. Subtracting 1.35 mmHg from the blood pressure measured in the arm or leg for each 1 cm of head elevation has been recommended to avoid cerebral hypoperfusion due to the effect of gravity. However, there have been no previous studies of the function of the kidney in the beach chair position, when it is below the heart, or on the regulation of blood flow to the kidney. Two mechanisms have been shown to be involved in renal autoregulation, i.e., the myogenic response and the tubuloglomerular feedback mechanism [16]. A piglet study [17] concluded that renovascular autoregulation is impaired in advance of cerebrovascular autoregulation during hemorrhagic shock. Taken together, these findings indicate cerebral blood flow control is more difficult than renal blood flow control in the beach chair position, so care is needed. Reflecting the results of this study, it is sufficient to monitor brain function and rScO2 in cases with MAP ≥ 60 mmHg, but renal damage may occur in cases with MAP ≤ 50 mmHg. Therefore, it is necessary to prevent the MAP from falling below 50 mmHg, and if it does drop below this level, active treatment to increase the blood pressure immediately is necessary.
This study had some limitations. First, it used a retrospective design, and consistent and accurate administration of fluids and vasoactive drugs may not have been achieved. However, the same surgeon performed the same operations, and the anesthesia team used the same guidelines. Second, the number of patients was different between the hypotension group (Group 1) and control group (Group 2), and there were differences in the demographic data; thus we performed correlation analysis. Third, there may be a measurement accuracy problem. It is not possible to know how accurate it is because the pressure is not measured directly through a catheter, but measured by NIBP. However, the non-surgical arm was fixed to a dedicated armrest, so the measurement was not interfered with by the surgeon or assistant. Renal blood flow and pressure in humans cannot yet be measured directly; new medical devices and equipment or research techniques are thus required [18].