Recruitment of renal functional reserve by intravenous amino acid loading in a sheep model of cardiopulmonary bypass

Abstract

Background: Cardiopulmonary bypass (CPB) may decrease the renal functional reserve (RFR). However, the temporal changes in RFR after during the recovery period after CPB remains unknown. We assessed RFR before and then weekly after CPB over four weeks following CPB in non-anaesthetised sheep.

Methods: In 10 Merino ewes, amino acids were infused before CPB and weekly for four weeks to assess RFR. At each assessment, we measured renal blood flow (RBF), renal oxygen delivery (RDO₂), creatinine clearance and medullary and cortical oxygenation. Histological assessment was performed at 4 weeks.

Results: Before CPB, amino acid infusion increased RBF from (mean ± SD) 6.60 ± 1.64 to 8.56 ± 1.80 mL/kg/min, and RDO₂ from 0.80 ± 0.28 to 1.12 ± 0.37 mL O₂/kg/min. These renal macro-circulatory responses remained consistent across all weekly assessments after CPB. Amino acid infusion also increased creatinine clearance (from 62.5 ± 15.0 to 110 ± 30.6 mL/h pre-CPB) throughout the study period. RFR remained unchanged over time (P = 0.53). However, compared with pre-CPB values, medullary (33.9 ± 9.0 pre-CPB to 15.1 ± 13.2 mmHg at 4 weeks, P = 0.0068) and cortical tissue PO₂ (46.0 ± 14.2 to 17.2 ± 6.5 mmHg, P = 0.0029) decreased over time. Furthermore, the response of the medullary (but not cortical) PO₂ to amino acid infusion changed over time (P = 0.0064). While medullary PO₂ did not change in response to amino acid infusion pre-CPB and at one week after CPB, it appeared to fall from two weeks thereafter (P = 0.039 and 0.091 at weeks 2 and 3, respectively). Despite preserved RFR, sheep exposed to CPB showed greater peritubular inflammation, interstitial fibrosis and tubular casts compared with healthy controls (P = 0.007, 0.021, 0.007, respectively).

Conclusions: In this large mammalian model of CPB, weekly amino acid administration consistently recruited RFR over four weeks, despite the presence of histological injury. However, it was associated with the development of renal medullary hypoxia after two weeks. These findings highlight the complexity of the pathophysiological response of the kidney to CPB.

Keywords: Acute kidney disease; Acute kidney injury; Amino acids; Cardiac surgery; Cardiopulmonary bypass; Chronic kidney disease; Renal functional reserve.

Fig. 1

Schematic description of the study. Abbreviations: AA, amino acids; CPB, cardiopulmonary bypass; RFR, renal functional reserve

Fig. 2

Changes in renal haemodynamics following amino acid infusion. (a–e) Renal blood flow; (f–j) renal vascular conductance. Amino acids were infused over the 0–30 min period. Sample sizes are indicated in each panel, as some data were unavailable due to equipment failure or test subject loss. P-values were obtained using a mixed-effects model with a Greenhouse–Geisser correction applied to the main effect of time. Data are presented as mean ± SD. CPB cardiopulmonary bypass

Fig. 3

Renal microcirculation. (a) Baseline medullary tissue oxygen tension (PO₂); (b) Comparison of baseline vs. post-AA infusion medullary PO₂; (c) Changes in medullary PO₂ relative to baseline; (d) Baseline cortical PO₂; (e) Comparison of baseline vs. post-AA infusion cortical PO₂; (f) Changes in cortical PO₂ relative to baseline. Sample sizes for medullary and cortical PO₂ are shown in panels a and d, respectively, as some data were unavailable due to equipment failure or test subject loss. P-values were obtained using a mixed-effects model with a Greenhouse–Geisser correction for the main effect of time (a, c, d, and f), or by performing multiple paired t-tests with Holm-Šidák correction (b and d). Data are presented as mean ± SD. * P < 0.05. AA, amino acids; CPB, cardiopulmonary bypass

Fig. 4

Renal functional indices. a Changes in baseline plasma creatinine pre- and post-CPB. Sample sizes are indicated in each panel, as some data were unavailable due to equipment failure or test subject loss. P-values were obtained using a mixed-effects model with a Greenhouse–Geisser correction applied to the main effect of time. b Resting (baseline) and maximum glomerular filtration rates (GFR) recruited by amino acid infusion. P-values were obtained using a paired t-test (*P < 0.05). c Changes in renal functional reserve, calculated as maximum minus resting GFR. P-values were obtained using a mixed-effects model with a Greenhouse–Geisser correction applied to the main effect of time. All data are presented as mean ± SD. Abbreviation: CPB, cardiopulmonary bypass; GFR, glomerular filtration rates

Fig. 5

Representative images of histological analysis. (a, b) Normal kidney tissue; (c, d) moderate peritubular inflammation; (e, f) moderate interstitial fibrosis; (g, h) hyaline casts. White scale bars represent 100 µm in panels a, b, c, e, and f, and 10 µm in all other panels