Respiratory quotient as an early predictor of length of stay after cardiac surgery: A prospective observational study

Abstract

Background and objectives: This study aimed to investigate the respiratory quotient (RQ) levels and its trend in the early postoperative period of patients with heart disease. Additionally, we explored factors influencing RQ and evaluated the predictive capabilities of RQ and lactic acid for various outcomes.

Methods and study design: In this prospective observational study, participants included were heart disease patients aged 18-80 years who underwent elective open-heart surgery and were subsequently admitted to the cardiothoracic surgery ICU post-operation. Indirect calorimetry (IC) measurements were conducted on patients dur-ing the first three days after surgery to assess their RQ levels. Clinical data, including personal information, postoperative characteristics, and duration of surgery, were documented based on the patient's medical history.

Results: In this study, 135 patients, of whom 57.04% were male, underwent a total of 247 IC measurements following cardiac surgery. On the first, second, and third days of admission to the ICU, the RQ values were 0.77±0.09, 0.80±0.07, and 0.78±0.05, respectively. The ROC curve analysis showed that on the first day of admission to the ICU, RQ was a better predictor of prolonged mechanical ventilation, LOS in ICU, and LOS in hospital compared to lactate level. However, on the second day of ICU admission, neither RQ nor lactate level could predict longer durations of mechanical ventilation, LOS in ICU, and LOS in hospital.

Conclusions: RQ may serve as a potential predictor for LOS in patients after cardiac surgery.

Keywords: serum lactate; cardiac surgery; indirect calorimetry; length of stay; respiratory quotient.

Figure 1

RQ of cardiac patients after operation. RQ1: respiratory quotient on the 1st day of admission to ICU; RQ2: respiratory quotient on the 2nd day of admission to ICU; RQ3: respiratory quotient on the 3rd day of admission to ICU. Repeated measured ANOVA (based on Generalized Linear Model) was used, and the mixed-effects (maximum likelihood) model was used for missing values. Tukey's method was used for post hoc comparisons. The p-value refers to the result of post hoc comparisons using Tukey's method.

Figure 2

Scatter plots of RQ. RQ: respiratory quotient; r2 and p-value are statistical values of linear regression

Figure 3

Heat Map of RQ. Each block signifies a pair of variables, and the color intensity within the block denotes the strength and direction of their relationship. A darker hue represents a stronger positive correlation, while a lighter shade suggests a weaker or no correlation. The correlation coefficient is expressed by the color depth of the block; Lactate 1, glucose 1, REE 1, HR1, Body temperature 1, RQ1 refer to the values of parameters on the day after operation; Lactate 2, glucose 2, REE 2, HR2, Body temperature 2, RQ2 refer to the values of parameters on the second day after operation; PN: energy intake of intravenous infusion fluid on the first day after surgery

Figure 4

ROC curve for lactate and RQ. RQ1 refers to the respiratory quotient on the day of admission to the ICU; RQ2 refers to the respiratory quotient on the second day of admission to the ICU; Lactate1 refers to the lactic acid level on the day of admission to the ICU, and the interval between the determination time and RQ1 was less than 2 h; Lactate2 refers to the lactic acid level on the second day of admission to the ICU, and the interval between the determination time and RQ1 was less than 2 h. LOS: length of stay. The median values of DMV, LOS in ICU, and LOS in hospital were set as cut-off points, and those larger than the median were set as positive outcomes