Non-invasive echocardiographic studies in mice - Influence of anesthetic regimen

Transgenic murine models of cardiovascular disease offer great potential insights regarding mechanisms of human disease, but efficient and reliable methods for phenotype evaluation are necessary. We employed non-invasive echocardiography to evaluate hemodynamic parameters in mice, and evaluated statistical reliability of these parameters with respect to anesthesia regimen. Male CF-1 mice received inhaled halothane (0.25-0.75% in 95% O₂) or ketamine/xylazine (80/10mg/kg i.p.) and 2-dimensional, M-mode, and Doppler ultrasound imaging were used to assess cardiac contractility and aortic flow velocities. Halothane was more convenient and reliable with respect to rate of induction, reversal, and control of anesthetic depth. At comparable levels of anesthesia, ketamine/xylazine produced significant reductions in heart rate (308 ± 14 vs. 501 ± 14 bpm, p<0.001), left ventricular fractional shortening (41.7 ± 1.3 vs. 49.3 ± 1.0 %, p<0.001), and cardiac output (7.6 ± 0.5 vs. 11.5 ± 0.6 ml/min, p<0.001) when compared to halothane inhalation. No change in stroke volume or peak aortic velocity was observed. Correlation analyses revealed highly significant positive relationships between heart rate and fractional shortening (r= 0.61, p<0.002) and cardiac output (r= 0.88, p<0.001) but no relation to stroke volume or aortic velocity. Variability of intra-animal and intra-group parameter estimation were frequently 2-fold larger for ketamine/xylazine anesthesia vs. halothane. Statistical power analysis showed the increased measurement error for ketamine/xylazine leads to much larger numbers of mice/group to achieve identical statistical sensitivity. These data further illustrate the feasibility of echocardiography for rapid, non-invasive cardiovascular assessment in mice. However, several obtainable parameters are highly sensitive to both heart rate and anesthetic used and the choice and control of anesthetic are critical for physiologically relevant performance parameters and maximal ability to detect statistical differences among groups. Thus, for these non-invasive studies, inhalation anesthesia with agents such as halothane is superior to anesthesia induced by ketamine/xylazine administration.