Background and Goal of Study: 

Cardio-pulmonary Exercise Testing (CPET) is a well-validated method of assessing cardiorespiratory performance. Measurements of oxygen consumption (VO2) recorded at peak exercise & Anaerobic Threshold (AT) have been found in previous studies to identify those at highest risk of post-operative complications  The ultrasonic cardiac output monitor (USCOM) has been used as a non-invasive method of assessing cardiac function for several years. Recent work has demonstrated that the USCOM can be used to estimate inotropy (cardiac contractility), a well-recognised but seldom-quantified concept in cardiac physiology. Calculating inotropy in addition to the more routine haemodynamic measurements recorded by USCOM may provide a tool that could help in pre-operative risk profiling & haemodynamic optimisation peri-operatively. 

We assessed baseline & change (Δ) in USCOM-obtained haemodynamic values pre- & post-CPET (bike ergometer), in high-risk pre-operative patients. Correlation with CPET markers was then performed.

Materials and Methods: 
After ethical approval, 51 patients, under consideration for major non-cardiac surgery gave consent & were prospectively recruited to this observational study. USCOM measurements were obtained immediately pre- and post-CPET in the upright position.  The difference between these values was calculated in order to find the percentage Δ in cardiac index (ΔCI) pre- & post-CPET, & inotropy reserve (peak exercise minus resting value). Inotropy reserve & ΔCI were compared with peak VO2 (ml/kg/min) & VO2 at AT.

Results and Discussion: 

Mean age was 73(±8), (15 female, 36 male). Pre-CPET values (mean±SD) were: SV 49(±14) ml, HR 79(±15) beats per minute, (CI 2(±0.75) l/min/m-2, mean arterial pressure (MAP) 105(±15.5) mmHg & Inotropy 1.14(±0.4) Wm-2. Post-CPET mean ΔSV was 14%, ΔCI 68%, ΔHR 49% & ΔMAP 4%. Mean O2 consumption at AT was 12.5(±2.9) ml/kg/min. 

In those patients (n=30) with a VO2 at AT ≥11 ml/kg/min mean ΔCI was significantly higher vs the group (n=21) with a VO2 <11 (or who did not reach AT) (table 2) (P = 0.007). This was predominantly due to a rise in HR rather than SV. There was a significant difference demonstrated in both mean ΔCI & inotropy reserve in patients who achieved a peak VO2 >14 ml/kg/min vs ≤14 ml/kg/min (table 3).

ΔCI pre & post CPET correlated with VO2 in those patients who reached AT & with peak VO2 (table 4). Inotropy reserve still demonstrated positive correlation with these two CPET variables though not to the same degree. There was no significant correlation noted between static haemodynamic parameters and peak VO2 & VO2 at AT.


Conclusion(s): 
In a cohort of high-risk individuals, ΔCI correlates well with traditional CPET markers of cardiovascular performance. USCOM assessment is simple to perform & holds promise as an adjunct to CPET or use where the later is not available.
 
References

Smith BE, Madigan VM. Non-invasive method for rapid bedside estimation of inotropy: theory and preliminary clinical validation. Br. J. Anaesth. (2013) 111 (4):580-588.

Older P, Smith R, Courtney P, Hone R. Preoperative evaluation of cardiac failure and ischemia in elderly patients by cardiopulmonary exercise testing. Chest 1993; 104: 701–4

Utility of non-invasive cardiac output measurements during cardiopulmonary exercise testing in pre-operative risk assessment