MR augmented cardiopulmonary exercise testing - a novel method of assessing cardiovascular function

Background

Reduced exercise capacity is a common feature of many cardiovascular diseases. Quantitative assessment of exercise capacity is usually achieved by measuring peak oxygen consumption (VO₂). However, measuring peak VO₂ alone neglects the different components of reduced exercise capacity: namely reduced cardiac output (CO) and oxygen extraction (ΔcO₂). A better approach would be to simultaneously measure VO₂ and CO and then calculate ΔcO₂. This could be achieved using MR augmented cardio-pulmonary exercise testing (MR-CPET)

The aims of this study were to demonstrate: 1) MR-CPET is feasible and well tolerated, 2) peak VO₂ in the MR scanner correlates with conventional peak VO₂, and 3) variation in peak VO₂ is related to both peak CO and peak oxygen extraction (ΔcO₂) as calculated by the Fick equation.

Methods

17 healthy volunteers (21-55 years) underwent MR-CPET. Exercise was performed on MR-compatible ergometer (Lode, Groningen, The Netherlands) and VO₂ was assessed using a commercial respiratory gas analyzer (Ultima, MedGraphics, St. Paul, USA) with a modified sampling tube that was MR compatible. Aortic flow was continuously measured using a previously validated UNFOLD-SENSE spiral PCMR sequence. Images were reconstructed using a graphical processing units card and analyzed using an in-house plug-ins for OsiriX software. Conventional CPET was also performed within 2 weeks of MR-CPET.

For both test, participants were asked to rate i) concern ii) comfort and iii) perceived helplessness.

Results

15 out of 17 volunteers completed exercise; exclusions were due to claustrophobia (n=1) and inability to master exercise technique (n=1). Reported concern and discomfort was higher with MR-CPET, although still within acceptable limits.

Peak VO₂, peak VCO₂ and VE showed strong correlation between conventional CPET and MR-CPET: VO₂ peak (r=0.94, p<0.001); VCO₂ (r=0.87, p<0.001); VE (r=0.88, p<0.001). Resting and peak values VO₂, CO, HR, SV and ΔcO₂ are shown in table 1. Multiple linear regression analysis demonstrated that both peak CO and ΔcO₂ were independent predictors of peak VO₂ measured during MR-CPET (beta=0.73 and 0.38 respectively, p<0.0001) and conventional CPET (beta=0.78, 0.28 respectively, p<0.0001). Representative VO₂, CO and ΔcO₂ are shown in figure 1.

Table 1 Resting and peak values during MR-CPET

Figure 1

Conclusions

MR-CPET is feasible, well tolerated and demonstrates physiology not apparent with conventional CPET. In this study, we have shown that MR-CPET allows assessment of the differing contributions of CO and ΔcO₂ to variation in peak VO₂. We believe that will be useful in understanding to origin of reduced exercise capacity in cardiac disease.

Funding

British Heart Foundation, Great Ormond Street Children's Charity.