We have successfully designed, constructed, and tested an MRI-compatible cycle ergometer for cardiac assessments, which takes into account between-subject variations, to accurately quantify workload power. The ergometer was designed to fit into a 1.5 T MRI scanner (Magneto Avanto; Siemens), and is adjustable for a relatively wide range of heights.
This ergometer has the potential to assist physiological studies looking at cardiac responses to dynamic exercise in a variety of populations. Moreover, the dose of exercise (workload) generated can be more precisely adjusted, which allows for the assessment of patients with varying degrees of fitness [10
]. Our ergometer is also useful in studies targeting fixed heart rates during MRI assessments, particularly valuable when assessing cardiac function. For instance studies have shown that to predict the development of diabetic nephropathy exercise tests with fixed heart rate were preferable than fixed workload in type 1 diabetic individuals due to the differences in physiological responses such as blood pressure which were associated with the development of micro- and macroalbuminuria [15
A number of cardiac impairments are only evident when the heart is exposed to stress. Heart rate or rate pressure product (heart rate × systolic blood pressure) have the strongest association with myocardial work of all non-invasive measures [16
]. Thus, steady-state quantification of heart rate is important during cardiac function studies. An advantage of the fixed resistance bike used in this study is that, when presented with a target heart rate, the patient can increase or decrease pedalling cadence or frequency to adjust their heart rate. In contrast, the investigators must change workload or resistance for the patient when using a fixed workload ergometer.
Heart rate control is also critical in the assessment of diastolic filling properties. Peak early mitral valve inflow velocity, an indicator of diastolic function, is inversely proportional to heart rate [17
]. This is largely due to the preferential reduction in diastolic duration during tachycardia [18
]. Therefore, accurate comparisons of diastolic function in different groups require equal heart rates (i.e. diastolic durations). Previous investigations using fixed workload ergometers have described standard deviations of up to 17 beats per minute around the targeted heart rates [10
]. In contrast, the standard deviation around our target heart rates was 1 beat (see Table ).
Comparison of our results with two previous studies in healthy individuals using commercial cycle ergometer
A disadvantage of the fixed resistance machine used in this study is that it is more difficult to establish a steady state workload. Indeed fixed workload machines are ideally engineered for this purpose (workload is unaffected by cadence). The fixed resistance machine used in this study is poorly suited for reproducing a relative workload (e.g.% maximal watts) because of the dependence on pedalling cadence. However, this can be readily achieved by using a metronome to provide a pedalling cadence at which any resistance can be used to develop a desired workload with minimal difficulty.
This MRI cycle ergometer demonstrated the value in obtaining MRI images during exercise at a fixed heart rate. In particular it enables identification of stroke volume which varied between 70-120 ml at rest increasing up to 200 ml during exercise, depending on ventricular morphology and fitness level [19
]. As exercise intensity increases, stroke volume in physically unconditioned individuals increases gradually to a plateau at approximately 120 bpm [20
], or 40% of
]. Conversely, stroke volume continues to increase progressively until maximum heart rate in elite athletes [20
]. The physiological responses obtained using our MRI-compatible cycle ergometer and protocols were comparable to previous studies in which an increase in heart rate and stroke volume increases cardiac output during exercise [7
] (Table ). Roest et al. used a commercial MRI cycle ergometer in healthy individuals (MRI cardiac ergometer, Lode BV, Groningen, The Netherlands) [7
]. The first study examined left and right ventricular function in a group of healthy volunteers (mean age of 17.5 years) [7
] (Table ). The exercise images were obtained at 60% of maximal workload using 10 short-axis images for the left ventricular analysis. The second study involved individuals of similar age to our study participants, and the left ventricular volumes were obtained from 10 consecutive short axis scans, which were acquired during exercise at a workload corresponding to 60% of maximal oxygen consumption [8
]. Even though the exercise workloads (approximately 130 W vs. 96 W) and heart rates (approximately 120 beats/min vs. 110 beats/min) were higher in their studies, the hemodynamic responses were similar. This is because Roest et al. participants were exercising at fixed workloads relative to their maximal capacity, while ours were exercising at a fixed heart rate of 110 beats/min. Moreover, our participants were comparatively less fit, requiring a lower workload to trigger an increase in heart rate.
Typically with MRI compatible cycle bikes the workload is either set at a constant level for all individuals or manually adjusted for each individual to increase and maintain heart rate at a predetermined criterion level such as 65% of maximum heart rate. According to Laperriere and colleagues 1989 a potential problem arises when manual adjustments of workload are used to produce and maintain specified heart rate. The workload value needed to maintain a predetermined heart rate percentage of maximum heart rate for a group of individuals, will be different for individuals with different pre-existing fitness levels [10
]. Also, fatigue, even after a few minutes, could trigger a change in workload. Therefore, workload requires adjustments as heart rate is increased to the desired level, and may require continuous fine adjustments to maintain heart rate at target level [10
In summary, the MRI-compatible cycle ergometer constructed by our research group allows for accurate and reproducible exercise cardiac assessments at tightly regulated heart rates, while continuously recording workload.