Enabling low-cost exercise CMR through the PIPER: a half-Pipe Exercise Rig

Background: Exercise stress cardiovascular magnetic resonance (CMR) is a powerful method for identifying cardiac dysfunction¹. Its widespread clinical adoption does, however, remain hampered by the high cost (~50k USD) and time-consuming setups (~15min) of commercial CMR-compatible exercise equipment’s. Virsinskaite et al recently addressed this by developing a portable exercise apparatus that facilitates physical activity through alternating leg flexions and extensions within two plastic half-pipes, applying it to patients with pulmonary arterial hypertension². In this study, we manufactured a similar half-pipe exercise rig (named PIPER), and assessed its ability to reproduce cardiopulmonary physiological responses comparable to those achieved by a commercial CMR ergometer.

Methods: We built the PIPER in-house following specifications by Virsinskaite et al², but with a mounting solution using the patient table railings to support both flat and curved table designs.

We included 11 healthy subjects (30±8years, 4 female) who underwent cardiopulmonary exercise CMR at 0.55T (FreeMax, Siemens). Each subject was imaged at rest and thrice during exercise: twice using the PIPER and once using the CardioStep module (Ergospect), randomizing the exercise module order (Figure 1). Load was modulated by encouraging leg flexions in pace with a playlist with strides increasing from 60 to 100 bpm, streamed through the scanner headphones. During each phase, free-breathing short-axis cine³ and dynamic lung water⁴ imaging were performed to assess cardiac output and lung water density. In addition, heart rate (HR), peripheral capillary oxygen saturation, and brachial blood pressures were recorded at each phase. Measurements were compared across imaging phases.

Results: We built the PIPER for ~500 USD. It required 15s to setup. Exercise CMR yielded similar percent max HR by age for each exercise round (1^st PIPER: 56±7bpm; 2^nd PIPER: 58±7bpm; CardioStep: 58±12bpm, p >0.05). Figure 2 shows the changes in HR, lung water density, and cardiac output between rest and exercise, and Table 1 further summarizes the measured physiological parameters. HRs differed between rest (71±8bpm) and peak exercise (1^st PIPER: 107±15bpm, p< 0.0001; 2^nd PIPER:110±14bpm, p< 0.0001; CardioStep:110±24bpm, p=0.0008), but were consistent across all exercise rounds (p >0.05). We observed the same pattern for rest and exercise lung water density (1^st PIPER: 23±2.1% vs 25±2.8%, p=0.004; 2^nd PIPER: 23±2.8% vs 24±3.2%, p=0.0003; CardioStep: 23±2.0% vs 25±2.3%, p=0.0003). Cardiac output only increased for the first PIPER exercise round (Rest 7.0±1.5L/min vs 1^st PIPER 9.3±1.5L/min, p=0.01; 2^nd PIPER 8.6±2.6L/min, p=0.17; CardioStep: 7.2±1.9L/min, p=0.83). Exercise cines were, however, not analyzed in half the dataset due to technical issues.

Conclusion: The cardiovascular physiological response to exercise using the PIPER is repeatable and in parity with a commercial CMR-ergometer, thus offering a user-friendly low-cost option for exercise CMR.

Table 1. Cardiovascular physiological parameters at rest and exercise.

	Rest	Exercise 1st PIPER	Exercise 2nd PIPER	Exercise CardioStep
Percent max heart rate by age (%)	38±4.6	56±7.1****	58±6.8****	58±12***
SpO2 (%)	96±1.4	96±1.0 ns	96±0.92 ns	97±1.8 ns
Systolic blood pressure (mmHg)	114±13	132±17**	132±16**	125±13*
Diastolic blood pressure (mmHg)	62±13	78±15***	76±13**	72±14 ns

Values are represented as mean ± standard deviation. Difference to rest by one-way ANOVA: ns, non-significant; *, p< 0.050; **, p< 0.010; ***, p< 0.001, ****, p< 0.0001. SpO2, peripheral capillary oxygen saturation.Figure 1. Image of A) the in-house built half-pipe exercise rig (PIPER) and B) the commercially available CardioStep Module (Ergospect). C) The imaging protocol began with a short-axis (SAX) cine acquisition at rest, followed by a 6-minute dynamic lung water acquisition which comprised 3 minutes of rest and 3 minutes of exercise. Subsequently, another short-axis cine image was acquired during exercise. After subjects rested for at least 5 minutes to return to their resting heart rate, the lung water and exercise short-axis acquisitions were repeated twice. Half of the subjects exercised two rounds using the PIPER, followed by one round using the CardioStep. The other half started with the CardioStep before exercising two rounds with the PIPER.
Figure 2. Illustration of changes between rest and exercise in A) heart rate, B) lung water density, and C) cardiac output using the half-pipe exercise rig (PIPER) and CardioStep exercise devices. One-way ANOVA: ns, non-significant; *, p<0.050; **, p<0.010; ***, p<0.001, ****, p<0.0001.