Exercise-based cardiac rehabilitation (CR) is a class 1 indication for patients with cardiovascular disease (CVD) and represents a mainstay of therapy to reduce CVD risk factors while reducing rehospitalization and mortality.1 The coronavirus 2019 (COVID-19) pandemic resulted in closure of CR programs to in-person exercise training due to concerns for potential viral transmission.2,3 Some programs have considered offering home-based CR4,5 and instituting protective measures6; however, challenges remain. Exercise training, as conducted during CR, augments the generation of airborne respiratory droplets and aerosolized particles.7 In addition, the utilization of face masks, while effective at capturing large droplets, has shown minimal efficacy in reducing small particulate accumulation during exercise.7 Thus, there is an urgent need for guidance on alternative strategies that can be instituted by CR programs to mitigate the accumulation of airborne particles during exercise-based CR. In this study, we tested the efficacy of enclosed pods with continuous air filtration to mitigate the accumulation of airborne particles during exercise in a CR center setting.

METHODS

All experimental protocols were approved by the institutional review board at Mayo Clinic. Six healthy volunteers (three women; age: 31 +/- 3 yr; body mass index: 25 +/- 3 kg/m²) free of any known CVD, pulmonary, or metabolic disease completed two 30-min moderate-intensity continuous training sessions with (MICT-F) and without air filtration (MICT-NF) and a single 30-min high-intensity interval training session with air filtration (HIIT-F) in the CR center at Mayo Clinic (Rochester, MN). Each MICT session consisted of treadmill running at a self-selected speed and grade to achieve a rating of perceived exertion (RPE) of 11-13. The HIIT-F session consisted of six high-intensity intervals (target RPE: 16-18; 2 min), each followed by a low-intensity interval (target RPE: 10-12; 3 min). All exercise sessions were preceded by a 5-min warm-up, followed by a 5-min cooldown. Face masks were not utilized during any of the exercise sessions. Heart rate was recorded during the final minute of exercise.

All sessions were performed in a custom-manufactured exercise pod housed inside the CR center. The pod was an enclosed structure with a single doorway and internal dimensions of 124 x 124 x 144 inches (1281 ft).3 An OptiClean Dual-Mode (Carrier) air scrubber and negative air machine was incorporated into the pod and provided 99.97% high-efficiency particulate air filtration of particles >=0.3 [mu]M at 713 ft³/min (~33 air exchanges/hr). Airborne particles were measured inside the pod and at a centralized location within the CR center using a light-scattering 985 Particle Counter (Fluke) capable of quantifying airborne particles between 0.3-10 [mu]M. Particle counters were positioned opposite the air filtration system inside the pod and near the middle of the CR center ~15 ft from the pod doorway. Air samples were taken every 10 sec at baseline (>10 min) and during each exercise session. The accumulation of aerosols (0.3-0.5 [mu]M),8 droplets (0.5-1.0 [mu]M),8 and cumulative particle counts (0.3-10 [mu]M) was calculated as changes from baseline and time averaged into 5-min bins to determine peak particle accumulation for each condition.

Data are presented as mean +/- SD, and statistical significance was set at P < .05. Nonparametric distribution of measurements was assumed. A Friedman test was used to detect differences in peak particle accumulation among exercise conditions. If significant, Wilcoxon signed rank tests with Bonferroni correction applied ([alpha] = .025) were used to compare both MICT-F and HIIT-F with MICT-NF.

RESULTS

The Table shows baseline particle densities as well as peak particle accumulation in the CR center and exercise pod during all exercise sessions. The greatest increase in cumulative particle density in the CR center was only ~10% above baseline levels and occurred during HIIT-F (Table). No differences in peak aerosol (P = .25), droplet (P = .57), or cumulative particle accumulation (P = .43) were detected in the CR center among exercise conditions. Aerosol particle accumulation inside the exercise pod was significantly less during both MICT-F and HIIT-F than during MICT-NF (P = .016 and P = .028, respectively; Table). Aerosol droplet particle accumulation inside the exercise pod was significantly less during both MICT-F and HIIT-F than during MICT-NF (P = .032 and P = .046, respectively; Table). Similarly, droplet particle accumulation inside the exercise pod was significantly less during both MICT-F and HIIT-F than during MICT-NF (P = .041 and P = .047, respectively; Table). Cumulative particle accumulation inside the exercise pod was also significantly less during both MICT-F and HIIT-F than during MICT-NF (P = .026 and P = .043, respectively; Table). Finally, heart rate tended to be higher at end of the exercise during HIIT-F (162 +/- 23 bpm; P = .07) than at MICT-NF (147 +/- 25 bpm) and MICT-F (149 +/- 17 bpm).

Table Baseline Particle Densities and Peak Particle Accumulation in the CR Center and Exercise Poda

DISCUSSION

This study provides clear evidence in support of using enclosed pods with air filtration to contain and mitigate airborne particles generated during exercise in a center-based CR setting. Considering respiratory droplets and aerosolized particles are key pathways for viral transmission,9-11 identifying strategies to mitigate the accumulation of airborne particles is a critical step in safely reopening and maintaining normal operations of CR programs to in-person patient care while minimizing the risk of COVID-19 transmission. The enclosed exercise pod used in this study effectively contained airborne particles generated during exercise. Moreover, incorporating air filtration successfully mitigated the accumulation of airborne particles inside the pod. These findings provide a potential strategy for safely reestablishing outpatient exercise-based CR, which is critical for reducing risk factors, rehospitalization, and mortality in patients with CVD.1

A potential limitation of this study is the use of young, healthy volunteers. The self-selected exercise intensities in which our participants engaged are likely much greater than patients in a CR program. Therefore, the tidal volume, respiratory rate, overall minute ventilation, expiratory pressure, and, presumably, generation of airborne particulate matter are likely much greater in this study than would be expected by older patients with CVD engaging in CR. In addition, face masks were not worn during the exercise sessions conducted in this study; however, currently available data suggest masking does not negatively impact the physiologic response to exercise and may be appropriate in CR settings.12

In conclusion, airborne particles generated during exercise in a CR center can be successfully contained by enclosed exercise pods. Furthermore, incorporating air filtration systems effectively mitigates the accumulation of respiratory droplets and aerosols.

Shane M. Hammer, PhD

Joshua R. Smith, PhD

Eric J. Bruhn, MA

Randal J. Thomas, MD

Thomas P. Olson, PhD

Division of Preventive Cardiology

Department of Cardiovascular Medicine

Mayo Clinic

Rochester, Minnesota

ACKNOWLEDGMENTS

This work was supported by National Institutes of Health awards: T32 HL07111 to S.M.H. and J.R.S.; K12 HD065987 to J.R.S.; and R01 NR018832 to T.P.O.

REFERENCES