Year

2022

Degree Name

Master of Philosophy

Department

School of Medicine

Abstract

Background: Immobility and physical inactivity following coronary artery bypass graft (CABG) surgery can lead to significant functional decline, due to a combination of central haemodynamic and peripheral tissue changes. In the immediate post-operative recovery, there is opportunity for exercise interventions that target skeletal muscle health. Eccentric cycling describes the use of a motor-driven, self-pedalling bicycle ergometer to allow participants to resist the turning of the cycle wheel and thus perform repetitive muscle lengthening contractions. This exercise modality requires lower cardiorespiratory demand than workload-matched concentric exercises such as walking or traditional concentric cycling, while still engaging skeletal muscle. Outpatients with cardiorespiratory and other chronic disease have experienced clinical improvements following low-load eccentric cycling exercise, however there is a paucity of ergometers available for use in a hospital setting, where maintaining haemodynamic stability and minimising patient relocation is imperative. As such, eccentric cycling has not yet been studied in a hospitalised patient cohort, where its value in the immediate post-operative environment needs urgent attention.

Aims: This thesis is comprised of two main objectives. First, to design and construct an eccentric cycle ergometer specifically tailored to deliver low loads to patients in an acute care hospital setting. Second, to investigate whether eccentric cycling, provided at the bedside, can be practically and safely performed in the acute recovery after cardiac surgery, by observing the heamodynamic and peripheral muscle oxygen utilisation characteristics, and compare peripheral skeletal muscle oxygenation to walking at hospital discharge.

Methods: In study 1, an eccentric cycle ergometer (125 W AC) was custom-designed and built to be used by hospitalised patients, to perform low workload eccentric cycling, using visual feedback (power). The completed mobile eccentric cycle ergometer was tested using eight (n=8) healthy adult participants in a laboratory setting. After confirmation of eccentric workload load capacity, study 2 enrolled hospital patients (age; 61.6±10.3 years, BMI; 28.6±6.2 m2; 23 males and 1 female) to perform repeated bouts of low-intensity eccentric cycling following uncomplicated CABG. Pre and postoperative heart rate, blood pressure, arterial and skeletal muscle oxygen saturation were collected using continuous 12-lead ECG, automated sphygmomanometry and near-infrared spectroscopy respectively. Participants performed 10-minute exercise sessions, under supervision, on up to three occasions, commencing from the third postoperative day until hospital discharge. Workload was self-prescribed based upon ratings of perceived exertion using a maximum of 4/10 as a cut off. Functional capacity was assessed using a 20-meter walk for gait speed performed at hospital discharge.

Results: In study 1, all participants completed the protocol in its entirety with no mechanical issues with the ergometer. Mean power output for the two eccentric workloads was 31.1±5.7 W and 56.6±8.8 W respectively. Heart rate (rest: 68±13bpm) and minute ventilation (rest: 12.4±3.5 L.min-1) increased incrementally with workload 1 (HR: 83±16bpm MV: 21.76±6.5 L.min-1, p<0.001 v rest) and workload 2 (HR: 94±14 bpm MV: 26.5±8.9 L.min-1, p<0.001 v rest) while peripheral arterial oxygen saturation (98±1%) and local muscle oxygen saturation of the quadriceps muscle (89±5%) was sustained for both workloads. Participants did not report any muscle soreness following the exercise. The eccentric cycle ergometer performance was deemed reliable for the feasibility study in the post-operative CABG patient cohort.

In study 2, mean workload was assessed during consecutive eccentric cycling sessions, with a maximum of three eccentric cycling sessions performed prior to hospital discharge. The mean workload significantly increased during the second and third eccentric cycling sessions, albeit partly as a factor of increased cycling cadence. By the commencement of the third eccentric cycling session, cadence approached the pre-determined limit for safety (30 rpm) and was maintained over the duration of 10 minutes. Rate of perceived exertion increased in line with the workload, but was always retained at or below 4/10, over the course of the 10 minute bout. In session one, heart rate significantly increased from rest (87±11 bpm) to a maximum mean of 93±11 bpm (P<0.05) and this response was equivalent in session two and three (<10bpm despite increased workloads in those 10 min bouts). Muscle oxygen saturation was not disturbed from resting values by the eccentric workloads, maintaining a consistent tissue saturation index of 30-35% whilst arterial oxygen saturation was preserved (>95%).

Conclusion: An eccentric cycle ergometer for the hospital environment was successfully designed and constructed and was able to deliver consistent low-workloads to a healthy adult population. Following transfer of this ergometer to a hospital setting, 24 patients performed repeated bouts of eccentric cycling using workloads up to 40 watts, with no significant change in skeletal muscle oxygenation despite increasing workloads. No adverse medical events were encountered during the eccentric cycling exercise. This is the first study of hospitalised patients performing eccentric cycling exercise. The potential for improved patient outcomes, including the attenuation of physical capacity loss, may now be addressed in a range of hospitalised patients with low physiologic reserve.

FoR codes (2020)

320101 Cardiology (incl. cardiovascular diseases), 320803 Systems physiology, 420702 Exercise physiology

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.