Temporal Changes of Cardiac Structure, Function, and Mechanics During Sub-acute Cervical and Thoracolumbar Spinal Cord Injury in Humans: A Case-Series

Individuals with cervical spinal cord injury (SCI) experience deleterious changes in cardiac structure and function. However, knowledge on when cardiac alterations occur and whether this is dependent upon neurological level of injury remains to be determined. Transthoracic echocardiography was used to assess left ventricular structure, function, and mechanics in 10 male individuals (median age 34 years, lower and upper quartiles 32–50) with cervical (n = 5, c-SCI) or thoracolumbar (n = 5, tl-SCI) motor-complete SCI at 3- and 6-months post-injury. Compared to the 3-month assessment, individuals with c-SCI displayed structural, functional, and mechanical changes during the 6-month assessment, including significant reductions in end diastolic volume [121 mL (104–139) vs. 101 mL (99–133), P = 0.043], stroke volume [75 mL (61–85) vs. 60 mL (58–80), P = 0.042], myocardial contractile velocity (S') [0.11 m/s (0.10–0.13) vs. 0.09 m/s (0.08–0.10), P = 0.043], and peak diastolic longitudinal strain rate [1.29°/s (1.23–1.34) vs. 1.07°/s (0.95–1.15), P = 0.043], and increased early diastolic filling over early myocardial relaxation velocity (E/E') ratio [5.64 (4.71–7.72) vs. 7.48 (6.42–8.42), P = 0.043]. These indices did not significantly change in individuals with tl-SCI between time points. Ejection fraction was different between individuals with c-SCI and tl-SCI at 3 [61% (57–63) vs. 54% (52–55), P < 0.01] and 6 months [58% (57–62) vs. 55% (52–56), P < 0.01], though values were considered normal. These results demonstrate that individuals with c-SCI exhibit significant reductions in cardiac function from 3 to 6 months post-injury, whereas individuals with tl-SCI do not, suggesting the need for early rehabilitation to minimize cardiac consequences in this specific population.

Individuals with cervical spinal cord injury (SCI) experience deleterious changes in cardiac structure and function. However, knowledge on when cardiac alterations occur and whether this is dependent upon neurological level of injury remains to be determined. Transthoracic echocardiography was used to assess left ventricular structure, function, and mechanics in 10 male individuals (median age 34 years, lower and upper quartiles 32-50) with cervical (n = 5, c-SCI) or thoracolumbar (n = 5, tl-SCI) motor-complete SCI at 3-and 6-months post-injury. Compared to the 3-month assessment, individuals with c-SCI displayed structural, functional, and mechanical changes during the 6-month assessment, including significant reductions in end diastolic volume [ 15), P = 0.043], and increased early diastolic filling over early myocardial relaxation velocity (E/E') ratio [5.64 (4.71-7.72) vs. 7.48 (6.42-8.42), P = 0.043]. These indices did not significantly change in individuals with tl-SCI between time points. Ejection fraction was different between individuals with c-SCI and tl-SCI at 3 [61% (57-63) vs. 54% (52-55), P < 0.01] and 6 months [58% (57-62) vs. 55% (52-56), P < 0.01], though values were considered normal. These results demonstrate that individuals with c-SCI exhibit significant reductions in cardiac function from 3 to 6 months post-injury, whereas individuals with tl-SCI do not, suggesting the need for early rehabilitation to minimize cardiac consequences in this specific population.

INTRODUCTION
Left ventricular (LV) atrophy and decreased volumes are well documented in individuals with chronic spinal cord injury (SCI) (1). Decreased functional outcomes can result from impaired supraspinal sympathetic control over the cardiovascular system, such that cardiac responses to stress or exercise are diminished in individuals with high-level SCI (i.e., injury above the 6th thoracic spinal cord level) compared with non-injured individuals (2). Compounded with the higher prevalence of physical inactivity in the SCI population (3), it is perhaps unsurprising that cardiovascular disease (CVD) is the leading cause of morbidity and mortality in this population (4). These aforementioned factors have more substantial effects for individuals with cervical SCI compared to thoracolumbar SCI due to the combination of impaired sympathetic control to the heart (5), lack of skeletal muscle pump (i.e., decrease in venous return) (6), and greater functional impairments limiting physical activity (7). These factors predispose individuals with cervical SCI to daily cardiovascular disturbances, consequentially resulting in outcomes such as autonomic dysreflexia, orthostatic hypotension, and pooling of blood below the level of injury (8). Ultimately, previous literature suggests that these events can lead to detrimental cardiac consequences (9)(10)(11).
Similar to SCI, microgravity exposure during spaceflight has been associated with an overall physiological decline, providing a potential model to better understand the alterations in these specific populations over time (12). While the time course of changes in the autonomic nervous system following spaceflight and SCI have been described (12), temporal echocardiographic changes shortly after SCI have not been followed longitudinally. This is surprising as other cardiac changes, such as an increased prevalence of arrhythmias, occur in the months following SCI (13). Furthermore, previous research has demonstrated reduced echocardiographic indices of LV systolic function in individuals with cervical SCI at the chronic stage of injury (>1 year) compared to non-injured individuals (1), potentially due to the disrupted sympathetic input to the heart originating from T1 to T5 (14). Recently, a crosssectional study revealed associations between increased time since injury and reduced LV size and function; these alterations have been linked to consequences such as congestive heart failure and increased mortality (15). Ultimately, the cardiac consequences (1) of decreased cardiac reserve has important clinical implications as it can limit the ability of an individual to perform regular activities of daily living (16). Furthermore, characterizing the time frame of structural and functional cardiac alterations may provide further insight into the increased risk of CVD in those with SCI and aid in the timing of cardiac rehabilitation interventions.
No study to date has examined the longitudinal changes in cardiac function post-SCI in the sub-acute setting (i.e., within the first 6 months), stratifying for neurological level of injury (NLI). Thus, the aim of this present longitudinal case series was to investigate changes in LV structure, function, and mechanics, in individuals with cervical SCI and thoracolumbar SCI at 3-and 6-months post-injury using transthoracic echocardiography (TTE) (17).

Participants
Clinical protocols were approved by the University of British Columbia Clinical Research Ethics Board (H13-03072) and conducted in accordance with the second Helsinki Declaration. The participants provided written informed consent prior to data collection. The NLI and the severity of SCI were classified according to the International Standards for Neurological Classification of SCI (ISNCSCI) by a trained physician [i.e., providing an American Spinal Injury Association Impairment Scale (AIS) grade] (18). Participants were asked to report the frequency and duration of moderate (some physical effort) and heavy (maximum physical effort) intensity leisure time physical activity (LTPA) over the preceding 7 days using the validated LTPA questionnaire for individuals with SCI (LTPAQ-SCI) (19). These data provide an indication of the volume of weekly LTPA performed by participants. The validity of the LTPAQ-SCI is comparable to questionnaires used in the general population (19). Exclusion criteria for all participants included any history of CVD and any language or cognitive barrier that prevented the participant from following English instructions.

Echocardiography
TTE was performed on a commercially available ultrasound (Vivid 7/i; GE Medical, Horton, Norway) and stored for offline analysis using specialized computer software (EchoPAC; GE Healthcare, Horton, Norway) according to the recommendations of the American Society for Echocardiography (ASE) (20), by a single analyzer, blinded to time point and group.
The average of three cardiac cycles was used to determine LV structure and functional indices. Measures of LV structure at end-diastole and end-systole were reported from the parasternal long-axis views, and used to derive relative wall thickness (21). Volumetric measurements and systolic functions were derived from the apical four-and two-chamber views using the modified Simpson's biplane method. Cardiac output (Q) was calculated as the product of stroke volume (SV) and heart rate (HR) and systolic myocardial velocity (S') was derived from pulsed-wave Doppler. LV diastolic indices were calculated using early septal relaxation velocity (E'), early (E) and late (A) transmitral flow, and early-to-late transmitral filling velocity (E/A) ratio derived from pulsed-wave Doppler. The E/E' ratio was calculated to estimate LV filling pressure. Deceleration time and isovolumetric relaxation time were also determined on the spectral Doppler trace. LV mass index was calculated with the Devereux method and indexed to body surface area using the DuBois method (22,23).
Indices of LV mechanics were derived from apical fourchamber and parasternal short-axis images at the level of the mitral valve (basal), papillary muscle (mid), and apex (apical). Images were analyzed using 2D speckle-tracking software in accordance with recommended guidelines (24). Raw speckletracking traces were imported into customized post-processing software (2D Strain Analysis Tool, Stuttgart, Germany), and data was interpolated into 600 points in systole and 600 points in diastole using a cubic spline algorithm. Peak strain and strain rate in systole and diastole were determined for each parasternal short-axis view (radial, circumferential) and the apical fourchamber view (longitudinal). Basal and apical peak rotation and rotation rate in systole and diastole were determined. Twist was determined as the maximum value obtained when subtracting the frame-by-frame basal rotation from the frame-by-frame apical rotation. Torsion, a measure of twist normalized to LV chamber size, was calculated by dividing peak twist by the LV end-diastolic length.

Statistical Analysis
Data are presented as raw values and percentages. Statistical analyses were performed using Statistical Package for Social Science software (SPSS Version 27, IBM, Chicago, IL, USA) with statistical significance set at P ≤ 0.05. Wilcoxon signed rank test were used to analyze outcomes between time points within groups. Group differences were analyzed using the Mann Whitney U-test. Results are presented as median with upper and lower quartiles (i.e., 25-75%). Graphical representations were made in Prism (Version 9.

Changes in Cardiac Structure, Function, and Mechanics
There were no differences between the cervical SCI and thoracolumbar SCI groups for echocardiographic measures at 3-or 6-month time points, except for EF (P < 0.01), which was still within the normal range of 52-72% for males as per ASE guidelines (20). LV mechanics parameters can be    (25). (E) Early diastolic filling over early myocardial relaxation velocity (E/E') ratio was significantly increased in the cervical group at 6-vs. 3-months, yet not in the thoracolumbar group. (F) Median peak diastolic longitudinal strain rate was significantly lower at 6-vs. 3-months in the cervical group, yet not in the thoracolumbar group. (G,H) Global longitudinal strain standardized to cardiac cycle length for all participants at the 3-and 6-month time points for the cervical (G) and thoracolumbar (H) SCI groups; no differences were observed within groups. C, cervical; L, lumbar; S, sacral; T, thoracic; TTE, transthoracic echocardiography. * Cervical 6-months different from cervical 3-months (P < 0.05). Data are displayed individually (i.e., each triangle represents one individual) and grouped (i.e., box-and-whisker plot). found in Table 3. For LV systolic mechanics, apical rotation was higher in the thoracolumbar group at 6-months compared to 3-months, whereas radial strain rate at the mid-level was lower.

DISCUSSION
To our knowledge, this is the first study to investigate the time-course of cardiac changes in humans with sub-acute SCI (i.e., serial echocardiography measurements performed within the first 6 months post injury). While a recent meta-analysis demonstrated that structural and functional changes are apparent for individuals with chronic SCI compared to non-injured individuals (1), these current findings suggest changes may occur during the sub-acute stage post cervical SCI. The novel aim of this study was to longitudinally track the same individuals in each group over a 3-month period in the sub-acute stage of injury. The reduced cardiac capacity for the cervical SCI group may limit the ability of these individuals to perform regular activities of daily living (16), consequently putting these individuals at a greater risk for developing chronic diseases (i.e., CVD) (26) and impacting aspects of health-related quality of life.
Reductions in LV structural and functional cardiac indices were found for individuals with cervical SCI between 3-and 6months post-injury, whereas these changes were not apparent for individuals with thoracolumbar SCI. As the LTPA measures were highly variable but somewhat comparable amongst the cervical SCI and thoracolumbar SCI participants, the most likely explanation for the differences between groups can be attributed to the disrupted supraspinal regulation of sympathetic activity to the heart following cervical SCI (5). To support this, our group has previously published translational work suggesting the mechanism behind the reduction in cardiac function is underpinned by interrupted bulbospinal sympathetic control (27). Conversely, cardiac sympathetic control is preserved in those with thoracolumbar SCI. It has been demonstrated in individuals with chronic SCI that a higher NLI is likely to result in more devastating consequences for cardiac function (28). However, these longitudinal cardiac adaptations, assessed using echocardiography, have never previously been captured in the first 6 months following human SCI with the same individuals.
Despite reductions in LV structure and global function in the cervical SCI group, indices of LV systolic mechanics were similar over time. Perhaps this can be attributed to a mechanical compensation post-SCI to maintain systolic function in the cervical SCI group. The observation of preserved systolic mechanics and reduced SV has been previously reported (29), and is thought to be attributed to reduced afterload from the chronically hypotensive state of individuals with SCI, even with reductions in preload.

Leisure Time Physical Activity
Despite recommended exercise guidelines to improve cardiometabolic health following SCI (3), 40 and 50% of the SCI participants at 3-and 6-months post-injury, respectively, self-reported 0 min of moderate-to-heavy intensity LTPA per week. This is in agreement with previous literature revealing that 50% of community-dwelling individuals with SCI do not perform any LTPA (30). While the LTPAQ-SCI does not include activities of daily living in its assessment, literature suggests that activities of daily living are not associated with cardiovascular fitness in this population (31) and are therefore unlikely to have an impact on the cardiac indices measured in this study. During in-patient rehabilitation in Canada, participants have self-reported performing > 60 min of vigorous-intensity physical activity per day (31). However, the same study indicated that only 5 min per day was spent performing physical activity > 40%    heart rate reserve . Therefore, self-reported minutes of moderateto-vigorous-intensity physical activity do not reflect actual time spent performing activities at higher intensities as measured objectively via a heart rate monitor. Therefore, In-keeping with other results, it could be implied that individuals with SCI do not reach an effective cardiovascular training threshold to improve neurologic, cardiovascular, or musculoskeletal health during inpatient therapy (31). This suggests further work to optimize rehabilitation strategies is warranted. Previous literature suggests the cardiovascular deconditioning following SCI can be improved with exercise training (2). However, given the reduced upper-extremity motor scores (range     , individuals with cervical SCI can struggle to perform volitional upper-body exercise of a sufficient intensity or volume to offset CVD risk. These reduced upper-extremity motor scores may reflect an overestimation of reported LTPA, though this suggestion is outside the scope of our study. Thus, individuals with cervical SCI may require alternative forms of exercise to prevent cardiac decline. Pre-clinical rodent models have shown improvements in LVIDd, EDV, and SV following passive hindlimb exercise (32), suggesting passive lower-limb manipulation or stimulation (33) may improve cardiac function (34).
We contend that disrupted sympathetic control to the heart and blood vessels after cervical SCI (5) is primarily responsible for the accelerated cardiac decline in these participants. Physical activity may mitigate this to some degree (35), but the effectiveness of volitional upper-body exercise, especially in individuals with cervical SCI during the sub-acute phase requires further research. Additional work is required to promote exercise in humans with SCI in the sub-acute setting and investigate the efficacy of different exercise intensities and modalities to improve or maintain cardiac health.

Future Directions
Our novel findings, while preliminary and requiring verification with a larger cohort, reveal that significant cardiac changes occur in the sub-acute period following cervical SCI. It would be of interest to track these individuals into the chronic period of injury to observe whether these changes plateau or decline further. In addition to exercise, early pharmacological strategies may mitigate this cardiac decline, which potentially has implications for the risk of developing CVD. A recent preclinical study investigating the use of dobutamine in the acute setting to augment LV contractility, appears to preserve cardiac function in the chronic setting post-SCI (36). Furthermore, novel neuromodulation strategies, such as the application of epidural stimulation (37) to facilitate supraspinal control of the sympathetic nervous system could complement acute rehabilitation strategies. Although the cardiac indices measured in our participants are somewhat variable and within the normal range compared to clinical guidelines (20), the major strength of our study is the longitudinal design, which tracks the same participants during this novel sub-acute period.

Study Limitations
Although our groups are comparable for demographic characteristics, the progression of each type of injury can only be truly compared by measuring indices prior to injury. This is an implausible study design; therefore, pre-clinical models would help to complement these findings. Furthermore, while NLI and severity of SCI were reported using validated methods, autonomic completeness of the injury was not measured. However, associations between sensorimotor and sympathetic impairments have previously been observed in non-athletic individuals with SCI (38). Consequently, it is not possible to state with certainty that these findings are due to disrupted supraspinal cardiac control. MH-LTPA variations also emphasize the potential limitations of using self-report questionnaires. Participants were taken from a sample of inpatients at a single, local rehabilitation center who had access to transportation for testing appointments following discharge, therefore potentially subjecting the study to a degree of sampling bias. These individuals were not highly trained athletes or deconditioned before injury, therefore our sample represents individuals from an urban developed country that have sustained a SCI. Future research should incorporate validated, research-grade multisensor devices (39) to precisely quantify physical activity levels and to ascertain how the modification of certain physical activity dimensions might alter cardiac indices in the sub-acute setting. TTE recommendations to scan participants in the left-lateral supine position to avoid foreshortening (20) may present a limitation, as reductions to preload in an upright position (1) are common in individuals with SCI and should be considered for future studies. Finally, the role of vasoactive medications on these cardiac findings would be of interest in future studies as the optimal dose of these pharmaceutical strategies are still being investigated, particularly in the sub-acute period following injury (40).

CONCLUSION
Our data demonstrate a decline in several cardiac indices using TTE following cervical SCI, as early as 6-months postinjury. A lack of descending sympathetic control of the heart may be responsible for the rapid adaptive changes in cardiac indices. Worryingly, these changes occur over a relatively short time frame following cervical SCI. This highlights the need to investigate the effectiveness of different therapeutic strategies to mitigate the decline of cardiovascular function in this population during the sub-acute period.

DATA AVAILABILITY STATEMENT
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

ETHICS STATEMENT
The studies involving human participants were reviewed and approved by University of British Columbia Clinical Research Ethics Board (H13-03072). The patients/participants provided their written informed consent to participate in this study.

AUTHOR CONTRIBUTIONS
MW and AK had full access to all the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis. SB, TN, MW, KC, and AK contributed to conception and experimental design. SB acquired the data. SB, TN, and MW analyzed the data. SB, TN, KC, CW, TT, MW, and AK interpreted the data. SB and TN drafted the manuscript. SB, MW, TN, KC, CW, TT, and AK contributed to editing the final version of the manuscript. AK is the principle investigator of the project. TN and MW co-supervised the project. All authors contributed to the article and approved the submitted version.