Jason S. Au

Reduced common carotid artery longitudinal wall motion and intramural shear strain in individuals with elevated cardiovascular disease risk using speckle tracking

Longitudinal motion of the intima–media and adventitia layers of the common carotid artery (CCA) wall were assessed with ultrasound speckle tracking in seven individuals with spinal cord injury (SCI), who are considered at increased risk of cardiovascular disease, and in seven able‐bodied participants. CCA longitudinal wall displacement and intramural shear strain were compared to traditional markers of arterial health, including CCA stiffness and intima–media thickness (IMT). For each cardiac cycle, longitudinal CCA wall motion was characterized by bidirectional movement patterns containing motion retrograde to blood flow during systole, followed by antegrade motion during diastole. Relative displacement of the intima–media versus the adventitia was used to calculate longitudinal intramural shear strain and provided insight to local arterial wall properties. The retrograde intramural shear strain was smaller in individuals with SCI by 60·2% (P<0·05) compared to able‐bodied participants, showing smaller peak displacements in both the intima–media (P<0·05) and adventitia (P<0·05). In the antegrade direction, there were no group differences in either longitudinal displacements or shear strain. The group differences observed in the retrograde wall motion phase were greater than those observed for CCA stiffness or IMT and were found to be independent of both indices, indicating indices of the retrograde phase intramural shear strain may be a novel and sensitive marker of vascular health. Our findings demonstrate that assessment of longitudinal arterial wall shear strain may provide valuable insight into vascular structure and function and may hold potential for the early detection of cardiovascular disease.

Carotid artery longitudinal wall motion is associated with local blood velocity and left ventricular rotational, but not longitudinal, mechanics

Recent studies have identified a predictable movement pattern of the common carotid artery wall in the longitudinal direction. While there is evidence that the magnitude of this carotid artery longitudinal wall motion (CALM) is sensitive to cardiovascular health status, little is known about the determinants of CALM. The purpose of this integrative study was to evaluate the contribution of left ventricular (LV) cardiac motion and local blood velocity to CALM. Simultaneous ultrasound measurements of CALM, common carotid artery mean blood velocity (MBV), and left ventricular motion were performed in ten young, healthy individuals (6 males; 22 ± 1 years). Peak anterograde CALM occurred at a similar time as peak MBV (18.57 ± 3.98% vs. 18.53 ± 2.81% cardiac cycle; t‐test: P = 0.94; ICC: 0.79, P < 0.01). The timing of maximum retrograde CALM displacement was different, but related, to both peak apical (41.00 ± 7.81% vs. 35.33 ± 5.79% cardiac cycle; t‐test: P < 0.01; ICC: 0.79, P < 0.01) and basal rotation (41.80 ± 6.12% vs. 37.30 ± 5.66% cardiac cycle; t‐test: P < 0.01; ICC: 0.74, P < 0.01) with peak cardiac displacements preceding peak CALM displacements in both cases. The association between basal rotation and retrograde CALM was further supported by strong correlations between their peak magnitudes (r = −0.70, P = 0.02), whereas the magnitude of septal longitudinal displacement was not associated with peak CALM (r = 0.11, P = 0.77). These results suggest that the rotational mechanical movement of the LV base may be closely associated with longitudinal mechanics in the carotid artery. This finding may have important implications for interpreting the complex relationship between ventricular and vascular function.

Impact of shear rate pattern on upper and lower limb conduit artery endothelial function in both spinal cord‐injured and able‐bodied men

What is the central question of this study? This study addresses the following two central questions. (i) What is the impact of vascular deconditioning after spinal cord injury (SCI) on shear rate patterns and endothelial function? (ii) What is the impact of acutely altered shear rate on endothelial function in both SCI and able‐bodied control subjects? What is the main finding and its importance? Two main findings in the present study were as follows: (i) reduced superficial femoral artery endothelial function in the SCI group; and (ii) acutely altered shear rate decreased endothelial function in both SCI and able‐bodied control subjects. These findings may shed some light on future interventions taking into account these regulatory mechanisms.

Arterial Stiffness Is Reduced Regardless of Resistance Training Load in Young Men.

Purpose Discrepancies regarding the effect of resistance exercise training (RET) on arterial stiffness have led to uncertainty regarding the effect of RET on cardiovascular health. Confounding our understanding in this area are the roles of the following: load (heavier vs lighter), participant cardiovascular health, and arterial stiffness assessment method. We aimed to investigate the effects of a heavier versus a lighter load resistance training protocol on both central and local arterial stiffness in previously trained young men. Methods Participants were randomly assigned to a 12-wk supervised whole-body RET program consisting of three sets of 8–12 repetitions (heavier-load, lower-repetition [LR]; n = 16) or three sets of 20–25 repetitions (lighter-load, higher-repetition [HR]; n = 16) to volitional failure, or a control group who maintained their regular activity habits (n = 14). Central arterial stiffness (carotid-femoral pulse wave velocity), local arterial stiffness (common carotid arterial distensibility), and left ventricular mass were measured before and after 12 wk of RET. Results There was a reduction in carotid-femoral pulse wave velocity in both LR (6.24 ± 0.56 vs 5.77 ± 0.76 m·s−1; P < 0.05) and HR (6.42 ± 0.70 vs 5.72 ± 0.60 m·s−1; P < 0.05) with no changes in carotid distensibility (P = 0.52) or left ventricular mass allometrically scaled to fat-free mass (P = 0.60). There were no changes in any variable in the control group. Conclusions Using current criterion-standard assessment methods, central arterial stiffness was reduced after resistance training, regardless of the load lifted during RET, with no effect on local carotid artery distensibility or left ventricular mass. Our results support a role of RET in the promotion of positive adaptations in vascular function.

Lower limb conduit artery endothelial responses to acute upper limb exercise in spinal cord injured and able‐bodied men

Vascular improvements in the nonactive regions during exercise are likely primarily mediated by increased shear rate (SR). Individuals with spinal cord injury (SCI) experience sublesional vascular deconditioning and could potentially benefit from upper body exercise‐induced increases in lower body SR. The present study utilized a single bout of incremental arm‐crank exercise to generate exercise‐induced SR changes in the superficial femoral artery in an effort to evaluate the acute postexercise impact on superficial femoral artery endothelial function via flow‐mediated dilation (FMD), and determine regulatory factors in the nonactive legs of individuals with and without SCI. Eight individuals with SCI and eight age, sex, and waist‐circumference‐matched able‐bodied (AB) controls participated. Nine minutes of incremental arm‐crank exercise increased superficial femoral artery anterograde SR (P = 0.02 and P < 0.01), retrograde SR (P < 0.01 and P < 0.01), and oscillatory shear index (OSI) (P < 0.001 and P < 0.001) in both SCI and AB, respectively. However, these SR alterations resulted in acute postexercise increases in FMD in the AB group only (SCI 6.0 ± 1.2% to 6.3 ± 2.7%, P = 0.74; AB 7.5 ± 1.4% to 11.2 ± 1.4%, P = 0.03). While arm exercise has many cardiovascular benefits and results in changes in SR patterns in the nonactive legs, these changes are not sufficient to induce acute changes in FMD among individuals with SCI, and therefore are less likely to stimulate exercise training‐associated improvements in nonactive limb endothelial function. Understanding the role of SR patterns on FMD brings us closer to designing effective strategies to combat impaired vascular function in both healthy and clinical populations.

Cardiac and haemodynamic influence on carotid artery longitudinal wall motion

What is the central question of this study? Carotid artery longitudinal wall motion (CALM) is a bidirectional forward and backward motion of the arterial wall; however, there is no evidence in humans for what controls CALM despite proposals for pulse pressure, left ventricular motion and shear rate. What is the main finding and its importance? Carotid artery longitudinal wall motion responses were heterogeneous when manipulating sympathetic activation and endothelium‐independent vasodilatation, leading to non‐significant group responses. However, individual CALM responses were associated with left ventricular rotation and shear rate. These findings are important when interpreting changes in CALM in humans with acute or chronic experimental designs.

Modeling Perceived Exertion during Graded Arm Cycling Exercise in Spinal Cord Injury

Purpose RPE may be useful for exercise testing and prescription in individuals with spinal cord injury (SCI), although the roles of differentiated central and peripheral fatigue during exercise are not clear. We aimed to model differentiated RPE responses during graded arm cycling in individuals with SCI and to describe their relationship to cardiorespiratory outcomes. Methods Thirty-six individuals with SCI (13 paraplegia and 23 tetraplegia) completed a maximal graded arm cycling exercise test to volitional exhaustion (5 W·min−1 paraplegia; 10 W·min−1 tetraplegia). Participants were asked to report central RPE (CRPE) and peripheral RPE (PRPE) every minute using the Borg category ratio (CR10) scale until termination of exercise. Heart rate and breath-by-breath respiratory outcomes were collected throughout the exercise test. Ventilatory threshold (VT) was assessed using the ventilatory equivalents method. Results Cardiorespiratory indices increased linearly during graded arm exercise. By contrast, both CRPE and PRPE responses were best fit to a quadratic model with positively accelerating growth in individuals with paraplegia (P < 0.01) and tetraplegia (P < 0.05). PRPE developed faster than CRPE in individuals with tetraplegia (P < 0.01). Individuals with paraplegia had accelerated CRPE (P < 0.05) and PRPE (P < 0.05) responses compared with tetraplegia, but not when considering only individuals who reached VT. PRPE was higher than CPRE only in the late stages (80%–100% test duration; P < 0.05) in both groups when only considering individuals who reached VT. Conclusions PRPE develops faster than CRPE in individuals with tetraplegia in a nonlinear fashion, despite linear increases in cardiorespiratory responses during graded arm cycling. Although there is promise to use differentiated RPE for exercise testing and prescription within the SCI population, our results indicate that there are differences in how individuals with tetraplegia perceive peripheral versus central exertion.

Predicting peak oxygen uptake from submaximal exercise after spinal cord injury

The purpose of this study was to determine the validity of the 6-min arm ergometry test (6MAT) in predicting peak oxygen consumption (V̇O2peak) in individuals with chronic spinal cord injury (SCI). Fifty-two individuals with chronic SCI (age, 38 ± 10 years; American Spinal Injury Association Impairment Scale (AIS), A-D; neurological level of injury (NLI), C1-L2; years post-injury (YPI), 13 ± 10 years) completed an incremental arm ergometry V̇O2peak test and a submaximal 6MAT. Oxygen consumption (V̇O2) data from both tests were used to create a predictive equation with regression analysis. Subsequently, a cross-validation group comprising an additional 10 individuals with SCI (age, 39 ± 13 years; AIS, A-D; NLI, C3-L3; YPI, 9 ± 9 years) were used to determine the predictive power of the equation. All participants were able to complete both the V̇O2peak and 6MAT assessments. Regression analysis yielded the following equation to predict V̇O2peak from end-stage 6MAT V̇O2: V̇O2peak (mL·kg(-1)·min(-1)) = 1.501(6MAT V̇O2) - 0.940. Correlation between measured and predicted V̇O2peak was excellent (r = 0.89). No significant difference was found between measured (17.41 ± 7.44 mL·kg(-1)·min(-1)) and predicted (17.42 ± 6.61 mL·kg(-1)·min(-1)) V̇O2peak (p = 0.97). When cross-validated with a sample of 10 individuals with SCI, correlation between measured and predicted V̇O2peak remained high (r = 0.89), with no differences between measured (18.81 ± 8.35 mL·kg(-1)·min(-1)) and predicted (18.73 ± 7.27 mL·kg(-1)·min(-1)) V̇O2peak (p = 0.75). Results suggest that 6MAT V̇O2 can be used to predict V̇O2peak among individuals with chronic SCI. The 6MAT should be used as a clinical tool for assessing aerobic capacity when peak exercise testing is not feasible.

An assessment of intra-individual variability in carotid artery longitudinal wall motion: recommendations for data acquisition

OBJECTIVE To assess the intra-individual variation in carotid artery longitudinal wall motion (CALM) in healthy adults in order to determine the amount of data required to generate a representative measurement of CALM. APPROACH We conducted an analysis of 27 healthy men to determine whether calculation of resting individual CALM outcomes is dependent on the number of averaged heart cycles. CALM was assessed at rest, 1-2 cm proximal to the right carotid bifurcation during a breath hold and was segmented into three motion displacements: systolic anterograde CALM, systolic retrograde CALM, and diastolic CALM. A 2D measure of total carotid artery motion (RALength) was also determined from the longitudinal and radial displacements. Outcomes were averaged discretely using two, three, four, five, and six consecutive heart cycles to assess the impact of additional data on intra-individual coefficients of variation (CV%) and intra-class correlations (ICC). MAIN RESULTS Calculated means were similar between all heart cycle averaging for CALM displacements (all P  >  0.05), though the two-heart cycle average of RALength (P  =  0.06) was reduced in comparison to all other averages. Averaging data from four heart cycles was sufficient to generate a plateau in the variability in resting CALM displacements, such that within-subject ICC values all reached  >0.90 and CV% were similar to previously reported day-to-day variability in healthy adults. SIGNIFICANCE There is variability in beat-to-beat measures of CALM that should be considered when designing protocols for data collection and analysis. We suggest that four consecutive heart cycles should be averaged to generate representative resting CALM outcomes in humans. While indices of variability were reduced when assessing outcomes generated from two to four heart cycles, no further improvements were observed when more heart cycles were included, indicating that averaging more than four heart cycles is likely not required.

Assessing Ventilatory Threshold in Individuals With Motor-Complete Spinal Cord Injury

OBJECTIVE To assess the feasibility of measuring ventilatory threshold (VT) in higher-level motor-complete spinal cord injury (SCI) using 4 different analysis methods based on noninvasive gas exchange. DESIGN Observational. SETTING Laboratory testing. PARTICIPANTS Individuals with C4-T6 motor-complete SCI (16 paraplegia, 22 tetraplegia; American Spinal Injury Association Impairment Scale A/B; 42±10 years old). INTERVENTIONS Not applicable. MAIN OUTCOME VT from a graded arm cycling test to volitional exhaustion using 4 methods: ventilatory equivalents, excess CO2, V-slope, and combined method. RESULTS VT could be identified in all individuals with paraplegia, but in only 68% of individuals with tetraplegia. Individuals without observable VT completed the graded exercise test with lower ventilatory rate, peak power output, and peak oxygen consumption (Vo2peak) (all P<.05), compared to those with a detectable VT. Bland-Altman plots indicate minimal bias between methods (range: 0.01-0.03 L/min), with 95% limits of agreement of the difference within 0.25 L/min. Absolute V.o2 at VT with individual methods were all correlated to peak power output (r>0.74; P<.01) and Vo2peak (r>0.91; P<.01), with negligible differences between methods. CONCLUSIONS The assessment of VT is a feasible alternative to peak exercise testing for aerobic fitness in individuals with higher-level, motor-complete SCI, although care should be taken when interpreting VT in individuals with tetraplegia who have lower cardiorespiratory fitness and lower peak power outputs.