Guglielmo Antonutto

The interplay of central and peripheral factors in limiting maximal O2 consumption in man after prolonged bed rest

1 The effects of bed rest on the cardiovascular and muscular parameters which affect maximal O2 consumption (VO2,max) were studied. The fractional limitation of VO2,max imposed by these parameters after bed rest was analysed. 2 The VO2,max, by standard procedure, and the maximal cardiac output (Q̇max), by the pulse contour method, were measured during graded cyclo‐ergometric exercise on seven subjects before and after a 42‐day head‐down tilt bed rest. Blood haemoglobin concentration ([Hb]) and arterialized blood gas analysis were determined at the highest work load. 3 Muscle fibre types, oxidative enzyme activities, and capillary and mitochondrial densities were measured on biopsy samples from the vastus lateralis muscle before and at the end of bed rest. The measure of muscle cross‐sectional area (CSA) by NMR imaging at the level of biopsy site allowed computation of muscle oxidative capacity and capillary length. 4 The VO2max was reduced after bed rest (−16.6%). The concomitant decreases in Q̇max (−30.8%), essentially due to a change in stroke volume, and in [Hb] led to a huge decrease in O2 delivery (−39.7%). 5 Fibre type distribution was unaffected by bed rest. The decrease in fibre area corresponded to the significant reduction in muscle CSA (−17%). The volume density of mitochondria was reduced after bed rest (−16.6%), as were the oxidative enzyme activities (−11%). The total mitochondrial volume was reduced by 28.5%. Capillary density was unchanged. Total capillary length was 22.2% lower after bed rest, due to muscle atrophy. 6 The interaction between these muscular and cardiovascular changes led to a smaller reduction in VO2max than in cardiovascular O2 transport. Yet the latter appears to play the greatest role in limiting VO2max after bed rest (>70% of overall limitation), the remaining fraction being shared between peripheral O2 diffusion and utilization.

Correction of cardiac output obtained by Modelflow from finger pulse pressure profiles with a respiratory method in humans

The beat-by-beat non-invasive assessment of cardiac output (Q litre x min(-1)) based on the arterial pulse pressure analysis called Modelflow can be a very useful tool for quantifying the cardiovascular adjustments occurring in exercising humans. Q was measured in nine young subjects at rest and during steady-state cycling exercise performed at 50, 100, 150 and 200 W by using Modelflow applied to the Portapres non-invasive pulse wave (Q(Modelflow)) and by means of the open-circuit acetylene uptake (Q(C2H2)). Q values were correlated linearly ( r = 0.784), but Bland-Altman analysis revealed that mean Q(Modelflow) - Q(C2H2) difference (bias) was equal to 1.83 litre x min(-1) with an S.D. (precision) of 4.11 litre x min(-1), and 95% limits of agreement were relatively large, i.e. from -6.23 to +9.89 litre x min(-1). Q(Modelflow) values were then multiplied by individual calibrating factors obtained by dividing Q(C2H2) by Q(Modelflow) for each subject measured at 150 W to obtain corrected Q(Modelflow) (Qcorrected) values. Qcorrected values were compared with the corresponding Q(C2H2) values, with values at 150 W ignored. Data were correlated linearly ( r = 0.931) and were not significantly different. The bias and precision were found to be 0.24 litre x min(-1) and 3.48 litre x min(-1) respectively, and 95% limits of agreement ranged from -6.58 to +7.05 litre x min(-1). In conclusion, after correction by an independent method, Modelflow was found to be a reliable and accurate procedure for measuring Q in humans at rest and exercise, and it can be proposed for routine purposes.

The effect of laparoscopic cholecystectomy on cardiovascular function and pulmonary gas exchange

Hemodynamic changes, pulmonary CO2 elimination (VECO2) and gas exchange were evaluated during laparoscopic cholecystectomy.An algorithm to calculate inspired ventilation (VI) needed to maintain constant PaCO2 was also developed. In 12 ASA physical status I patients undergoing laparoscopic cholecystectomy, heart rate (HR), mean arterial pressure (MAP), cardiac index (CI), and systemic vascular resistance index (SVRI) were measured by the analysis of a radial artery pressure profile before, during, and after CO2 insufflation. Alveolar-arterial oxygen pressure gradient (P(A-a)O2), physiological and alveolar ventilatory dead space fractions (VDphys/VT; VDalv/VT), and PaCO2 were measured as well. VECO2 was assessed every minute in the patients maintained in the head-up position. HR did not significantly change during pneumoperitoneum, whereas MAP showed a transient increase (24.9%; P < 0.05) after CO2 insufflation. CI remained stable during pneumoperitoneum, but increased (25.0%; P < 0.05) after deflation. As a consequence, SVRI transiently increased after CO2 insufflation and decreased by 15.8% (P < 0.05) 5 min after deflation. P(A-a)O (2) increased slightly (P < 0.05) with increased anesthesia time. VDphys/VT and VDalv/VT did not change after pneumoperitoneum onset, but VDalv/VT decreased after CO2 deflation (13.4%; P < 0.05). VECO2 increased (decreased) after a monoexponential time course during (after) CO2 insufflation in 8 of 12 patients. The mean time constants (t) of the monoexponential functions were 26.3 and 15.4 min during and after pneumoperitoneum. A monoexponential time course was shown also by PaCO2 during CO2 insufflation (tau = 27.8 min). Finally, the VI needed to maintain PaCO2 at a selected value could be calculated by the following algorithm: VI = [0.448 centered dot (1 - e-t/tau) + 2.52] centered dot (VA centered dot PaCO (2) centered dot 713)-1, where VA corresponds to alveolar ventilation and t must be chosen according to the pneumoperitoneum phase. We conclude that CO2 insufflation in the abdominal cavity does not induce significant changes in cardiopulmonary function in ASA physical status I patients. The algorithm proposed seems to be a useful tool for the anesthesiologists to maintain constant PaCO2 during all surgical procedures. (Anesth Analg 1996;83:134-40)

The Hemodynamic and Metabolic Effects of Tourniquet Application During Knee Surgery

We evaluated the effects of tourniquet application on the cardiovascular system and metabolism in 10 young men undergoing knee surgery with general anesthesia. The duration of inflation was from 75 to 108 min. Heart rate, mean arterial pressure, cardiac index (CI) by pulse contour method, and systemic vascular resistance were measured before, during, and after tourniquet inflation. pH, Pao2, Paco2, and lactate blood concentrations were also measured. &OV0312;o2 and &OV0312;co2 were assessed every minute from tracheal intubation up to 15 min after tourniquet deflation and &OV0312;o2 in excess of the basal value over the 15 min after deflation (&OV0312;o2exc) was calculated. Mean arterial pressure increased 26% (P < 0.05) during inflation and returned to basal values after deflation. CI did not change immediately after inflation; although, thereafter, it increased 18% (P < 0.05). Five minutes after deflation, CI further increased to a value 40% higher than the basal value. Therefore, systemic vascular resistance increased 20% suddenly after inflation (P < 0.05) and decreased 18% after deflation (P < 0.05). &OV0312;o2 and &OV0312;co2 remained stable during inflation and increased (P < 0.05) after deflation. &OV0312;o2exc depended on duration of tourniquet inflation time (Tisch) (P < 0.05). After deflation, Paco2 and lactate increased (P < 0.05) while Tisch increased. We conclude that tourniquet application induces modifications of the cardiovascular system and metabolism, which depend on tourniquet phase and on Tisch. Whether these modifications could be relevant in patients with poor physical conditions is not known. Implications The clinical effects of tourniquet application were evaluated in 10 young men undergoing knee surgery. Our data indicate that tourniquet application causes hemodynamic and metabolic changes which may become clinically relevant after a long period of tourniquet inflation, particularly in patients with concomitant cardiovascular diseases.

Effects of body size, body density, gender and growth on underwater torque

Two forces act on a human body motionless in water: weight (W) and buoyancy (B). They are applied to the center of mass (CM) and to the center of volume (CV) of the subject, respectively. CM and CV do not coincide; this generates a torque that is a measure of the tendency of the upper part of the body to rise, rotating around its center of mass. To quantify this tendency, Pendergast & Craig defined ‘underwater torque’ (T1) as the product of the net force with which the feet of a subject lying horizontally in water tend to sink, times the distance between the feet and the center of volume of the lungs. In this paper we have investigated: (a) the relationships between T1 and body weight (BW), height (H), body surface area (BS), body density (BD) and leg density (LD) in a group of 30 subjects (group A, 14 females and 16 males, age range 16‐50 years); and (b) the effect of gender and growth on T1 in a group of 110 subjects (group B, 67 girls and 43 boys, age range 12‐17 years). In group A, T1 was found to be linearly related with BW (r= 0.833, P < 0.001), H (r= 0.803, P < 0.001), BS (r= 0.866, P < 0.001), BD (r= 0.617, P < 0.001) and LD (r= 0.549, P < 0.005). A multiple linear regression analysis showed that BS and BD explained about 85% of the variability of T1 (r2= 0.85). In group B, T1 was found to increase linearly with age (r= 0.47, P < 0.01), the increasing rate being three times higher in boys compared with girls. As a consequence, the T1 ratio between boys and girls increased with age, from 1.69 at 13 years to 2.04 at 16 years.

Time-frequency analysis and estimation of muscle fiber conduction velocity from surface EMG signals during explosive dynamic contractions

Time-frequency analysis of the surface electromyographic (EMG) signal is used to assess muscle fiber membrane properties during dynamic contractions. The aim of this study was to compare the direct estimation of average muscle fiber conduction velocity (CV) with instantaneous mean frequency (iMNF) of surface EMG signals in isometric and explosive dynamic contractions. The muscles investigated were the vastus lateralis and medialis of both thighs in 12 male subjects. The isometric contractions were at linearly increasing force (0-100% of the maximal voluntary contraction in 10s). The explosive contractions were performed on a multipurpose ergometer-dynamometer (MED). The subject, sitting on the MED, performed six explosive contractions, separated by 2 min rest, by pushing against two force platforms and thrusting himself backwards with the maximum possible speed, while completely extending his legs. The estimated CV significantly increased with force in both the isometric (mean+/-S.D., from 3.24+/-0.34 to 5.12+/-0.31 m/s for vastus lateralis and from 3.17+/-0.26 to 5.11+/-0.34 m/s for vastus medialis, with force in the range 10-100% of the maximal voluntary contraction level) and explosive contractions (from 4.36+/-0.49 to 5.00+/-0.47 m/s for vastus lateralis, and from 4.32+/-0.46 to 4.94+/-0.44 m/s for vastus medialis, with force in the range 17.5-100% of maximal thrusting force). Moreover, estimated CV was not significantly different at the maximal force in the two exercises. On the contrary, iMNF, computed from the Choi-Williams time-frequency transform, was significantly lower in the explosive (57.7+/-8.2 and 66.5+/-10.3 Hz for vastus laterialis and medialis, respectively) than in the isometric exercises (73.7+/-9.2 and 75.0+/-8.5 Hz for vastus laterialis and medialis, respectively) and did not change with force in any of the conditions. It was concluded that EMG spectral features provide different information with respect to average muscle fiber CV in dynamic contractions. Thus, in general, they cannot be used to infer CV changes during the exertion of a dynamic task. A joint analysis of CV and EMG spectral features is necessary in this type of contractions.

Muscle-fiber conduction velocity estimated from surface EMG signals during explosive dynamic contractions.

Muscle‐fiber conduction velocity (CV) was estimated from surface electromyographic (EMG) signals during isometric contractions and during short (150–200 ms), explosive, dynamic exercises. Surface EMG signals were recorded with four linear adhesive arrays from the vastus lateralis and medialis muscles of 12 healthy subjects. Isometric contractions were at linearly increasing force from 0% to 100% of the maximum. The dynamic contractions consisted of explosive efforts of the lower limb on a sledge ergometer. For the explosive contractions, muscle‐fiber CV was estimated in seven time‐windows located along the ascending time interval of the force. There was a significant correlation between CV values during the isometric ramp and explosive contractions (R = 0.75). Moreover, CV estimates increased significantly from (mean ± SD) 4.32 ± 0.46 m/s to 4.97 ± 0.45 m/s during the increasing‐force explosive task. It was concluded that CV can be estimated reliably during dynamic tasks involving fast limb movements and that, in these contractions, it may provide important information on motor‐unit control properties. Muscle Nerve 29: 823–833, 2004

Noninvasive assessment of cardiac output from arterial pressure profiles during exercise

The stroke volume of the left ventricle (SV) was assessed in nine young men (mean age 22.2, ranging from 20 to 25 years) during cycle ergometer upright exercise at exercise intensities from 60 to 150 W (about 20% to 80% of individual maximal aerobic power). The SV was calculated from noninvasive tracings of the arterial blood pressure, determined from photoplethysmograph records and compared to the SV determined simultaneously by pulsed Doppler echocardiography (PDE). Given the relationshipSV =As·Z−1 in whichAS is the area underneath the systolic pressure profile (in millimetres of mercury and second), andZ (in millimetres of mercury and second per millilitre) is the apparent hydraulic impedance of the circulatory system, a prerequisite for the assessment of SV from the photoplethysmograph tracings is a knowledge of Z. The experimental value of Z (hereafter defined Z*) was calculated by dividing AS (from the finger photoplethysmograph) by SV as obtained by PDE. When the whole group of subjects was considered, Z* was not greatly affected by the exercise intensity: it amounted to 0.089 (SD 0.028;n = 36). The Z was also estimated independently of any parameter other than heart rate (HR), mean (MAP) and pulse (PP) arterial blood pressure obtained from the photoplethysmograph. A computerized statistical method allowed us to interpolate the experimental values ofZ*, HR, PP and MAP by the equationZm = a·(b + c·HR + d·PP + e·MAP)−1, thus obtaining the coefficients a to e. The mean percentage error betweenZm (calculated from the coefficients obtained andZm was 21.8 (SD 14.3)%. However, it was observed that, in a given subject,Z* was significantly affected by the exercise intensity. Therefore, to improve the estimate ofZ a second algorithm was developed to update the experimental value ofZ determined initially at rest (Zin). This updated value (Zcor) ofZ was calculated asZcor =Zin· [(f/(i + g·(HR/HRin) + h·(PP/PPin) + 1· (MAP/MAPin], whereHRin,PPin,MAPin,HR,PP,MAP are the above parameters at rest and during exercise, respectively. Also in this case, the coefficients f to 1 were determined by a computerized statistical method usingZ* as the experimental reference. The values ofZcor so obtained allowed us to calculate SV from arterial pulse contour analysis asSVF =AS·Zcor/−1. The mean percentage error between theSVF obtained and the values simultaneously determined by PDE, was 10.0 (SD 8.7)%. It is concluded that the SV of the left ventricle, and hence cardiac output, can be determined during exercise from photoplethysmograph tracings with reasonable accuracy, provided that an initial estimate of SV at rest is made by means an independent high quality reference method.

Effects of prolonged bed rest on cardiovascular oxygen transport during submaximal exercise in humans

Abstract The hypothesis was tested that prolonged bed rest impairs O2 transport during exercise, which implies a lowering of cardiac output Q˙c and O2 delivery (aO2). The following parameters were determined in five males at rest and at the steady-state of the 100-W exercise before (B) and after (A) 42-day bed rest with head-down tilt at −6°: O2 consumption (V˙O2), by a standard open-circuit method; Q˙c, by the pressure pulse contour method, heart rate ( fc), stroke volume (Qh), arterial O2 saturation, blood haemoglobin concentration ([Hb]), arterial O2 concentration (CaO2), and Q˙aO2. The V˙O2 was the same in A and in B, as was the resting fc. The fc at 100 W was higher in A than in B (+17.5%). The Qh was markedly reduced (−27.7% and −22.2% at rest and 100 W, respectively). The Q˙c was lower in A than in B [−27.6% and −7.8% (NS) at rest and 100 W, respectively]. The CaO2 was lower in A than in B because of the reduction in [Hb]. Thus also Q˙aO2 was lower in A than in B (−32.0% and −11.9% at rest and at 100 W, respectively). The present results would suggest a down-regulation of the O2 transport system after bed rest.

Effects of an Uphill Marathon on Running Mechanics and Lower-Limb Muscle Fatigue.

PURPOSE To investigate the effects of an uphill marathon (43 km, 3063-m elevation gain) on running mechanics and neuromuscular fatigue in lower-limb muscles. METHODS Maximal mechanical power of lower limbs (MMP), temporal tensiomyographic (TMG) parameters, and muscle-belly displacement (Dm) were determined in the vastus lateralis muscle before and after the competition in 18 runners (age 42.8 ± 9.9 y, body mass 70.1 ± 7.3 kg, maximal oxygen uptake 55.5 ± 7.5 mL · kg-1 · min-1). Contact (tc) and aerial (ta) times, step frequency (f), and running velocity (v) were measured at 3, 14, and 30 km and after the finish line (POST). Peak vertical ground-reaction force (Fmax), vertical displacement of the center of mass (Δz), leg-length change (ΔL), and vertical (kvert) and leg (kleg) stiffness were calculated. RESULTS MMP was inversely related with race time (r = -.56, P = .016), tc (r = -.61, P = .008), and Δz (r = -.57, P = .012) and directly related with Fmax (r = .59, P = .010), ta (r = .48, P = .040), and kvert (r = .51, P = .027). In the fastest subgroup (n = 9) the following parameters were lower in POST (P < .05) than at km 3: ta (-14.1% ± 17.8%), Fmax (-6.2% ± 6.4%), kvert (-17.5% ± 17.2%), and kleg (-11.4% ± 10.9%). The slowest subgroup (n = 9) showed changes (P < .05) at km 30 and POST in Fmax (-5.5% ± 4.9% and -5.3% ± 4.1%), ta (-20.5% ± 16.2% and -21.5% ± 14.4%), tc (5.5% ± 7.5% and 3.2% ± 5.2%), kvert (-14.0% ± 12.8% and -11.8% ± 10.0%), and kleg (-8.9% ± 11.5% and -11.9% ± 12%). TMG temporal parameters decreased in all runners (-27.35% ± 18.0%, P < .001), while Dm increased (24.0% ± 35.0%, P = .005), showing lower-limb stiffness and higher muscle sensibility to the electrical stimulus. CONCLUSIONS Greater MMP was related with smaller changes in running mechanics induced by fatigue. Thus, lower-limb power training could improve running performance in uphill marathons.