Mean blood pressure assessment during post-exercise: result from two different methods of calculation.

Author:Sainas, Gianmarco
Position:Research Article - Report
 
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Introduction

Mean arterial blood pressure (MBP) is the average blood pressure in a subject during the cardiac cycle. Inasmuch as this parameter represents the force to sustain organ perfusion, it should be considered a fundamental cardiovascular variable. During dynamic exercise MBP usually slightly rises, depending on the mode and intensity of exercise (Crisafulli et al., 2005; 2006a; 2006b; 2007; Higginbotham et al., 1986; Lewis et al., 1983; Miles et al., 1984). Importantly, an acute hypotensive effect of dynamic exercise has been several times reported in the period immediately after exercise (Fleg and Lakatta, 1986; Kenney and Seals, 1993; McDonald et al., 2000; Nottin et al., 2002; Piepoli and Coats, 1994; Raine et al., 2001). Even though the precise mechanism of this hypotension is still to be clarified, it has been mostly ascribed to an acute vasodilation due to exercise (Halliwill et al., 2013). It is to be noticed that usually MBP has been calculated with a standard formula (SF), which considers as constant the proportion between diastolic (DBP) and systolic (SBP) blood pressure during the cardiac cycle. In detail, this formula is as follows: MBP= DBP + 1/3 (SBP - DBP) (Crisafulli et al., 2003a; Crisafulli et al., 2003b; de Almeida et al., 2010; Coats et al., 1989; Kilgour et al., 1995; Nottin et al., 2002; Raine et al., 2001). This assumption is based upon the fact that at rest the proportion between systolic and diastolic periods of the cardiac cycle is approximately 1/3 and 2/3 for the systole and the diastole respectively. However, this is only a crude estimation. When tachycardia occurs, as during exercise and recovery, this proportion is lost, as diastole shortens more than systole. It becomes that the SF may lead to a systematic error in MBP assessment.

This occurrence has been already demonstrated during exercise (Moran et al., 1995; Rogers and Oosthuyuse, 2000), where a substantial error (i.e. underestimation) due to the SF has been detected and it has been found to be linearly correlated with heart rate (HR). Authors of the quoted study proposed a corrected formula (CF), where MBP is calculated taking into consideration changes in the diastolic and systolic periods by exercise-induced tachycardia. In particular, the fraction of systole (FS) from the heart cycle was assessed and MBP was then calculated from DBP and the pulse pressure (PP) adjusted for FS as follows: DBP+FSPP (Moran et al., 1995; Rogers and Oosthuyuse, 2000).

However, to the best of our knowledge, none has to date investigated on the difference between the SF and the CF during exercise recovery. This study was devised to compare the SF and CF in the estimation of MBP after bouts of exercise at different intensities. Given the supposed hypotensive effect and its potential role in the treatment of hypertension, this investigation would be useful in a clinical perspective in order to correctly verify and quantify this hypotensive effect.

Methods

Subjects

Ten healthy males, whose mean [+ or -] standard deviation (SD) of age, height, and mass were 29.6 [+ or -] 5.7 years, 1.73 [+ or -] 0.04 m, and 72.6 [+ or -] 8.2 kg respectively, volunteered to take part in this study. All volunteers gave written informed consent to participate in the present investigation, which was performed according to the declaration of Helsinki and approved by the local ethics committee. All subjects were physically active who trained 4 to 6 hours per week and were free of any known cardiovascular or pulmonary disease. None of them reported suffering from post-exercise intolerance or hypotension and none was assuming any kind of medication.

Experimental design

Before entering the study, each subject underwent a preliminary incremental test on an electromagnetically-braked cycle-ergometer (CUSTO Med, Ottobrunn, Germany) to assess the workload at anaerobic threshold (Wat) and the maximum workload achievable (Wmax). This test consisted of a linear increase of work load of 20W/min, starting from 20W, at a pedalling frequency of 60 rpm, up to volitional exhaustion (i.e. the point at which the subject was unable to maintain a pedalling rate of at least 50 rpm). Oxygen uptake (VO2) and carbon dioxide output (VCO2) were measured throughout this preliminary test and Wat was determined by using the V-slope method (Beaver et al., 1986). The mean [+ or -] SD values of maximum HR and Wmax reached were 195 [+ or -] 4 bpm and 248.4 [+ or -] 45.3 W respectively, while maximum values of V[O.sub.2] and VC[O.sub.2] were 3.13 [+ or -] 0.27 and 4.18 [+ or -] 0.31 l x [min.sup.-1] respectively. Wat occurred at 179.6 [+ or -] 24.7 W (i.e. 72.2% of Wmax).

In separate days from this preliminary test (the interval was at least 3 days), each subject underwent the following study protocol, randomly assigned to eliminate any order effect:

Recovery after exercise performed above Wat (130%Wat test): this test consisted of a period of three minutes of rest in the upright position seated on the cycle-ergometer, in order to obtain baseline data; a warm-up of three minutes pedalling at 60 rpm against a resistance of 40W; this period was then followed by exercise, performed against a resistance equivalent to 130% of Wat for 10 minutes or till exhaustion, i.e. the point at which the subject was unable to maintain a pedalling rate of at least 50 rpm. The mean power output and duration of the 130%Wat test were 233.5 [+ or -] 52.3 W and 264.4 [+ or -] 142.4 s respectively. After the 130%Wmax test, recovery was studied for 30 minutes: three minutes were active recovery pedalling at 60 rpm against 40W, followed by 27 minutes of passive recovery during which the subject remained on the bicycle without moving the legs.

Recovery after exercise performed below Wat (70%Wat test): the same rest-warm-up-exercise-recovery protocol employed in the 130%Wat test was used, but the exercise was performed against a resistance equivalent to 70% of Wat and it always lasted 10 minutes. The mean power output of the 70%Wmax test was 124.5 [+ or -] 27.9 W.

A similar protocol was already used to study differences in cardiovascular response after tasks of various intensities (Crisafulli et al., 2006a). In particular, we chose these two exercise intensities to have mild-moderate and moderate-heavy effort intensities. All experiments were conducted between 9 A.M. and 2 P.M. in a temperature-controlled room (room temperature set at 22[degrees]C and relative humidity 50%) and were spaced from each other by at least three days. Subjects had a light meal at least two hours before exercising.

Hemodynamic measurements

Hemodynamic parameters during the recovery sessions of the 130%Wat and the 70%Wat tests were collected by means of an impedance cardiograph (NCCOM 3, BoMed Inc., Irvine, CA), a device which allows continuous non-invasive cardiodynamic measuring during rest, exercise, and recovery. This method has been employed in similar studies dealing with hemodynamics during exercise and recovery (Crisafulli et al., 2000; 2003a; 2004; 2006a; 2006b; 2007; Kilgour et al., 1995; Miles et al., 1984; Moore et al., 1992; Richard et al., 2001; Takahashi and Miyamoto, 1998). This technique assumes that, when an electrical current circulates through the thorax, the pulsatile aortic blood flow induces a proportional fluctuation in electrical conductivity. Therefore, changes in thoracic electrical impedance during systole are representative of stroke volume (SV) (Bernestein, 1986). Our group has previously used the impedance method in similar experimental setting in order to evaluate hemodynamics during recovery following submaximal, maximal, and supramaximal exercise and detailed descriptions of its rationale and application can be found in these reports (Crisafulli et al., 2003b; 2004; 2006a; 2007; 2011). Briefly, NCCOM 3 was connected to the subject by arranging eight commercially available electrodes: two pairs were thoracic and cervical sensing electrodes, while two other pairs were sensing electrodes placed above the cervical and below the thoracic pairs. By means of a digital chart recorder (ADInstruments, PowerLab 8sp, Castle Hill, Australia) NCCOM 3-derived analog traces of electrocardiogram, thorax impedance (Z0), and Z0 first derivative (dZ/dt) were stored and offline cleaned from signals affected by movement and respiratory artifacts. Traces were then analysed offline, paying particular attention to deriving hemodynamic variables only from traces not affected by impedance artifacts. This signal processing procedure has been previously employed and, although time-consuming, it allows the obtaining of reliable and reproducible cardiodynamic data estimation during various kind of exercise and recovery (Crisafulli et al., 2003a; 2003b; 2004; 2006a; 2006b; 2007; 2011). Stroke volume was assessed by using the Sramek-Bernstein equation (Bernestein et al., 1986):

SV = (VEPT x Z0-1) x dZ/dtmax x VET

where VEPT is the volume of electrical participating tissue and was derived using a nomogram from sex, height, and weight of the subject; Z0 was the thorax impedance measured at the end of cardiac diastole; VET was the left ventricular ejection time, measured as the interval between the beginning and the minimum of the deflection in dZ/dt trace during systole (Crisafulli et al., 2000; Crisafulli et al., 2001). HR was calculated as the reciprocal of the electrocardiogram R-R interval and cardiac...

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