Heart rate and electrical conduction
One of the simplest CV parameters to assess is heart rate; normally recorded as the number of complete cardiac cycles (or beats) per minute. We often assess resting heart as well as exercise heart rates. Care, of course, must be taken with heart rate interpretation in CV disease patients who may be taking medications with a chronotropic effect (e.g. beta blockers).
Heart rate is normally assessed by palpation, short-range telemetry or an electrocardiogram (ECG). Heart rate assessment via palpation can occur at any superficial artery but is commonly measured at the carotid or radial arteries. This is a simple method that is invaluable in the field or laboratory setting. Care must be taken to avoid palpating with the thumb as it often has its own strong pulse that can be mistaken for the pulse of the subject at rest. Care must also be taken not to apply too much pressure when palpating the carotid artery because of the proximity of the carotid bodies. Only one carotid artery should be palpated at a time as there is a risk of collapse if both carotid arteries are simultaneously occluded. Second, heart rate can be displayed via short-range telemetry involving an electrical sensing system on the chest wall and a receiving/display unit often worn as a wristwatch. The system detects electrical peaks (R wave of the ECG) and then displays a heart rate that is normally averaged over a few seconds. Such devices are quite flexible and can often store data for downloading after exercise.
These systems are considered to be very accurate and are in common use in exer-cise physiology laboratories, gymnasiums, sports clubs and rehabilitation units.
The most common method of heart rate assessment in clinical practice is the ECG (see Figure 17.1). An ECG requires a number of electrodes to be placed at specific sites on the chest wall that can generate a range of electrical traces that represent different ‘views’ of the overall electrical activity of all the cardiomyocytes.
A basic understanding of ECG nomenclature and waveforms is crucial even to the assessment of something as simple as heart rate. It is essential to manually check the heart rate display on ECG machines (check paper speed and count squares between adjacent R waves). This is especially important if the
rhythm is irregular. The PQRST nomenclature of the standard ECG represents the normal pattern of electrical conduction through the heart as viewed from lead II in a standard 12-lead configuration. This conduction pathway begins at the Sino-Atrial (SA) node, passes across the atria (which produces the P wave) to the Atrio-Ventricular (AV) node (junction) where the signal passes through specialised fibres that slow the signal conduction (bundle of His; and results in the delay between P and R waves) on through the left and right bundle branches and ending at the Purkinje fibres (which produce the QRS complex).
The T wave represents ventricular repolarisation with atrial repolarisation occurring within the QRS complex. The passage of electrical depolarisation along this pathway results in sinus rhythm and normal cardiac function.
The cardiac cycle at rest lasts ~0.8 s reflecting a normal resting heart rate of c.70 beats·min1. During exercise, heart rate rises in parallel with exercise intensity associated with a number of neural and hormonal changes. Maximal heart rate in normally healthy individuals is commonly associated with age (max HR ≈220-age) although the exact equation is still debated and contains significant individual variability.
Arrhythmias (sometimes termed dysrhythmias) are abnormal heart rhythms associated with abnormalities in the electrical conduction system and are common in a range of CV diseases. Arrhythmias often result in sub-optimal cardiac function and can, of course, significantly impact upon the assessment of heart rate. Extensive ECG interpretation is beyond the scope of this text and 158 KEITH GEORGE ET AL.
Figure 17.1 An exemplar ‘12-lead’ ECG
lies within clinical boundaries. We would urge those with a real and strong interest in this area to consult appropriate texts (e.g. Wagner, 2001) and seek out appropriate clinically supervised training.
Stroke volume and cardiac output
Cardiac output is defined as the volume of blood ejected from the ventricle, per unit time. In adult humans, cardiac output ranges from 4 to 7 l·min1at rest and can achieve a three-to six-fold increase during intense exercise. A more restricted range of outputs is seen in the diseased heart, making the assessment of cardiac output a useful diagnostic indicator. Cardiac output can be simply described by:
Q.
T SV · fc
where SV stroke volume in ml·beat and fc the frequency of cardiac cycles, or heart rate, in beats·min1.
Measurement of cardiac output during exercise presents a difficult challenge to exercise scientists. Measurements need to be applied rapidly and with ease if accurate measurements are to be taken. The method used should be capable of detecting beat-to-beat changes in cardiac output.
Invasive methods
Generally considered the standard for the determination of cardiac output, the direct Fick method requires the estimation of whole body oxygen uptake from expired air measurements and the sampling of arterial and mixed venous blood, via catheterisation, for oxygen concentration. The risks associated with catheterisation as well as the slow response (only valid in steady-state exercise) make it generally unsuitable for use during heavy exercise.
Indicator dilution techniques use an indirect Fick method where the substance to be measured, a metabolically inert dye or radioiodine labelled albumin, is injected as a bolus into the systemic circulation. The bolus becomes diluted in the returning venous blood and by taking arterial blood samples at frequent intervals the concentration of injectate in the arterial plasma can be plotted against time and cardiac output can be estimated as the area under the concentration–time curve.
It is now more common to use a thermodilution method, whereby a fixed volume of cold solution (e.g. NaCl, D5 W or autologous blood) is injected into the right atrium and the change in blood temperature (dilution) is measured continuously by a thermistor mounted on a pulmonary artery catheter. Cardiac output is determined from the area under the temperature–
time curve. Both indicator dilution and thermal dilution methods can be used with acceptable accuracy during exercise, with error reported to be less than5%.
Non-invasive methods
Blood flow velocity can be computed from either continuous-wave or pulsed-wave Doppler echocardiography. Measurements are taken in the left ventricular outflow tract (apical position) or the ascending aorta (suprasternal position).
Evaluation of the flow–velocity time curve for each cardiac cycle provides a measure of blood flow, which when allied to an M-mode estimation of aortic cross-sectional area is equivalent to stroke volume and hence can be used to estimate cardiac output. Stroke volume is calculated as:
SV CSA · FVI
where CSA aortic cross-sectional area at the site of velocity measurement in cm2and FVI flow – velocity integral or stroke distance in cm.
The non-invasive nature and speed of this method provide advantages over other methods but it does suffer from limitations of calibration and noise.
The major limitation of the use of Doppler in the accurate assessment of car-diac output is that of aliasing which is of particular concern when measuring cardiac output during or immediately following vigorous exercise. Accuracy of the method during exercise may be as low as 44% (Coats, 1990). Thus, Doppler methods offer a safe but only moderately reproducible and accurate method for measuring cardiac output.
Impedance cardiography relies on a direct correlation between blood flow through a body segment and the changes in bioimpedance across that segment to estimate SV and thus cardiac output. Simply stated, the increase in blood volume and velocity in the aorta during ventricular systole causes a decrease in bioimpedance that can be detected by electrodes placed at either end of the thoracic cavity. In this way SV can be estimated from the Sramek equation (Sramek et al., 1983):
SV VEPT · VET · [EVI/TFI]
where VEPT physical volume of electrically participating tissue in ml, VET the ventricular ejection time in s, EVI ejection velocity index in ·s1 and TFI thoracic fluid index [the total bioimpedance between the sensing electrodes] in .
Belardinelli et al. (1996) found no significant differences in cardiac output values determined by impedance cardiography, thermodilution, and direct Fick methods at rest and over a wide range of exercise workloads. Agreement between the methods was between 0.01 and 0.04 l·min1at rest and between 0.2 and 0.5 l·min1at peak exercise (≈80–140 W). The method may become less reliable at higher intensities (~180 W) of exercise (Moore et al., 1992).
Gas exchange methods are a common non-invasive technique for the estimation of cardiac output. The rate of uptake or excretion of physiological gasses (e.g. O2, CO2) or inert soluble gasses (e.g. acetylene, nitrous oxide, Freon), determined from analysis of alveolar gas exchange, can be used to estimate pulmonary capillary blood flow (Q.
c) without the need for blood sam-pling. The most commonly used gas exchange technique is the CO2rebreathing 160 KEITH GEORGE ET AL.
method. Rebreathing a gas mixture containing CO2is well tolerated by both trained and untrained subjects. The rebreathing method employs an indirect Fick principle such that:
Instead of measuring the concentrations of CO2in arterial blood, is estimated by measuring the partial pressure of CO2( ) in alveolar gas and converting it to the equivalent arterial gas concentration with the use of a CO2 dissociation curve. PCO2can be easily converted to provided pH and oxyhaemoglobin concentration are known or are unchanged during the meas-urement (McHardy, 1967). Modern metabolic analysis systems perform this computation automatically. Estimation of is somewhat more challeng-ing and is achieved by measurchalleng-ing the PCO2of expired air during a rebreathing manoeuvre.
The two methods of the CO2rebreathing technique commonly employed use either an equilibrium technique (Collier, 1956) or an exponential technique (Defares, 1958). In the equilibrium method, rebreathing a CO2 mixture reverses the normal – concentration gradient bringing the expired CO2 measurement into equilibrium with that in the blood passing through the pulmonary circulation. In the exponential method, the subject rebreathes a gas mixture with an initial CO2concentration of 5% and a volume at least equal to the subject’s tidal volume (VT). Estimation of is achieved by plotting a best-fit line through a series of end-tidal CO2( ) data points.
Carbon dioxide rebreathing procedures have been shown to provide acceptable levels of accuracy and precision in the estimation of Q.
T during steady-state exercise (Reybrouk et al., 1978). Indeed, both accuracy (Mahler et al., 1985) and reliability (Nugent et al., 1994) appear to improve during exercise. Measurements during non-steady-state exercise have, however, produced mixed results and further investigation of the technique is necessary before its accuracy can be confirmed.
Cardiovascular structures
A vast number of cardiac structures have been assessed in clinical practice using a broad array of techniques. In this chapter we limit our discussion primarily to left ventricular assessment. Likewise, we have limited the brief description of techniques to the most common clinical imaging technique: ultrasound (echocardiography) as well as providing some reference to radionuclide angiog-raphy and magnetic resonance imaging (MRI).
Clinical assessment of cardiac structures advanced significantly with the practical application of ultrasound imaging that became widespread in the 1970s. Initially with M-mode (see Figure 17.2) and subsequently with 2-D and 3-D sector echocardiography, clinicians were able to differentiate and measure PETCO2
PV–CO2 PaCO2
P– VCO2
C– VCO2
CCO2 PCO2
CaCO2
Q.c
V.CO2 C –
VCO2 CaCO2
parameters such as LV volume at end-diastole and end-systole, LV wall thickness and LV mass. From these measurements came the ability to estimate some functional variables such as stroke volume and ejection fraction. Other structural variables that can be assessed include some aspects of the major vessels leading to and from the chambers of the heart. In newer machines, with better resolution, portions of the cardiac arterial tree can be imaged. There are a number of studies that have demonstrated good validity for ultrasound measures (e.g. Devereux and Reichek, 1977) when compared with autopsy studies and both intra- and inter-tester reliability is acceptable in the hands of a skilled technician (Stefadouros and Canedo, 1977) working with standard imaging and measurement guidelines (e.g. Schiller et al., 1989).
Radionuclide techniques (e.g. radionuclide angiography) include a range of tests/procedures that utilise radioactive ‘agents’ that are injected into the subject, as a contrast medium for viewing cardiac chambers and arterial lumens when scanning. A common use of radionuclide testing is to examine the coro-nary arteries in suspected corocoro-nary artery disease. Other uses have included the assessment of global left ventricular function in health, exercise and sporting contexts (e.g. Spina et al., 1993). Whilst highly accurate (e.g. Borges-Neto et al., 1997) it has some limitations with the most accurate data for LV func-tion gathered from ‘multiple-pass’ scanning that restricts its use within exercise interventions to steady-state (often in the supine position). MRI is a powerful tool for assessing CV structures and function. The combination of a magnetic field with radiofrequency energy produces images of cardiovascular tissues that reflect their different hydrogen (mainly water content), thus providing clear tissue differentiation. The resolution and clarity of images are better than other techniques (Bottini et al., 1995) and no harmful effects of the testing have been documented. Cost, technical and clinical requirements are such that both radionuclide techniques and MRI are likely to remain a clinical and/or research tool.
162 KEITH GEORGE ET AL.
Figure 17.2 An example of an M-mode echocardiogram across the left ventricle