Isovolumic Contraction

Heart sounds Sound Si occurs at the beginning of systole as the mitral and tricuspid valves close S2 occurs at the beginning of diastole as the aortic and pulmonary valves close. These points should be in line with the beginning of electrical depolarization (QRS) and the end of repolarization (T), respectively, on the ECG trace. The duration of Si matches the duration of isovolumic contraction (IVC) and that of S2 matches that of isovolumic relaxation (IVR). Mark the vertical lines on the plot to demonstrate this fact. 

Left Ventricle (LV) A simple inverted U curve is drawn that has its baseline between 0 and 5 mmHg and its peak at 120 mmHg. During diastole, its pressure must be less than that of the CVP to enable forward flow. It only increases above CVP during systole. The curve between points A and B demonstrates why the initial contraction is isovolumic. The LV pressure is greater than CVP so the mitral valve must be closed, but it is less than aortic pressure so the aortic valve must also be closed. The same is true of the curve between points C and D with regards to IVR. 

Other new echocardiographic methods have been employed in evaluation of PAH. One of them is strain imaging, which measures deformation over time it has been used in concert with TAPSE and RV Tei index (isovolumic contraction time and relaxation time/ejec-tion time [TVt - RVt]/RVt), and has been shown to be reduced in patients with acute pulmonary embolus and improved after stabilization (37). Using parameters of the eccentricity index, which highlight RV/LV interdependence, it was shown that an eccentricity index less than median was associated with improved survival and freedom from a composite endpoint (38). Lastly, the newer technique of vector velocity imaging applied to the right ventricle is promising as a quantitative assessment of RV function and reserve. 

Suga H, Sagawa K (1974) Instantaneous pressure-volume relationships and their ratio in the excised, supported canine left ventricle. Circ Res 35 117-126 Taylor RR, Covell JW, Ross J Jr (1969) Volume-tension diagrams of ejecting and isovolumic contractions in left ventricle. Am J Physiol 216 1097-1102 Weber KT, Janicki JS, Hefner LL (1976) Left ventricular force-length relations of isovolumic and ejecting contractions. Am J Physiol 231 337-343... 

FEIGL I have a little trouble with your terms elastance and resistance . It seems to me that elastance is similar to what cardiac muscle mechanics people might be tempted to call the length-strength relationship, since you ve derived it from isovolumic contractions. Similarly, resistance would be called the force-velocity relationship. What is your reason for choosing the terms elastence and resistance ... 

BEYAR I would like to ask about the relationship of the maximum elastance to the heart rate. It is known from physiologic measurements that in isovolumic contractions the pressure which the left ventricle develops is higher with the increase in rate that means, a higher elastance value is induced by accelerating the heart rate. In one of your papers you report the effect of the heart rate on the elastance value, saying that the elastance function shortens in duration but is quite steady in magnitude. That means it doesn t increase with the heart rate. Would you like to comment about these two discrepancies ... 

Some of the applicable muscle models include the Maxwell, Voigt, Hill and Carlson models (Figure 1). In particular, the Carlson (1957) equation is used in much of this work to describe the stress-velocity relationship of cardiac muscle over the entire cardiac cycle. Min et al. (1978) found very little difference in analyzing ventricular dynamics when he alternately used Carlson s equation only during isotonic contraction and Hill s equation during isovolumic contraction. 

Wong YK (1973) Myocardial mechanics application of sliding-filament theory to isovolumic contraction of the left ventricle. J Biomechanics 6 565-581... 

Burns JW, Coveil JW (1972) Myocardial oxygen consumption during isotonic and isovolumic contractions in the intact heart. Am J Physiol 223 6 1491-1497 Cross CE, Rieben PA, Salisbury PF (1970) Coronary driving pressure and vasomotor tonus or determinants of coronary blood flow. Circ Res 9 589-600... 

FEIGL But there is a shape change during isovolumic contraction —... 

Coulson RL (1976) Energetics of isovolumic contractions of the isolated rabbit heart. J Physiol London 260 45-53... 

---