**5. Afterload**

Afterload is defined as the resistance to LV ejection. Major components are SVR and AEDP. AEDP is the resistance the LV has to overcome in order to open the aortic valve. Ninety percent of myocardial oxygen consumption takes place during the isovolumetric

Intra-Aortic Balloon Counterpulsation Therapy

**7. Ventricular function** 

and Its Role in Optimizing Outcomes in Cardiac Surgery 47

In order for us to appreciate the need for the IAB, we need to understand ventricular function, end diastolic volume and the Fick equation. Ventricular function in critical care is assessed by the Ejection Fraction (EF). EF is defined as the volume of blood ejected per beat (stroke volume or SV) divided by the volume in the LV prior to ejection, end diastolic volume (EDV). Stroke

EF = EDV-ESV/EDV= SV/EDV Ventricular function is graded into four groups by two dimensional echocardiography

> **LV Grade Ejection Fraction**  Grade 1 LV ≥ 55% Grade 2 LV 45 to 54% Grade 3 LV 30 to 44% Grade 4 LV ≤ 30%

**Diastolic volume index**: EDV indexed to the patients body surface area (BSA) is another measure of ventricular performance. A normal index is considered less than 100ml/m². In patients with regurgitant valves and volume overload, the end diastolic volume index

O2 consumption= C.O X O2 content difference (sao2-svo2) C.O = O2 consumption/O2 content difference (sao2-svo2) Therefore CO is inversely proportional to O2 content difference, assuming that O2 consumption remains constant. A normal value for arterio-venous oxygen difference would be less than 5 ml/dl or 50ml/L. A low cardiac output state would encourage increased oxygen extraction, resulting in widening of arterio-venous o2 content of greater than

Increase in the frequency of contraction increases C.O at a given filling pressure. This is called the staircase (Bowditch) effect. Reduction in diastolic time can result in reduction of ventricular filling, thus limiting C.O increase with tachycardia. An important corollary to remember is that the filling of coronary arteries takes place in diastole and tachycardia can

The major vessels of the coronary circulation are the left main coronary that divides into left anterior descending and circumflex branches, and the right main coronary artery. The left and right coronary arteries originate at the base of the aorta from openings called the

The myocardium is perfused during diastole through the coronary system (fig 6).

**The Fick Equation**: Defines the relationship between C.O and oxygen extraction.

vol equals EDV minus end systolic volume (ESV). The equation for EF would then be:

according to ejection fraction as shown in fig 5 (*Conolly HM & Kohl J 2012*).

Fig. 5. LV Grade and Corresponding Ejection Fraction

(EDVI) may be high despite preserved LV function.

5ml/dl*. (Hensley et al.,1995 )*

compromise diastolic coronary filling.

**9. Coronary circulation and anatomy** 

coronary ostia located just distal to the aortic valve leaflets.

**8. Heart rate** 

Fig. 3. Starling Law

contraction phase when the LV is trying to overcome the resistance caused by the AEDP in order to open the Aortic valve. The aim of counterpulsation is to lower AEDP, thereby reducing afterload and myocardial oxygen consumption during isovolumetric contraction (fig 4).

Fig. 4. Myocardial Oxygen Supply and Demand
