CardioVascular Indices

a) cardiac output & index

b) systemic vascular resistance

c) pulmonary vascular resistance

d) pulmonary capillary wedge pressure

a) Cardiac Output Normal resting CO: 4 — 8 litres/min

CO = HR x SV

Heart rate: influenced by:

1) Sympathetic nervous system (direct &/or indirect via circulating catecholamines)—> increased HR

2) Parasympathetic nervous system —> decreased HR

3) Distension of right atrium 2° overfilling —> increased HR

Stroke volume: determined by:

1) Preload [ventricular end diastolic pressure]; influenced by: duration of diastole, diastolic filling pressure, total blood volume, distribution of blood volume, atrial kick.

2) Afterload [SVR;PVR]: is determined primarily by the arterioles. Is increased by aortic stenosis, increased blood viscosity.

3) Contractility: ‘the ability of the myocardium to contract effectively’. Is increased by sympathetic stimulation, increased temperature & hypercalcemia. Is reduced by hypoxemia, hypercapnia, metabolic acidosis, infarction, ischaemia, hyperkalemia & hyponatremia.

Cardiac index Normal CI: 2.7 —4.3 litres/min/m²

Used to normalise cardiac output data for comparison of data between patients.

CI 1.8 - 2.2 l/min/m2 indicate clinical hypoperfusion

CI < 1.8 l/min/m2 indicate cardiogenic shock

b) Systemic Vascular Resistance Normal SVR: 1000 — 1300 dynes/sec/cm⁵

dynes/sec/cm⁵ [absolute resistance units]

A measure of peripheral blood vessel resistance to blood flow ∫ afterload

Hypovolemia —> increased; sepsis —> decreased

c) Pulmonary Vascular Resistance Normal PVR: 150 — 250 dynes/sec/cm⁵

dynes/sec/cm⁵ [absolute resistance units]

PVR is one sixth SVR.

Pulmonary hypertension, hypoxia, PE —> increased PVR

d) Pulmonary Artery Wedge Pressure 4— 12 mmHg

The LVEDP is a major determinant of left ventricle function. PAD gives an indication of LVEDP as during diastole there is no obstruction between the pulmonary artery and the left ventricle since the mitral valve is open. However in lung disease the PAD exceeds LVEDP due to raised PVR. Thus require to block the effects of pressure emanating from the right side of the heart —> use an occlusive balloon.

The PAWP reflects left atrial pressure and has similar contour and characteristics as the right atrial pressure.

Note that the PAWP exceeds LVEDP in: mitral valve dx, ventilator, pulmonary venous obstruction, left atrial myxoma.