Chapter 3

    After an extensive examination of the patient, review of history, electrocardiogram and chest roentgenograms, cardiac catheterization and selective angiography are necessary many times for accurate structural and hemodynamic analysis of the patient.
    The hemodynamic data will be collected via right and or left heart catheterization and in few cases via transseptal catheterization.
    The basic data is given by: pressure measurements, pressure curve analysis, blood oxygen determination at different heart levels and catheter position.
    After that basic information is collected, cardiac output, vascular resistances, valve orifice area and shunts can be calculated if necessary.
    For measurement of the cardiac output, the formula of Adolph Fick is more than accurate in most cases, given the values of total oxygen consumption, arterial blood oxygen content, and mixed venous blood content from pulmonary artery catheterization.
    For the evaluation of valve orifice area the Gorling and Gorling formula is probably the most accurate, given the volume rate of blood flow across the orifice and the pressure gradient across the valve orifice.
    For the calculation of the blood flow, the cardiac output and the diastolic filling period of systolic ejection fractions are necessary.
    Shunt calculation can be performed too, utilizing the Fick principle which is useful providing a quantitative index which is particularly important in assessing surgical possibilities. However, it should be underlined that none of the existing formulas for shunt calculations (Cournand, Bing, Bexter) are totally accurate.
    Indicator-dilution techniques for evaluation of cardiac output and quantitative shunt evaluation correlates well with those from the Fick's method, but a further analysis of its technique is beyond the scope of this presentation.

FICK OXYGEN METHOD FOR CALCULATION OF CARDIAC OUTPUT

CARDIAC OUTPUT (CO) =

Blood oxygen carrying capacity = Hb(gm %) x 1.34 (ml O₂/gm of Hb) x 10 = ml O₂/L blood

Oxygen content of venous or arterial blood = oxygen carrying capacity X % of saturation

Peripheral artery oxygen content = pulmonary venous content (if no intracardiac shunt is present)

CALCULATION OF THE DIFFERENT TYPES OF VASCULAR RESISTANCE

MPAp: mean pulmonary artery pressure
MLAp: mean left atrium pressure
PF: pulmonary blood flow

SAp: systemic arterial pressure
MRAp: mean right atrium pressure
SF: systemic blood flow

NORMAL VALUES FOR VASCULAR RESISTANCE

Pulmonary arteriolar resistance:

Total pulmonary resistance:

Systemic vascular resistance:

    The pulmonary vascular resistance does not necessarily correlate with left atrial pressure or pulmonary wedge pressure. With a high left atrial pressure (case of mitral stenosis, or Congestive Heart Failure, etc.) the P.V.R. can be elevated or be normal and at times be entirely out of proportion to the given left atrial pressure.
    An increase in the P.V.R. will protect the patient from pulmonary edema, but will favor the development of right ventricular failure. The right ventricle is a volume adapted chamber and the pulmonary vasculature in normal circumstances can manage large changes in volume of circulation without significant pressure variation in the pulmonary system). So any significant increase in the pulmonary vascular resistance may easily determine failure of the right ventricle. The right ventricle will easily adjust to changes in blood volumes but will easily fail in front to pressure changes.

GENERAL FORMULA IN STENOTIC VALVES:

A: area
F: volume rate of blood flow across orifice in ml/sec
C: orifice constant (different in each valve)
P₁ - P₂: pressure gradient across valve orifice

FORMULA TO CALCULATE FLOW:

CO: cardiac output
DFP: diastolic filling period
SEP: systolic ejection period or fraction

FORMULA TO CALCULATE MITRAL STENOSIS:

MVA: mitral valve area
LAP: left atrium
LVdMP: left ventricular diastolic mean pressure.

FORMULA TO CALCULATE AORTIC STENOSIS:

AVA: aortic valve area
LVsMP: left ventricular systolic mean pressure
AOsMP: aortic systolic mean pressure.

FORMULA TO CALCULATE PULMONARY VALVE STENOSIS:

PVA: pulmonary valve area
RVsMP: right ventricular systolic mean pressure
MPAsMP: mean pulmonary artery systolic mean pressure

CRITERIA FOR A SIGNIFICANT "STEP-UP" IN OXYGEN CONTENT:

CALCULATION OF LEFT TO RIGHT SHUNTS:

CALCULATION OF BIDIRECTIONAL SHUNTS:

    In case of impossibility of entering pulmonary vein, 98% X O₂ capacity can be used, as an equivalent figure. The use of standard indicator dilution curves permit a much more accurate detection and calculation of shunts (Indocyanine green, Ascorbic acid, Nitrous oxide, Krypton 85, Hydrogen, Freon).

EVALUATION OF VALVE STENOSIS BY CARDIAC CATHETERIZATION

	Normal	Symptoms Below	Critical Area
Mitral valve area	3.5-6.5 cm2	2.0 cm2	1 cm2
Aortic valve area	2.6-3.8 cm2	1.5 cm2	0.5 cm
Pulmonary valve area	2.5-3.8 cm2	1.8 cm2	0.5 cm
Tricuspid valve area	9-11 cm2	3 cm	?

    The assessment of the valve stenosis severity only by pressure gradient is not valid, unless correlated to blood flow.
    In this regard the Gorling and Gorling formula considers the different factors in proper proportion.

EVALUATION OF MITRAL VALVE AREA

Three different variables are to be taken into proper account.

1. Mitral valve gradient (MVG)
2. Cardiac output
3. Diastolic filling period (summation to diastolic times of minute)

where:
MVF: Mitral valve flow
MVG: Mitral valve gradient

    The gradient alone can be entirely misleading in assessing the Mitral area. For example, a gradient of 28 mm Hg can be seen in a valve of 2.2 cm² if the MVF is 320 ml/sec. The same gradient can be noted in a different patient with a critical Mitral stenosis of 1.0 cm² if MVF is 170 ml/sec.
    Never decide on surgical treatment of mitral valve stenosis only on the basis of valve gradient.
    The larger the flow through a given stenotic valve the larger the gradient.
    Severe mitral or aortic stenosis in a patient with low cardiac output will have probably only a moderate gradient.
    Moderate stenosis in a patient with a high cardiac output can give rise to large valve gradients, giving the false sensation of a grave and probable surgical lesion.
    A patient with minimal mitral stenosis may have a gradient for example of 4 mm of Hg if his heart rate is 100 per minute and the same person can exhibit a gradient of 15 mm of Hg a few minutes after if his heart rate drops to 50 per minute.
    To calculate mitral valve area proceed as follows:

1. Determine mitral valve gradient, recording if possibly simultaneously pulmonary wedge pressure and with a second catheter left ventricular pressure.

    Calculate the gradient if possible by planimetry or determining the value of three different gradients during the same cycle calculating the average, then do the same with several cycles and determine an average value.

2. Determine cardiac output by the Fick Principle or any other of the current methods.
3. Determine the Diastolic filling period in case of Mitral Stenosis or Systolic ejection period in case of Aortic Stenosis.

EVALUATION OF THE AORTIC VALVE AREA

    As in the case of Mitral Stenosis, the sole consideration of the Aortic gradient can be entirely misleading and lead to erroneous assessment of the stenosis with grave consequences in the patient management.

    The main hemodynamic consequence, regardless of the anatomical type of Aortic Stenosis, is the systolic pressure overloading of the left ventricle.
    The left ventricle will increase its work attempting to maintain the cardiac output and the stroke volume.
    Left ventricular compliance is diminished and left ventricular hypertrophy supervenes. Left ventricular oxygen consumption increases in relation to the new levels of wall tension and oxygen supply determining the typical electrocardiographic changes of ischemia or left ventricular systolic overload seen in severe Aortic Stenosis.

where:
AVF: Aortic valve flow
AVG: Aortic valve gradient

    Similarly, the valve area can be calculated in Pulmonary and Tricuspid valves.

Simultaneous pressure recording from the left ventricle and aorta showing a large systolic gradient across the aortic valve which is represented in the picture by the shaded area.

Simultaneous pressure recording from the left ventricle and left atrium, showing diastolic gradient across the mitral valve. The shaded area, which represents the gradient, can be easily calculated by planimetry.