In combination, the pressure, flow and resistance variables are used to help the clinician in evaluation of the haemodynamic state. The blood gas and saturation measurements can be used to calculate metabolic indices such as oxygen delivery and utilisation. Examples of these calculations can be examined by adjusting the controls on the 'Calculators' page of the simulator.

Right heart pressures.
The measurable right-sided pressures are the central venous pressure and the pulmonary artery pressure. During passage of the catheter, the right ventricular pressure can also be noted, but it cannot be measured once the PAC is correctly positioned because the catheter does not have a lumen in its intraventricular portion.

The normal CVP waveform contains three peaks ('A', 'C', 'V') and two descents ('X', 'Y'). The 'A' wave occurs at the end of ventricular diastole and is the result of atrial contraction. The aetiology of the 'C' wave is debatable. It was originally attributed to transmission of the systolic pressure wave from the adjacent carotid artery, although more recent opinion suggests that it is due to tricuspid valve closure. The 'V' wave occurs as a result of venous filling of the atrium during late ventricular systole. These waves are shown in Figure 1. The peak of the 'A' wave coincides with the point of maximal filling of the right ventricle and is therefore the value which should be used for measurement of RVEDP.

The right ventricular pressure waveform is seen transiently as the catheter is passed from the atrium to the pulmonary artery. The waveform is shown in Figure 2.

Pulmonary Artery Pressure.
The pulmonary artery pressure waveform is seen after the catheter is passed through the pulmonary valve into the pulmonary artery. It should be noted that the peak of the systolic pressure wave coincides with the 'T' wave of the ECG. This is in contrast to the 'V' wave of the PAOP trace which is timed after the 'T' wave. A typical PAP waveform is shown in Figure 3.

Pulmonary Artery Occlusion Pressure.
As outlined in the section entitled 'Basic concepts', the PAOP is thought to reflect the left ventricular end-diastolic pressure (LVEDP) (Figure 4). LVEDP determines, in part, the amount of 'stretch' or 'preload' on the ventricular myocardial fibres before systole commences. This is an important determinant of cardiac performance because as preload rises, so the contractile force of the heart increases ¹ - the 'Frank-Starling' effect (Figure 5).

A normal PAOP tracing is shown in Figure 6. Note that, like a central venous pressure trace, 'A' and 'V' waves are present, but that a 'C' wave is usually not present. As with the CVP waveform, the peak of the 'A' wave coincides with the point of maximal filling of the left ventricle and is therefore the value which should be used for measurement of LVEDP.

Pulmonary Capillary Pressure.
The effective hydrostatic pressure in the pulmonary capillaries is greater than the pulmonary artery occlusion pressure. It can be measured by various techniques ^{2, 3}, but is most easily estimated by the method of Cope et al ² (Figure 7). The measurement is of theoretical interest, but rarely of practical significance.

Coronary Perfusion Pressure.
Blood flow to the left ventricle occurs principally during diastole (Figure 8) and is largely determined by the driving pressure (diastolic pressure minus left ventricular diastolic pressure), the diastolic interval and the state of coronary vasodilation. If PAOP is taken as a measure of LVEDP, CPP can be calculated according to the equation:

CPP = DAP - PAOP

It should be noted that the coronary circulation has considerable powers of autoregulation, with the level of the autoregulatory plateau being determined by myocardial VO2 (Figure 9). Interested readers may wish to read a recent review by Bourdarias ⁴.

Vascular Resistances.
The resistance of the systemic and pulmonary circulations can be calculated using data derived from a PAC. The systemic vascular resistance (SVR) is a measure of LV afterload and is an important determinant of the performance of the left heart. It is calculated in metric units according to the equation:

SVR = (MAP - RAP * 79.9)/CO

The pulmonary vascular resistance (PVR) is a measure of RV afterload. It is calculated in metric units according to the equation:

PVR = (MPAP - PAOP * 79.9)/CO

It is inappropriate to use any of the indices of cardiac performance without relating them in some way to body size. The most appropriate measure of size is body surface area (BSA) and all the flow related variables should generally be related to this and reported as indices. Calculated vascular resistances can be converted to indices by multiplying the absolute value of resistance by the body surface area. The normal values for the derived variables are shown in Table 2.

Cardiac Output.
The cardiac output measurement that is made with a PAC is, in fact, a thermodilution measurement of blood flow in the pulmonary artery (Figure 10). Provided that there are no anatomical right-to-left or left-to-right shunts, this flow can be safely equated with the left ventricular cardiac output. Output is calculated using the Stewart-Hamilton equation (Table 3). Cardiac index is then calculated by dividing the cardiac output by the body surface area.

Cardiac Work.
The external work of the heart can be measured by calculating left ventricular stroke work according to the equation:

SW = (MAP - PAOP) *SV * 0.0136

Where SW = Stroke work in g.m and 0.0136 is the mm Hg.cm3 to g.m conversion factor.

Blood gas and saturation measurements.
Blood gas and saturation measurements can be performed on specimens obtained from the superior vena cava and / or pulmonary artery (true mixed venous). If arterial blood gases are also available, metabolic variables, such as oxygen delivery (Table 4) and utilisation (VO2) (Table 5) can be calculated and the ratio of the two used to define the overall supply:demand relationship of the patient. If the amount of oxygen dissolved in the plasma is ignored, the quantity of oxygen delivered to the tissues each minute ('Flux') can be quickly calculated according to the equation:

DO2 = CO * Hb * 1.306 * SaO2

Where Hb is the haemoglobin concentration in gm/L, 1.306 is quantity of oxygen that 1gm of haemoglobin can hold ⁵ and SaO2 is the fractional saturation of haemoglobin at the time of measurement.

The inability of patients to generate a substantial DO2 ( > 700 ml/min) has been identified as a predictor of perioperative mortality ⁶ and the ability to achieve 'supranormal' oxygen delivery in response to therapy with inotropes, vasodilators or volume loading has been identified as a predictor of reduced mortality in both ICU and trauma patients ^{7, 8}.

Oxygen utilisation is a measure of the metabolic rate and is calculated according to the equation:

VO2 = CO * (CaO2 - CvO2)

The data can also be used in the calculation of lung function indices such as Qs/Qt (also known as 'venous admixture' or 'shunt fraction') (Table 6 ). The 'Calculators' page of the simulator can be used to calculate all of the variables above for any given set of conditions.

Special function catheters.
Special pulmonary artery catheters are available which allow the continuous measurement of mixed venous saturation and cardiac output and right ventricular function. The technique for determining these parameters is described fully in the section entitled 'Cardiac Output Measurement'.

1. Frank O: Zur Dynamik des herzmuskels. Ztschr fur Biol 32:370, 1895

2. Cope DK, Allison RC and Taylor AE A Simple Method to Determine Pulmonary Capillary Pressure. Anesthesiology 1987; 67:866-868

3. Holloway H, Perry M, Downey J, Parker J, Taylor A Estimation of effective pulmonary capillary pressure in intact lungs. J Appl Physiol 1983 Mar;54(3):846-51

4. Bourdarias JP Coronary reserve: concept and physiological variations. Eur Heart J, 16 Suppl I:2-6, 1995

5. Gregory IC The oxygen and carbon monoxide capacities of foetal and adult blood. J. Physiol. 236:625 (1974)

6. Orlando R, Nelson LD, Civetta JM: Invasive preoperative evaluation of high-risk patients. Crit Care Med 13:263, 1985

7. Shoemaker WC, Kram HB, Appel PL, et al: The efficacy of central venous and pulmonary artery catheters and therapy based upon them in reducing mortality and morbidity. Arch Surg 125:1332-1338, 1990

8. Fleming A, Bishop M, Shoemaker W. et al: Prospective trial of supranormal values as goals of resuscitation in severe trauma. Arch Surg 127:1175-1181, 1992

Last edited on: 13/11/2000