PULMONARY ARTERY PRESSURE

Transcription

1 6 INTENSIVE CARE Stroke volume x y RAP LAP LAP Atrial pressure Fig. 4.8 Left (LV) and right (RV) ventricular function curves in a patient with left ventricular dysfunction. Since the stroke volume of the two ventricles must be the same (except perhaps for a few beats during a period of circulatory adjustment), left atrial pressure (LAP) must be higher than right atrial pressure (RAP). Moreover, an increase in stroke volume (x) produced by intravascular volume expansion will be associated with a small rise in RAP (y), but a marked increase in LAP (z). but may occur in situations in which pulmonary vascular resistance (i.e. right ventricular afterload) is raised, such as acute respiratory failure and pulmonary hypertension, as well as in those with right ventricular ischaemia. These discrepancies between right and left ventricular performance can be exacerbated by the use of inotropic and vasoactive drugs. If there is a disparity in ventricular function after cardiac surgery the left atrium can be cannulated directly, but if the thorax is not open, some other means of determining left ventricular filling pressure is required. PULMONARY ARTERY PRESSURE In 193 pulmonary artery catheterization with a balloontipped, flow-directed catheter was pioneered in the animal laboratory (Lategola and Rahn, 193). Seventeen years later Swan and colleagues described the use of a similar catheter in humans (Swan et al., 197). This balloon flotation catheter allowed prompt and reliable catheterization of the pulmonary artery without the need for screening, and minimized the incidence of arrhythmias. Later, a catheter with a slightly increased diameter, a larger balloon, a proximal lumen for CVP measurement and a thermistor located near the tip was introduced, the thermistor allowing determination of cardiac output by the thermodilution technique (see below) (Forrester et al., 1972). Balloon flotation catheters have since been modified for other purposes, including cardiac pacing, pulmonary angiography, continuous monitoring of mixed venous oxygen saturation and determination of right ventricular ejection fraction (Zink et al., 24). More recently methods have been developed for the continuous measurement of cardiac output (see later). z RV LV Approaches Pulmonary artery catheters can be inserted centrally, through the femoral vein or via a vein in the antecubital fossa. The latter route is perhaps the most comfortable, but it may be difficult to advance the catheter beyond the shoulder region. Furthermore secure fixation is not easily achieved and involves some degree of immobilization of the arm. On the other hand, the complication rate, particularly the risk of pneumothorax, is reduced. The left infraclavicular approach to the subclavian vein conforms most closely to the natural curvature of the catheter, and secure fixation is more easily achieved at this site. Catheterization of the right internal jugular vein is, however, safer and is the shortest and most direct route to the right side of the heart. The procedure Because pulmonary artery catheters have to be introduced through a wide-bore cannula, a guidewire technique is used (see Fig. 4.4). First an incision is made in the anaesthetized skin and the vein is punctured with a needle or a standard intravenous cannula. A guidewire with a flexible, J-shaped tip is then introduced and the cannula or needle is removed. A tapered vein dilator is passed over the guidewire to ease subsequent passage of the cannula. The dilator carrying the wide-bore cannula is inserted over the guidewire, which is then withdrawn. (If the original skin incision is not sufficiently large and deep, pushing the dilator and cannula through the skin and subcutaneous tissues may prove difficult.) The dilator is then removed and the pulmonary artery catheter passed through the introducer into the vein. The introducer incorporates a valve mechanism, which prevents air embolism and spillage of blood after the dilator is removed and during insertion of the catheter. This introducer cannula should be left in situ and provides an additional central venous access point. A plastic sleeve is provided with most introducer kits, which protects a length of catheter, thereby maintaining its sterility. This can subsequently be manipulated if the catheter becomes misplaced, without risking contamination. PRECAUTIONS Before the pulmonary artery catheter is inserted: The balloon should be inflated with the recommended volume of air to check for leaks and to ensure that inflation is symmetrical. Confirm that the thermistor is functioning. The oximeter, if present, should be calibrated. The various lumens should be flushed with heparinized saline. The technique must be learnt under supervision because complications are inversely related to operator experience.

2 Assessment and monitoring of cardiovascular function 61 a 2 1 Right atrium b 2 1 Right ventricle c 2 1 Pulmonary artery d 2 1 Pulmonary artery occlusion pressure Fig. 4.9 Pressure waveforms as a pulmonary artery catheter is passed through the chambers of the heart into the wedge position. (a) Once in the thorax, respiratory oscillations are seen. The catheter should be advanced further towards the lower superior vena cava/right atrium when oscillations become more pronounced (1 2 cm of catheter inserted). The balloon should then be inflated and the catheter advanced. (b) In the right ventricle (2 3 cm) there is no dicrotic notch and the diastolic pressure is close to zero. The patient should be returned to the horizontal, or slight head-up, position before advancing the catheter further. (c) In the pulmonary artery (3 cm) a dicrotic notch appears and there is elevation of the diastolic pressure. The catheter should be advanced further with the balloon inflated. (d) Reappearance of a venous waveform indicates that the catheter is wedged. Stop advancing. The balloon should be deflated to obtain pulmonary artery pressure, and then inflated intermittently to obtain pulmonary artery wedge or occlusion pressure. INTRODUCING THE CATHETER Passage of the catheter from the major veins, through the chambers of the heart into the pulmonary artery and the wedge position, is monitored and guided by the pressure waveforms recorded from the distal lumen (Fig. 4.9). The catheter should not be advanced too rapidly since redundant loops may form in the right atrium or ventricle, with a risk of knotting. Radiographic control must be used if any difficulty is encountered and is most often required in those with a low cardiac output and/or a large heart. A chest radiograph should always be obtained to check the position of the catheter; the tip should be within 2 cm of the cardiac silhouette (Fig. 4.). Some recommend a lateral chest radiograph taken when the patient is supine to ensure that the tip of the catheter is in a posterior vessel. PRESSURE MEASUREMENTS Once in position, the balloon is deflated and pulmonary artery systolic pressure, end-diastolic pressure (PAEDP) and mean pulmonary artery pressure (PAP) can be obtained. The balloon is then inflated intermittently with the recommended volume of air (.8 1. ml), thereby propelling the catheter distally where it will impact in a medium-sized pulmonary artery and record pulmonary artery occlusion pressure (PAOP) (see Fig. 4.11). If a PAOP is obtained when the balloon is inflated with less than.8 ml of air, the catheter Fig. 4. Pulmonary artery flotation catheter correctly positioned in a patient with acute respiratory distress syndrome.

3 62 INTENSIVE CARE Balloon inflated Thermistor Distal lumen Ao PA Static blood LA Capillary bed RA Proximal lumen RV LV Pulmonary vein Fig Balloon flotation pulmonary artery catheter in the wedged position. There is now a continuous column of static blood between the tip of the catheter and the left atrium; pulmonary artery occlusion pressure (PAOP) is therefore usually closely related to left atrial pressure. LA, left atrium; LV, left ventricle; PA, pulmonary artery; RA, right atrium; RV, right ventricle. should be withdrawn a few centimetres to reduce the risk of pulmonary artery rupture or infarction (see later). All intravascular pressures should be measured relative to atmospheric pressure and should therefore be obtained at end-expiration. The digital pressure display does not provide an accurate end-expiratory value; occlusion pressure should therefore be determined from a continuous recording of the waveform using a cursor and inbuilt algorithms. In patients receiving a positive end-expiratory pressure (PEEP) (see Chapter 7), the recorded pressure will be incremented by an amount proportional to the level of PEEP. This effect is, however, difficult to quantify in the individual patient and depends on a number of factors, including lung compliance. PEEP should not be discontinued while obtaining pressure measurements because this alters the haemodynamic conditions unpredictably, there may be a sudden increase in venous return and there is usually a significant fall in P a O 2. It is important to remember that in obstructive airways disease there may be an intrinsic PEEP (see Chapter 8). INTERPRETATION When the pulmonary artery catheter is in the wedge position, flow ceases in the isolated segment of pulmonary vasculature and the pressure in the occluded vessels equilibrates with pulmonary venous pressure (Fig. 4.11). PAOP is therefore usually closely related to LAP. The measurement of PAOP is, however, prone to errors and misinterpretation. It is clearly essential to establish that a genuine PAOP reading has been obtained. When the catheter wedges : a relatively damped venous waveform should appear; respiratory oscillations should be apparent; the PAOP should be a few mmhg less than PAEDP; some check that it is possible to withdraw arterialized blood, although as much as 2 4 ml of blood may have to be aspirated to obtain a fully oxygenated sample. Occasionally, when the balloon is inflated, a PAOP trace is obtained only intermittently. This transitional waveform can occur when the fluctuations in PAP cause the catheter to wedge and unwedge in a branch of the pulmonary artery. The recorded PAOP may be higher than PAEDP if: the balloon is overinflated; the balloon inflates eccentrically and/or the catheter tip abuts the vessel wall; the waveform is abnormal (e.g. large V waves in mitral regurgitation or cannon waves in complete heart block). Once a genuine PAOP has been obtained, a number of potential causes of misinterpretation remain. The assumptions are that: PAOP = LAP = LVEDP = LVEDV

4 Assessment and monitoring of cardiovascular function 63 As discussed above, the relationship between LVEDP and LVEDV depends on the compliance of the ventricles, which is altered in many critically ill patients. Furthermore, LAP may not accurately reflect LVEDP in the presence of mitral valve disease, left atrial myxoma or severe left ventricular dysfunction. Finally, PAOP will only be equivalent to LAP when there is a continuous column of blood between the catheter tip and the left atrium (i.e. West s zone 3 conditions prevail; see Chapter 3). This will not be the case when the intervening pulmonary vessels are collapsed by intra-alveolar pressures that are higher than pulmonary venous pressure. This can occur in ventilated patients requiring high inflation pressures or extrinsic PEEP, and in those with airway obstruction, particularly if they are hypovolaemic and/or the catheter is in the upper zones of the lungs (West s zones 1 and 2; see Chapter 3). Under these circumstances the pressure recorded is a reflection of intra-alveolar, rather than left atrial, pressure. Non-zone 3 conditions are suggested: by the absence of normal cardiac oscillations in the PAOP trace; when PAOP exceeds PAEDP; if, during the application of increasing levels of PEEP, PAOP increases by more than half the PEEP increment. It has also been suggested that in sepsis Starling resistor effects in the pulmonary venous system may lead to a dissociation between PAOP and LAP, with PAOP tending to overestimate left ventricular filling pressures (Fang et al., 1996). In the supine patient a lateral chest radiograph taken with the catheter wedged can determine whether the tip is correctly positioned at or below left atrial level. Fortunately, because the catheter enters the right ventricle anteriorly through the tricuspid valve and leaves it to enter the pulmonary artery in a posterior direction, the curve on the catheter usually causes it to enter a posterior branch of the pulmonary artery supplying the right lower lobe. Also blood flow is greatest in zone 3, increasing the likelihood of the catheter floating into this region of the lung. The PAOP may also overestimate LVEDP if there is a tachycardia because premature closure of the mitral valve will increase the atrioventricular pressure gradient and there is limited time for equilibration between PAOP and LAP. If it proves impossible to obtain a satisfactory PAOP (e.g. if the balloon ruptures), it has been suggested that PAEDP can provide a reasonable guide to LAP. There is, however, normally a gradient of 1 3 mmhg between PAEDP and PAOP, and this is increased in the presence of pulmonary hypertension, in those with tachycardias and during rapid transfusion (Lappas et al., 1973). In general, PAEDP is an unreliable guide to left ventricular filling pressures in the critically ill. It must be emphasized that, as with CVP measurement, the response of PAEDP and PAOP to a fluid challenge is often of more significance than the absolute value. Also, as Table 4.2 Complications of pulmonary artery catheterization Haemorrhage/haematoma Pneumothorax Arrhythmias (during passage of catheter through the right ventricle, usually benign, can often be prevented with lidocaine) Sepsis (at insertion site, bacteraemia) Endocarditis (seldom recognized clinically) Knotting (catheter coils in right ventricle) Valve trauma (catheter withdrawn with balloon inflated, valves repeatedly closing on catheter) Thrombosis/embolism Pulmonary infarction (embolism or catheter remains in wedge position) Pulmonary artery rupture (frequently fatal) Balloon rupture/leak/embolism mentioned earlier in this chapter, it is important to appreciate that in general PAOP and CVP are rather poor guides to ventricular filling, intravascular volume, cardiac performance and the response to volume infusion, even in normal subjects (Kumar et al., 24). COMPLICATIONS There are a large number of complications associated with the use of pulmonary artery catheters, some of which may be extremely serious and even fatal (Table 4.2). Arrhythmias. These are more common when the larger thermodilution catheters are used and there is a higher incidence in patients with electrolyte disorders, acidosis or myocardial ischaemia. It is important to inflate the balloon to the recommended volume to conceal the tip of the catheter and prevent it irritating the endocardium during its passage through the right ventricle. Although fatal ventricular tachycardia has been reported (Sise et al., 1981), these arrhythmias are usually transient and consist of a few benign ventricular premature contractions, which stop as soon as the catheter enters the pulmonary artery. If troublesome, they can usually be suppressed with intravenous lidocaine, although in some very unstable patients it may be safer to abandon the procedure. Transient right bundle branch block occurs in up to % of patients, but usually resolves within 24 hours. In the presence of pre-existing left bundle branch block, this may precipitate complete atrioventricular block or asystole. Knotting. To avoid knotting, which is more common in those with enlarged cardiac chambers and in low-flow states, the catheter should not be advanced by more than 3 cm without observing a change in waveform (see Fig. 4.9).

5 64 INTENSIVE CARE Heart valve damage. Heart valves can be severely damaged if the operator withdraws the catheter without deflating the balloon. In the longer term, valve cusps can be progressively traumatized by repeated closure against the catheter. Pulmonary infarction. This may be related to thrombus formation in and around the catheter, but will also occur if the catheter remains in the wedge position for any length of time. The latter can be avoided by continuously displaying the PAP so that the spontaneous appearance of a wedge pressure (caused by softening and migration of the catheter) can be detected and remedied immediately. It is also important to minimize the length of time for which the catheter is intentionally wedged. Pulmonary artery rupture. Although rare, pulmonary artery rupture may be fatal due to intractable haemorrhage. This complication appears to be commoner in the elderly, particularly in those with pulmonary hypertension, and in those receiving anticoagulants. Pulmonary artery rupture is usually due to either continuous impaction of the catheter tip with erosion of the vessel wall, or rapid inflation of a distally placed balloon. It may also occur if the catheter is advanced with the balloon deflated or if a wedged catheter is flushed by hand. Management may include reversal of anticoagulation, application of high levels of PEEP, selective endobronchial intubation, transcatheter embolization and early surgical intervention, perhaps involving pulmonary resection. Mortality has been reported to be between 2 and 83%. Infection. Infections are more common with internal jugular placement, if the catheter remains in place for more than 4 days and when a catheter is reinserted at an old site. Infection of catheter-associated thrombosis can produce systemic sepsis in the absence of local inflammation. Infective endocarditis attributable to pulmonary artery catheterization is a rare event. Overview of the complications. Despite this rather formidable list of potential hazards, in practice haemodynamic monitoring using pulmonary artery catheters has an acceptably low morbidity and mortality (Sise et al., 1981). The majority of complications are closely related to user inexperience and their incidence has fallen as worldwide expertise has increased. There is now some concern, however, that the recent reduction in the use of pulmonary artery catheters may be associated with an increase in the incidence of complications due to lack of user experience. BLOOD VOLUME AND LUNG WATER MEASUREMENTS (Hudson and Beale, 2) COLD (circulation, oxygenation, lung water and liver function diagnosis) technique Cannulae are placed in the femoral, brachial or axillary artery, the pulmonary artery and/or a central vein. Thermodilution cardiac output measurements can be made by injecting a bolus of cold fluid into a central vein and recording from a thermistor incorporated into the tip of the arterial catheter. Such transpulmonary thermodilution measurements tend to overestimate cardiac output because of loss of indicator into the lungs. They are, however, more repeatable than the conventional method because the normal respiratory variation in stroke volume is integrated. The arterial catheter also has an optional oximetry probe for measurement of S a O 2 and a similar catheter can be placed in the pulmonary artery for monitoring S v O 2 and measuring cardiac output by thermodilution. A fibreoptic reflectance densitometer built into the arterial catheter measures changes in indocyanine green concentration following bolus injection, allowing an assessment of liver function based on the rate of disappearance of the dye. Since indocyanine green is bound to albumin and is therefore retained in the intravascular compartment, whereas cold distributes to the extravascular space, extravascular lung water (EVLW) and intrathoracic blood volume (ITBV) can be determined by the double-indicator technique. Global end-diastolic volume (i.e. the sum of the diastolic volumes of both left and right atria and ventricles) can also be estimated. Changes in global end-diastolic volume and ITBV correlate well with changes in cardiac output. This device is no longer available commercially. Pulse-induced continuous cardiac output (PiCCO) technique This more recent development allows measurement of EVLW and ITBV, as well as calculation of global end-diastolic volume, by analysing only the transpulmonary thermodilution curve recorded from a thermistor-tipped cannula sited in the femoral or brachial artery. The device also provides continuous estimates of cardiac output by pulse contour analysis (see below), with automatic recalibration being performed by intermittent thermodilution. The device is relatively non-invasive and can be used in conscious patients. Given the well-recognized limitations of pulmonary artery catheterization discussed elsewhere in this chapter, the concept of using direct estimates of cardiac filling and lung water to guide volume replacement and the administration of inotropes and vasoactive agents is undoubtedly attractive. In the light of experience with pulmonary artery catheters, however (see below), some are sceptical about the potential of these monitoring devices to improve outcome. Further studies are clearly required to establish the precise indications for these techniques and their influence on clinically relevant outcome measures. CARDIAC OUTPUT AND MYOCARDIAL FUNCTION As discussed in Chapter 3, cardiac output is a major determinant of oxygen delivery and is therefore one of the most clinically relevant haemodynamic variables. NON-INVASIVE TECHNIQUES FOR ASSESSING CARDIAC FUNCTION Over the years, there have been many attempts to develop clinically viable, non-invasive techniques for determining

6 References pp 6-64 Fang K, Krahmer RL, Rypins EB, et al. (1996) Starling resistor effects on pulmonary artery occlusion pressure in endotoxin shock provide inaccuracies in left ventricular compliance assessments. Critical Care Medicine 24: Zink W, Nöll J, Rauch H, et al. (24) Continuous assessment of right ventricular ejection fraction: new pulmonary artery catheter versus transoesophageal echocardiography. Anaesthesia 9: Forrester JS, Ganz W, Diamond G, et al. (1972) Thermodilution cardiac output determination with a single flow-directed catheter. American Heart Journal 83: Kumar A, Ariel R, Bunnell E, et al. (24) Pulmonary artery occlusion pressure and central venous pressure fail to predict ventricular filling volume, cardiac performance, or the response to volume infusion in normal subjects. Critical Care Medicine 32: Extracts 28 Elsevier Limited. All rights reserved. Lappas D, Lell WA, Gabel JC, et al. (1973) Indirect measurement of left-atrial pressure in surgical patients pulmonary capillary wedge and pulmonary artery diastolic pressures compared with leftatrial pressure. Anesthesiology 38: Lategola M, Rahn H (193) A self-guiding catheter for cardiac and pulmonary arterial catheterization and occlusion. Proceedings of the Society for Experimental Biology and Medicine 84: Sise MJ, Hollingsworth P, Brimm JE, et al. (1981) Complications of the flow-directed pulmonary artery catheter: a prospective analysis in 219 patients. Critical Care Medicine 9: Swan HJC, Ganz W, Forrester J, et al. (197) Catheterization of the heart in man with use of a flow-directed balloon-tipped catheter. New England Journal of Medicine 283: