The quantitative determination of hemodynamic function is crucial for managing critically ill patients. In this group, isolated clinical observation is frequently subjective and has a poor correlation with measured hemodynamic parameters. It is documented that even senior cardiologists are unable to predict weather a pulmonary capillary pressure ( PCP ) is greater or less than 18mmHg in 68% of the time. Futhermore, 30% of patients with a PCP > 18 do not have audible pulmonary rales, and 35% of those who do actually have a PCP < 18. Clearly a better method of evaluation is needed, as critically ill patients are very dynamic, changing volume status and filling pressures very quickly, so that a precise knowledge of their hemodynamic status at every minute does really help in managment.
The pulmonary artery ( PA or Swan-Ganz catheter ) flow directed catheter has provided intensivists with the ability to measure, at the bedside, pressures and flows within the cardiopulmonary system, and gain insights into complex physiologic problems. When used correctly, this device allows us to characterize the fluid status of patients with confusing clinical pictures, classify shock syndromes in hypotensive patients, evaluate responses to therapeutic manipulations of pressors and afterload reduction in patients with left ventricular dysfunction, and assess oxygen delivery in patients with sepsis syndrome. The key to proper use of the catheter lies in understanding how it works, what it really measures, how to interpret the information obtained, and how to avoid and recognize at an early stage potential complications associated with its application.
In this discussion, we will focus on topics related to PA catheter theory ( how does it work ), PA catheter in practice ( how and when to use ) and the correct interpretation of data as far as shock syndromes are concerned.
The device is actually a catheter inserted through a central vein ( preference for right internal jugular and left subclavian, which give the most direct access to the right heart ) that goes all the way to the a branch of the main pulmonary artery, passing by the right atrium ( RA ) and right ventricle ( RV ). It has two extremeties, a proximal one that is located in the RA and a distal one which has a balloon on its tip, located in a pulmonary artery branch. This balloon allows the catheter to be flow directed when it is inflated with air because of the generation of a drag force which pulls the catheter in the direction of flow, making its way to the pulmonary artery instead of going to peripheral veins, as blood in veins flow in that direction, in a manner similar to a small boat carried by the wind in the sea. Both the extremities have the ability to measure the pressures of the chambers they are into when connected to a recording system, so that we can document directly what's going on inside the cardiopulmonary system. Also, we can inject fluid in the proximal extremity and collect blood from the distal extremity, allowing respectively to calculate cardiac output ( by a thermodilution method in which a termistor located in the distal extremity measures the change in the temperature of the fluid that we have injected in the RA ) and make gas analysis of blood from the pulmonary artery ( mixed venous blood, which is the mixture of venous blood coming from the whole body to be oxygenated in the lungs ).
With the balloon inflated, the tip of the catheter is unexposed, and thus cannot injure the endothelium. The balloon will be pushed by the flowing blood until it reaches a pulmonary vessel of the same approximate diameter ( 1cm ). At that point, the edges of the balloon touch the vessel walls and the catheter becomes " wedged ". In this position, called the pulmonary artery occlusion pressure ( PAOP, because the artery is occluded by the balloon ), the most distal part of the catheter provides access to pressures on the left side of the heart. This is because at this point a sealed comunication is created between the catheter tip and the left atrium by the pulmonary capillaries and veins, so that it allow us to have an indirect access to the left side of the heart by a right sided catheterization. Importantly, we should never leave a balloon inflated permanently, which carries the risk of causing a pulmonary infarction as it occludes blood flow. The balloon should be inflated, accordingly, only during placement and when measuring PAOP, which takes only some seconds, leaving it deflated the rest of the time the catheter is in place and when withdrawing it.
Its sometimes said that a PA catheter provides an assessment of left ventricular preload by means of the PAOP. This is not truly correct. Preload is actually the volume which fills the left ventricle at end diastole, passively stretching the muscle fibers. The PA catheter measures pressures, not volumes. More specifically, with the balloon inflated and properly positioned, it measures pressures at a point which approximates ( is slightly greater than ) the mean pressure in the left atrium ( called the J point, which is in the bifurcation of a pulmonary vein following the occluded arterial branch ), which in turn approximates the left ventricular end diastolic filling pressure. So, if there is a significant pressure drop across the mitral valve, such as in mitral stenosis, mean left atrial pressure is a poor approximation of left ventricular end diastolic pressure. Thus, while the PAOP provides an index of preload, it does so in an indirect fashion, as LV pressure is related to LV volume through the compliance curve ( diastolic P-V filling curve ). These concepts are summarized below :
Another important concept is that PAOP reflects an approximation of the hidrostatic pressure in the pulmonary capillaries ( Pcap ), which is one of the driving forces that push fluid from the pulmonary circulation into the pulmonary intersticium and alveoli, thus causing pulmonary edema. Usually, pulmonary edema develops when the Pcap is > 25 torr. The relationship between PAOP and Pcap is expressed in the following formula :
Pcap = PAOP + 0.4 ( PAP mean - PAOP )
where PAP = pulmonary artery mean pressure
If we are to get an accurate measurement of PAOP, we have to remember the pulmonary West zones, in which the lung is physiologically divided based on gravitational differences in ventilation and perfusion. In zone I, alveolar pressure ( Palv ) is greater than pressure in the pulmonary artery branch ( Pa ) and in the pulmonary vein that follows it ( Pv ). In zone II, Palv > Pv, but is less than Pa. In zone III, blood flow is uninterrupted, allowing free comunication between the catheter tip and the distal vascular pressures, as Pa > Palv and Pv. This is the ideal zone to place a PA catheter, as if it is in zones I or II, pressures recorded could reflect more alveolar than vascular pressures. In the presence of high alveolar pressures, areas that function as zone III can revert to zones I or II, as can occur in settings of high PEEP pressures in mechanical ventilators or in hypovolemic patients ( < Pa ). The following characteristics enable us to determine if the tip of the catheter is actually in a zone III :
Clear waveforms ( not damped )
No high variations in PAOP waveforms ( = LA tracing ) during the respiratory cycle
PAP mean > PAOP
PAOP decreasing no more than 50% of a reduction in PEEP level
SO2 PAOP blood = SO2 arterial blood ( SO2 = oxygen saturation )
Having measured all that data related to pressures ( RA, RV, PA, PAOP ) and flow ( cardiac output ), we can, by means of hemodynamic relations, have access to metabolic information, which reflects the exchange of gases between the intracelular and intravascular compartments. This is actually the final common pathway of the whole process of both pump and oxygenation functions of the cardiopulmonary system. The following parameters should be routinely calculated.
where MAP = mean arterial pressure, PAP = pulmonary artery pressure, RAp = central venous pressure ( RA pressure ), PAOP = pulmonary artery occlusion pressure ), PA O2 = partial alveolar oxygen pressure, and Pa O2 = partial arterial oxygen pressure.
Indications for placement of a PA catheter can generally be divided in two broad categories, the most common from each is summarized below :
Cardiac Intensive Care Medicine :
Noncardiac Intensive Care Medicine :
One of the most important indications for placement of a PA catheter in an Intensive Care Unit is to better characterize a shock state. Although not synonimous with hypotension, the most common manifestation of a shock state is a relative decrease in blood pressure significant enough to cause organ hypoperfusion. Shock is a common problem among both medical and surgical patients, and if untreated or unresponsive to therapy evolves into multiple system organ failure and death. That are many different causes of shock, each requiring specific definitive therapy and often selective supportive therapy. So, its very important to be sure about the the kind of shock we are dealing with, as thats what guides the managment with specific strategies. The classification of shock syndromes involves four main groups :
1) Hypovolemic shock - hemorrhage, dehydration
2) Distributive shock - sepsis, anaphylaxis, adrenocorticoid insufficiency, SIRS
3) Cardiogenic shock - LV pump failure, RV infarct, papillary muscle rupture, ...
4) Obstructive shock - massive pulmonary embolus, cardiac tamponade, pneumothorax,...
Besides obstructive shock ( which has its typical clinical and hemodynamic manifestations, as pressure equalization in all four heart chambers in pericardial tamponade, for example ), we can use the data obtained from the PA catheter to detect 3 main hemodynamic profiles of shock, as follows :
| SYNDROME | CVP | PAOP | CO | SVR | VO2 | DO2 |
|---|---|---|---|---|---|---|
| Distributive | 0,< | 0,< | > | < | <,0,> | > |
| Cardiogenic | > | > | < | > | < | < |
| Hypovolemic | < | < | < | > | > | < |
Altough considered a relatively safe procedure, PA catheter placement is an invasive procedure done under pressure monitorization ( not direct visualization under fluoroscopy most of the time ), so as we can " see " where the catheter tip is, and as such has many potential complications, some of which can be fatal. Ideally, the most experienced person available should be the one to make the procedure. Complications are related both to achieving vascular access and to the catheter itself :
- VASCULAR COMPLICATIONS :
- COMPLICATIONS RELATED TO CATHTER :
In conclusion, since its introduction two decades ago, the PA catheter has brought many insights in physiology, pathophysiology of disease states and better patient care, enabling intensivists to treat critical conditions much more precisely and securely. Yet, as many of these conditions carry a high mortality rate, there are still many questions unanswered about its effect on mortality, with many critics questioning its use because it is costly, could not decrease mortality and is not innocuous. Even though, its indication are widening, and those who live in an intensive care environment feel clearly more secure in handling critical patients with some objective data, allowing definition of best therapy and evaluation of its effect. Obviously this is an open field for investigation that should interest those who care for critical patients, physiology and therapeutic innovations. I would be pleased to have your comments and questions in my email, so that we can meet the principal objective of these home page, which is real interaction between students and other health care workers around the world.
. Hall,J. ; Schmidt,G. ; Wood,L. , in " Principles of Critical Care ", McGraw Hill, 1992, pages 323- 342.
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