In this lecture we are going to talk about the following:
- SWANS catheter
- Cardiac output
- Pulmonary artery wedge pressure
- Atrium pressure (central venous pressure)
The main reason for to run a hemodynamics test is to measure four different things:
- Vascular capacity – how much pressure is going into the heart
- Blood volume – how much volume of blood the heart should push
- Pump effectiveness – deals with cardiac output, stroke volume, preload, afterload
- Tissue perfusion – concerned with the oxygen that the body consumes
SWANS catheter (pulmonary artery catheter), on the other hand, measures three things, namely:
- Cardiac output – how much blood the heart pushes in one minute
- Oxygen – how much oxygen is going out of the heart
To measure pressure, the doctor inserts the catheter into the right atrium (adjacent to the SA node) and inflates the balloon. Through natural force, the air follows the pressure of the fluid inside the heart and will rest at the pulmonary arteries. Take note that the right side of the heart is responsible for pumping blood directly into the lungs; therefore, the inserted catheter will measure the pressure from the body, into the lungs.
Now, remember, right-sided heart failure is equivalent to body failure. This means that the body does not have sufficient amount of pressure to pump blood into the lungs, causing blood to be forced back into the body, which then leads to edema.
Whereas, if there is a backflow of blood into the left ventricle and goes back into the lungs, there will be the presence of crackles, which is one of the symptoms of left-sided heart failure. Remember that left-sided heart failure can also be considered as a lung-heart failure because the fluid from the heart is being sent back into the lungs.
There are three specific ways to measure the pressure inside the heart.
- Right atrial pressure (central venous pressure) – this should be between 1 – 8 mmHg.
- PAP (pulmonary artery pressure) – always deflated; resulting in systolic pressure of 15-26 mmHg and a diastolic of 5 – 15 mmHg.
- Systolic pressure is the squeeze, the force that the heart exerts during contraction (depolarization) and into the lungs and the left ventricle.
- Diastolic pressure is the decompression or relaxation
- PAWP (pulmonary artery wedge pressure) – catheter inflation in the pulmonary artery down to the lungs. Inflation lasts for three to five seconds, cutting off circulation for a quick moment. This will provide a direct measurement of the pressure being back-flowed from the lungs to the prongs. The pressure should be between 4 – 12 mmHg. Furthermore, this measures left ventricular pressure and the diastolic pressure; these are just fancy words for left ventricle “filling time.”
Trivia: in a laboratory setting, this phenomenon is determined by brain natriuretic peptides (BNP). BNP inside the left ventricle helps in the stretching of this specific chamber.
If in case the PAWP is less than 4 mmHg, the client will experience hypovolemia or decreased pressure being pushed into the left ventricle. Hypervolemia, on the other hand, happens if PAWP is more than 12 mmHg; which is also indicative of left ventricular failure.
Example, if your client has 18 mmHg, there will be increased pressure on the left ventricle due to the pooling of blood inside the chamber which causes it to stretch further. The backflow will extend into the right ventricle.
Cardiac output (CO) measures the blood flowing into the heart. SWANS measures cardiac output through the thermodilution method which you can remember as the cold choo-choo train.
Take note that normal cardiac output is 4 – 8 L/min. In the thermodilution method, how is that measured with the catheter?
The doctor will pump 5 to 10 ml of cold, normal saline into the catheter, which goes through the heart. The measurement will depend on how fast or how long it took for normal saline to travel into the heart; thus, the cold choo-choo train.
This measurement basically focuses on how much oxygen is present in the heart and how much oxygen is going back into the lungs. Here, the SVO2 caliber in the catheter is used. Normal SVO2 is between 60% to 80% hemoglobin going back into the lungs. A fiber optic light is used in this type of measurement.
As a recap, a SWANS catheter measures a client’s:
- Cardiac Output
- Heart and blood volume
When is a SWANS catheter (pulmonary artery catheter) used?
SWANS catheter is inserted in clients who underwent cardiac surgery, usually post-CABG. Here, we wanted to observe how the heart is coping with the procedure. Aside from that, SWANS is also inserted in cases of heart failure wherein the doctor has already exhausted all pharmacologic measures like taking volume-depleting drugs which includes ACE inhibitors, Lasix (furosemide), beta-blockers, and calcium-channel blockers.
How is the SWANS catheter inserted?
The client is put on a supine or Trendelenburg position. The doctor will insert the catheter into the jugular vein of the neck or the subclavian vein right under the clavicle or collarbone. From there, the catheter will then be inserted in the right atrium of the heart.