Cardiac Catheterisation

The focus of the cardiac catheterisation study is on the right heart. RHC (right heart catheterisation) provides important information about the pressures in the different chambers of the heart and the blood flow through the heart and pulmonary artery. The test may involve the patient being admitted to hospital for a few days. If the patient is already being treated with warfarin, a drug that slows down the speed at which the blood normally clots, often this therapy will be stopped temporarily a few days before. This is done so that the patient does not bleed excessively during the procedure, although it should be said that some doctors do not wish patients to stop their warfarin.

RHC allows the healthcare team to obtain a precise measurement of the blood pressure in the heart and pulmonary artery. Occasionally, left heart catheterisation is required in addition to right heart catheterisation. This can for example allow the healthcare team to search for suspected congenital heart disease or to evaluate suspected left heart abnormalities (mitral valve disease, coronary artery disease etc).

RHC provides information that will confirm the diagnosis of PAH and confirm or exclude other causes of pulmonary hypertension.

Just before the procedure begins the patient is sometimes sedated (made sleepy). Adult patients are not usually given a general anaesthetic although children may be. The patient is connected to an ECG monitor and an “Oximeter” and the blood pressure is monitored closely. With the patient lying still, a thin flexible tube called a catheter is inserted through a vein in the groin or the neck and slowly advanced into the right atrium, right ventricle and then into the pulmonary artery. The catheter is a tube about one metre long and as thin as spaghetti. The catheter is specially designed to contain three or four lumens (hollow tubes) to allow the doctor to measure the blood pressure in several places at once. This is often done with the help of continuous X-ray pictures which are viewed on a TV monitor. The pressures are recorded many times by some very clever sensors on the end of the catheter. An estimate of cardiac output is also recorded. Several blood samples are often taken via the catheter to give specific measurement of the oxygen levels in various parts of the blood circulation system. Once the catheter is in position the doctor can start taking measurements. The important measurements are as follows:

Mean pulmonary artery pressure (mPAP)

• This is the average blood pressure in the main pulmonary arteries. If it is above 25 mmHg (while the patient is resting) then they are said to have pulmonary hypertension.  Some patients have mPAPs as high as 100+ mmHg. Sometimes patients may have a ”normal” mPAP while resting, but when they do very moderate exercises, like raising their legs or arms, then the pressure will rise. If it rises above 30mmHg, then this patient is said to have “PH on exercise”. However, there are no specific cut-off pressures that define a patient as having mild, moderate or severe disease. Disease severity is a decision that is arrived at after interpreting a number of test results, not just the RHC study.

Mean right atrial pressure (mRAP)

• The pressure in the right atrium is normally very low, around 5 mmHg. If this rises above 10 mmHg this is an indication that the heart is beginning to find it difficult to pump blood efficiently. However if patients are being treated with diuretics (water tablets) this result may be hard to interpret, as diuretics can hide the fact that a patients heart is pumping inefficiently.

Pulmonary capillary wedge pressure (PCWP / wedge)

• This measurement gives the doctor an idea of what the pressure is on the left side of the heart. This pressure is also generally low (around 5 mmHg). If this pressure is above about 15 mmHg this tells the doctor that the heart is having to push against this high pressure to get the blood through the lungs.

Cardiac output

• This measurement tells the doctor how much blood the patient’s heart pumps over one minute. This amount varies depending on how big the patient is. Therefore a measurement known as the cardiac index is often used. This measurement takes into account the patient’s weight and it takes into account the patient’s body surface area. A cardiac index below 2 L/min/m2 indicates that the heart is not pumping as much blood as it should.

Pulmonary vascular resistance (PVR)

• This number is calculated using the measurements described above. It gives an indication of how difficult it is for blood to flow through the lungs.

Mixed venous oxygen saturation

• The doctor may take a blood sample from the catheter and measure the oxygen saturation. This will give the doctor an idea of how much oxygen the patient’s muscles and organs are extracting from the blood.

Vasodilator Testing

• When all the studies described above have been carried out, they are repeated while the patient breathes in a high concentration of oxygen and a gas called nitric oxide in order to see if the blood vessels in the lung can dilate (open up). If the blood vessels dilate well the pulmonary vascular resistance falls. A big drop in resistance means that the vessels are still fairly pliable and not too obstructed. This result helps the doctor to determine a treatment regime for the patient. The greater the dilatation, the less powerful the medication needed.

Functional Classification

• Once PH is diagnosed, there is a need to classify the disease according to the “functional classification system” developed by the New York Heart Association. It is based on patient reports of how much activity they can comfortably undertake before symptoms start to become noticeable.
  
  
  
  • Class 1 describes patients who have no symptoms of any kind and for whom ordinary physical activity does not cause fatigue, palpitation, shortness of breath or chest pain.
  • Class 2 describes patients who are comfortable at rest but have symptoms with ordinary physical activity.
  • Class 3 describes to patients who are comfortable at rest but have symptoms with less-than-ordinary effort.
  • Class 4 is described patients who have symptoms while at rest.