Take home messages
- Don't switch it off
- The goal is to improve blood supply to the myocardium, while also reducing its oxygen demand
- Patients need to be anticoagulated
- Check renal function to ensure the renal arteries are not occluded
- Balloon pumps cause haemolysis
- If the timing isn't right, then the patient can become very unstable
What is counterpulsation?
The balloon sits in the descending aorta, inflates during diastole, and deflates during systole.
This means that systolic blood flow to the body is unimpeded, but as soon as the heart relaxes, the balloon inflates and displaces the blood in the aorta both forwards and backwards, encouraging the blood to flow down the coronary arteries.
Then when the balloon deflates again immediately prior to systole, it causes a slight negative pressure in the aorta, reducing afterload on the left ventricle by reducing the pressure gradient across the aortic valve.
What is the Windkessel effect?
- The 'Windkessel Effect' refers to the elastic recoil of the aortic root, generating pressure to squeeze blood down the coronary arteries in diastole. The balloon aims to supplement this.
Effects on the heart
- Reduces systolic pressure and increases diastolic pressure in the aorta
- Reduces end-diastolic pressure and afterload on the left ventricle
- Increases cardiac output and hopefully coronary blood flow
What factors affect how well the balloon works?
- The volume of the balloon - determines how much blood is displaced
- Aortic compliance - a more compliant aorta will distend, reducing the backpressure and producing less augmentation
- Heart rate - a shorter diastole means less balloon time and less augmentation
The Timing Sequence and Arterial Waveform
The dicrotic notch on the arterial waveform occurs when the aortic valve closes. The balloon inflates immediately after this, and then deflates again just before the valve opens in systole. This point is the lowest point of the arterial pressure waveform.
When the balloon inflates, it obstructs arterial blood flow, causing a rapid drop in blood pressure at the point of the dicrotic notch. However by increasing or augmenting diastolic pressure, it produces a large second peak, which is unsurprisingly labelled 'diastolic augmentation'.
Hopefully the diastolic augmentation will be higher than the systolic pressure.
You can then set the pump to assist every beat, every alternate beat, or any ratio that is required. As the patient's cardiovascular status improves, the support can be weaned off with less and less augmentation. It should never, however, be left deflated, as it is a huge thrombosis risk.
Indications and Contraindications for IABP
- Acute Myocardial Infarction
- Refractory left ventricular failure
- Cardiogenic shock not responding to pharmacological therapy
- Cardiac surgery and weaning from cardiac bypass
- Acute MR and VSD
- Infant with cardiac abnormalities
- Refractory unstable angina
- Aortic dissection
- Aortic regurgitation
- Previous Aortic stenting
- Chronic end stage heart failure deemed inoperable
- Severe uncontrolled sepsis
- Severe peripheral vascular disease
- Abdominal aortic aneurysm
- Anatomical abnormalities or previous arterial reconstructive surgery
The intra aortic balloon pump is divided in to two parts - the balloon itself and the control pump.
The polyethylene balloon is usually between 25 and 50ml and is loaded onto a double-lumen 8 - 9.5 Fr catheter.
The control pump coordinates the timing of inflation and deflation, and often uses helium for two reasons:
- Its very low density makes it ideal for rapid inflation and deflation
- It's very rapidly absorbed by the blood, which is helpful if the balloon ruptures
As you might imagine, inserting a large catheter into the aorta of a patient with heart problems, and then attempting to inflate and deflate a balloon to occlude the aorta in time with the heartbeat, can sometimes go wrong.
- Balloon rupture and gas embolus
- Haemolysis and thrombocytopenia
- Occlusion of cerebral or renal arterial supply
- Limb ischaemia
- Aortic trauma or dissection
- Bleeding and infection
What is the equation for coronary perfusion pressure?
- Coronary perfusion pressure (CPP) = ADP - LVEDP
- ADP = Aortic diastolic pressure
- LVEDP = Left ventricular end diastolic pressure