Can Tiny Bubbles Offer an Alternative to Catheters for Assessing Pressures Inside the Heart? Investigating Ultrasound Contrast Agents as Pressure Sensors Against Gold Standard Catheter Pressures in Cardiac Catheterisation Patients.

The goal of this clinical trial is to investigate if ultrasound contrast agents can be used to estimate filling pressures inside the heart in patients with suspected heart disease. The main questions it aims to answer are: * Is there a strong correlation between the contrast signal and filling pressures inside the heart? * What is the calibration approach to convert the contrast signal from dB to a measure of pressure in mmHg? Researchers will compare the contrast signal with reference pressures measured using a catheter to see if it can be used to quantify filling pressures inside the heart. Participants will: * Be exposed to a small amount of additional ionising radiation to guide a catheter in position inside the heart for reference pressures * Receive an ultrasound contrast agent at the clinically recommended dose and in line with clinical guidelines, via an intravenous line in their arm * Undergo contrast echocardiography - ultrasound scan of their heart with contrast * Undergo standard echocardiography - ultrasound scan of their heart without contrast