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

This study will record ultrasound contrast signals simultaneously with gold standard catheter pressure measurements inside the heart, to enable a direct comparison between the two methods. Complementary ultrasound data will be used to develop a personalised calibration solution to convert the contrast signal in dB to estimated pressure in mmHg. All study procedures will be completed on the same day as the participant's standard of care cardiac catheterisation and no additional study visits will be required.

Inclusion Criteria:

• Participant is willing and able to give informed consent to participate in the study
• Patients who require a cardiac catheterisation procedure as part of their standard medical care
• Good acoustic windows for echocardiography when lying flat on their back

Exclusion Criteria:

• Known previous allergy to SonoVue, used in ultrasound contrast scans
• Known allergy to any of the components of SonoVue microbubbles, for example, sulphur hexafluoride or polyethylene glycol (PEG), also known as macrogol, which is in bowel preparations used during colonoscopy and certain laxatives
• A hole in their heart that lets blood flow from the right side to the left, skipping the lungs
• Very high blood pressure in the arteries of the lungs (severe pulmonary hypertension)
• Uncontrolled high blood pressure (hypertension)
• Adult respiratory distress syndrome (ARDS; where severe lung inflammation prevents enough oxygen from reaching the body)
• Current use of the medicine dobutamine (used to treat heart failure), or have been advised not to take dobutamine
• Moderate to severe heart valve disease that could affect the catheter measurements
• Recent acute coronary syndrome or unstable ischaemic cardiac disease, where blood flow to the heart muscle is reduced
• Pregnant or may be pregnant
• Participation in a clinical trial of a medicine within the past four months, to avoid any possible interactions with SonoVue
• Participating in other research that would prolong their cardiac catheterisation procedure, to ensure that the overall process does not become too tiring or burdensome

De-identified data towards any publications and conference papers will be made publicly available on an online open access repository (e.g. Figshare) after the research output is accepted.