Inclusion Criteria:  MRI/computed tomography (CT) evidence of small vessel occlusion stroke(s)/lacunar stroke(s) (involving ≤2 cm in the acute phase and ≤1.5cm in the late phase) and/or confluent deep white matter hyperintensities (≥ grade 2 on Fazekas's scale). Clinical evidence of cerebral small vessel disease can be: a) small vessel occlusion stroke (lacunar stroke) syndrome with symptoms lasting > 24 hours, occurring < 5 years ago; OR b) transient ischemic attack (TIA) with symptoms lasting < 24 hours AND with MR-DWI imaging performed acutely showing small vessel occlusion stroke, occurring < 5 years ago; OR c) TIA with symptoms lasting < 24 hours AND no acute MRI-DWI lesion but MRI/CT evidence of CSVD with old small vessel occlusion stroke(s) (involving ≤1.5cm) and/or confluent deep white matter hyperintensities (≥ grade 2 on Fazekas's scale). Age ≥ 50 years.  Exclusion Criteria:  Known diagnosis of dementia, medically treated dementia, or under investigation for dementia Pregnancy or nursing Women of childbearing age not taking contraception Known cortical infarction (> 1.5 cm maximum diameter) Known carotid artery stenosis ≥ 50 % with Doppler ultrasound, CT angiography, or MRI angiography diagnosed within the last five years Known carotid or vertebral dissection as a cause of stroke Stroke after carotid or heart surgery Known hypercoagulable disease Systolic BP < 90 and/or diastolic BP < 50 Known severe renal impairment (eGFR < 30ml/min) Known severe hepatic impairment (Child-Pugh > B) History of non-arthritic anterior ischemic optic neuropathy Concomitant use of PDE5 inhibitors e.g. sildenafil, tadalafil, and vardenafil during trial period Patients receiving nicorandil and nitrates e.g. isosorbide mononitrate, isosorbide dinitrate, glyceryl trinitrate History of acute myocardial infarction in the last three months before trial intervention Body weight > 130kg Known cardiac failure (NYHA ≥ II) Known persistent or paroxysmal atrial fibrillation/flutter History of "sick sinus syndrome" or other supraventricular cardiac conduction conditions such as sinoatrial or atrioventricular block (2nd of 3rd degree) Other known cardiogenic cause of stroke Contraindication to CO2 challenge, eg severe respiratory disease MRI not tolerated or contraindicated Known monogenic causes of stroke i.e. CADASIL Unable to provide informed consent The participant does not wish to be informed about results from the MRI