CARDIAC CATHETERISATION IN LEFT TO RIGHT SHUNTS AND ASSESSMENT OF OPERABILITY R. Suresh Kumar Sreeja Pavithran Madras Medical Mission
Introduction • Detection, localisation and quantification of intracardiac shunts form an integral part of the hemodynamic evaluation of patients with congenital heart disease • Indication for catheterisation study - to quantify the shunt and to assess associated pulmonary hypertension. CLINICAL CARDIOLOGY UPDATE 2009
Right heart catheterization • Measure O2 saturation in the SVC and IVC • Record pressures and saturations in the mid lateral right atrium (RA), RV, main, right and left pulmonary arteries (PA) and pulmonary capillary wedge positions. • Record pressure, oxygen saturation and blood gas values in the descending and then ascending aorta. • If the pulmonary artery pressures are very high, assess the reversibility of pulmonary hypertension by administering 100% oxygen by mask to the patient for 10 minutes. • The oximetry run and pressure measurements are then repeated. CLINICAL CARDIOLOGY UPDATE 2009
Pressure Data • Pressure measurements are the most important hemodynamic data obtained during cardiac catheterization. • Before the catheterization is started, the pressure transducer is calibrated at the level of the heart so it reads zero at atmospheric pressure. • Pulmonary artery pressure • If there is a large VSD or PDA, the PA systolic pressure equals the aortic systolic pressure because there is no restriction of pressure from the left side to the right side of the heart CLINICAL CARDIOLOGY UPDATE 2009
Blood flow measurement: The cardiac output and shunts • The quantity of blood delivered to the systemic circulation per unit time is termed the cardiac output (L/min) • Measured by application of Fick’s principle Cardiac output = oxygen consumption (VO2) Arterio-venous oxygen difference CLINICAL CARDIOLOGY UPDATE 2009
Important terms - 1 • Cardiac output • the quantity of blood delivered to the systemic circulation per unit time, expressed as (L/min) • Cardiac index (CI) • cardiac input indexed to body surface area and is expressed as L/min/m2. The normal cardiac index is 2.4 – 4.4 L/min/m2 • Oxygen consumption (VO2) • the amount of oxygen taken up by the tissues and is usually expressed in ml/minute. • Normal oxygen consumption • 0-5 years - 140-200 ml/min (per m2) • 5-15 years - 110-160ml/min (per m2) CLINICAL CARDIOLOGY UPDATE 2009
Important terms - 2 • Oxygen consumption can be measured by using the polarographic method and the paramagnetic method. • For practical purposes, oxygen consumption is assumed based on the table of Lafarge and Miettinen • Oxygen capacity – maximal amount of oxygen that can be taken up by hemoglobin in blood. When the hemoglobin level in g/100ml is multiplied by 1.36, the oxygen capacity is derived, expressed as ml/dl. • Oxygen saturation – a measure of the proportion of oxygen actually combined with hemoglobin to the total amount of oxygen that can be taken up by hemoglobin in a blood sample.
Important terms - 3 • Oxygen content – amount of oxygen present in any blood sample in question, expressed as ml/L of blood and refers to the total quantity both combined with Hb and dissolved in plasma. • Arterio venous oxygen (AVO2) difference - amount of oxygen extracted from the circulation as the blood flows through a vascular bed, expressed in volumes percent (mlO2/100ml)
Detection of LR Intracardiac shunts • The oxygen content or percent saturation is measured in blood samples drawn sequentially from the pulmonary artery, RV, RA, SVC and IVC. • Significant step up in blood oxygen saturation or content denotes left to right shunt
Criteria for significant step up Atrial level > 9% Ventricular level > 6% Great artery level > 6%
Possible causes of step-up • RA - ASD, PAPVC, Ruptured Sinus of Valsalva to RA, VSD with TR, coronary fistula to RA • RV - VSD, PDA with PR, Coronary fistula to RV • Great Vessel (RV to PA) – PDA,Aortopulmonary window • A step down of >2% or an aortic saturation of <92% is suggestive of R L shunt.
Step-up Caveats • If significant step up present, the pulmonary and systemic blood flows and magnitude of L to R shunt are calculated • A pulmonary to systemic flow ratio of greater than 2.0:1 usually indicates a large L to R shunt
Calculations in left to right shunts Calculation of oxygen content and A-V oxygen difference • Step 1: Oxygen carrying capacity: Hemoglobin (gm/dl) x 1.36 (ml of O2/gm of Hb) x 10 = ____mlO2/L of blood • Step 2: Saturation of arterial (BA,FA,AO) blood = ________% • Step 3: Oxygen content of arterial blood: O2 capacity x % saturation = _------ml/L (step 1) (step 2) • Step 4: Saturation of mixed venous blood = [3SVC O2 sat+IVC O2sat%] 4
Calculations - 2 • Step 5: O2 content of mixed venous blood: O2 capacity x % saturation = _____ml/L (step 1) (step 4) • Step 6: AVO2 difference: Arterial O2 content – Venous O2 content = ____ml/L (step 3) (step 5)
Calculation of pulmonary and systemic blood flow • Qp is the amount of blood flowing through the pulmonary capillary bed (pulmonary flow) Qp = O2 consumption (VO2) (ml/min) (L/min) PVO2 - PA O2 content content (ml/L) (ml/L) = VO2 (Pvsat – PAsat) (O2 capacity)
Calculation of pulmonary and systemic blood flow - 2 • Qs is the amount of blood flowing through the systemic capillaries (systemic flow) Qs = VO2 Arterial O2 - MVO2 content (ml/L) content (ml/L) = VO2 (SAsat – MVsat) (O2 capacity) MVO2 content = 3(SVCO2 content) + 1(IVC O2 content) 4
Calculation of pulmonary and systemic blood flow - 3 • Calculation of Lt to Rt shunt • L R Shunt = Qp – Qs (L/min) • Qp / Qs • Qp = SA sat – MV sat Qs PV sat – PA sat • A Qp/Qs <1.5 a small L to R shunt 2.0 a large L to R shunt surgical repair to prevent late pulmonary vascular disease as well as other complications of prolonged circulatory overload
Calculation of flow during oxygen administration • PO2 must be measured to correct for dissolved oxygen. Dissolved O2 is about 0.3 ml/dl or 3ml/L • When a patient is breathing room air and systemic arterial or pulmonary venous PO2 is about 100mmHg, then O2 sat is almost 100% and dissolved O2 is only about 3ml/L • If there is moderately large left to right shunt, with a relatively high O2 saturation of 85% in pulmonary artery and if VO2 is 150ml/mt. and Hb 14.5 g/dl, then O2 capacity = 14.5 x 1.36 x 10 = 200ml/L PVO2 content = 100/100 x 200 + 3 = 203 ml/L PaO2 content = 85/100 x 200 + 2.5 = 172.5ml/L
Calculation of flow during oxygen administration - 2 Qp = 150/203 – 172.5 = 4.9 L/min If dissolved O2 not taken into account PVO2 = 200ml/L and CPaO2 = 170ml/L Qp = 5.0 L/min Thus little error is produced during breathing in room air. If patient is breathing 100% O2 and pulmonary arterial saturation is 92% PO2 in pulm. Venous blood = 550mmHg PO2 in pulm. Arterial blood = 100mmHg
Calculation of flow during oxygen administration - 3 PVO2 = 100/100 x 200 + 3x5.5 (dissolved) = 216.5ml/L PAO2 = 92/100x200 + 3x1 (dissolved) = 187ml/L Qp = 150/216.5 – 187.0 = 5.1 L/min If dissolved O2 is not taken into consideration PVO2 = 200ml/L , PAO2 = 184ml/L Qp = 150/200 - 184 = 9.4L/min This represents an overestimate of pulmonary blood flow of almost 100%
Resistance • Calculated based on Poiseuille’s law R = Pin – Pout Q Q = flow, Pin = mean inflow pressure, Pout = mean outflow pressure and R = resistance Normal systemic vascular resistance - 20 mmHg/L/min/m2 (Range 15 – 30 mmHg/L/min/m2). Normal pulmonary vascular resistance - 1 –3 mmHg/L/min/m2 (Wood Units).
Formula for calculating resistance Systemic vascular resistance (SVR) = Aortic mean pressure - RA mean pressure Qs Pulmonary vascular resistance (PVR) = PA mean pressure - LA/PCW mean pressure Qp In children, vascular resistance is normalised for body surface area (BSA), thus giving a resistance index. SVR index (SVRI) = SVR x BSA PVR index (PVRI) = PVR x BSA
Assessment of operability • A Qp/Qs <1.5 small left to right shunt , may be left alone. 2.0 large left to right shunt surgical repair to prevent late pulmonary vascular disease as well as other complications of prolonged circulatory overload. • A Qp/Qs <1 indicates that there is a resting right to left shunt, which may preclude surgical repair • The ratio between PVR and SVR and the absolute PVRI is used as a criterion for assessing operability in L R shunts.
PVRI PVR / SVR INTERPRETATION <8 WU 0.25 – 0.5 Operable (moderate pulmonary vascular disease) 8 – 12 WU 0.5 – 0.75 Borderline operability (high risk) >12 WU >0.75 Inoperable (severe pulmonary vascular disease) Operability - 2
Response to Oxygen • Significant fall in the diastolic pressures (by about 10 mmHg) and mean pressure by about 5mmHg - reversible pulmonary artery pressures and hence operability.
FACTORS THAT MAY CHANGE THE PULMONARY VASCULAR RESISTANCE
Limitations and errors in calculating flows and shunts by the Fick method • Absence of a steady state during the collection of blood samples • A mixed venous sample may not be obtainable when there is a shunt of systemic arterial or pulmonary venous blood into the systemic veins. When one or more pulmonary veins drain into SVC / IVC, venous blood must be collected before their entry. • Oxygen saturations in the ascending and descending aorta may be different when there is a right to left shunt through the ductus arteriosus. • Pulmonary arterial oxygen saturation may be different in the left and right pulmonary artery in patients with patent ductus arteriosus.
Limitations - 2 • Assumed values for oxygen consumption. • When the arteriovenous difference is large, the errors inherent in measuring oxygen content or saturation do not result in major errors in calculation of flow. However when arteriovenous difference is small, small errors in measurement may result in large errors of flow measurement. • Calculations of flow during oxygen administration – Here if the dissolved oxygen is not considered, it leads to an overestimation of pulmonary blood flow. • Lacks sensitivity – small shunts are not consistently detected • Influenced by the blood Hb concentration. • Improper collection of mixed venous blood sample (e.g. air bubbles) or dilution of the sample with too much heparinised saline solution.
12 year old with findings of atrial septal defect Diagnosis? Supra cardiac TAPVC
3 year old, cyanosed since birth Diagnosis?TGA with Intact vent septum
3 years old, cyanotic, clinical diagnosis TOF Consistent with TOF? Diagnosis: TOF with restrictive VSD
8 months old, 6.5 kg, Down’s syndromeEcho: Atrioventricular septal defect , PAH On 10’ of O2 Qp Qs Qp/Qs PVR In room air5.0 2.8 1.8 9.1 On 10’ of oxygen 11.7 2.8 4.2 3.2