Thread by @jkurtz02

My first attempt at a tweetatorial. A🧵 on pulmonary overcirculation
#FOAMed #cardiotwitter #pedscards #CHD @cardionerds @UofLPeds @UofLPedsCCM 1/20

The term pulmonary overcirculation or overcirculation gets used a lot when discussing kids with CHD, usually without a specific set of criteria. If a patient is not progressing as expected, is hospitalized, or has a complication frequently overcirculation is blamed. 2/20

But what is really occurring when we refer to overcirculation? In a structurally normal 🫀, the pulmonary and systemic circuits work in series. The blood flow to the 🫁 (Qp) is the same as it is to the body (Qs). 3/20

The amount of blood flow (cardiac index, CI) is determined by several factors. The body adjusts to provide enough blood flow to maintain adequate oxygen delivery to the tissue. 4/20

In a structurally normal 🫀 when CI is unable to be maintained it can present as heart failure or shock. 5/20

In someone with CHD and a shunt (synthetic or natural) there is not necessarily a 1:1 balance of Qp and Qs. 6/20

If Qp>Qs then the patient technically has “pulmonary overcirulation” this can lead to numerous symptoms and these symptoms are what are being referenced when blaming overcirculation. 7/20

However, the issue may not be that there is too much pulmonary blood flow, but that the systemic circuit is underperfused i.e. low cardiac output. This is an important distinction, and we should be careful to only say overcirulation when there is ⬆️ pulmonary blood flow. 8/20

What may this imbalance present as? ⬆️ pulmonary blood flow: pulmonary edema and heart failure or ⬇️systemic flow: shock, NEC/gut hypoperfusion, low cardiac output syndrome, or heart failure. This is a broad range of symptoms. 9/20

We frequently attribute the symptoms or complications to overcirculation, but these could also be related to myocyte dysfunction, valvular disease, non-cardiac disease, prematurity, or numerous other co-morbidities. 10/20

While overcirculation is frequently blamed empirically, we can measure the relative amounts of Qp and Qs and determine if overcirculation is truly occurring. i.e. ⬆️ pulmonary blood flow versus ⬇️ systemic blood flow or another cause. 11/20

The Fick principle can be used to determine the flow through a system based on using oxygen as an indicator. This can be done for both the systemic and pulmonary circuits. 12/20

Essentially the Fick principle in this case is that if the amount of oxygen a system consumes (VO2) is known and the amount of change in oxygen content is known the amount of flow needed to supply that oxygen can be derived. 13/20

On room air the dissolved oxygen in blood is negligible and so the oxygen content of blood is based on the amount of Hgb (carrying oxygen) and the O2 saturation of the Hgb. 14/20

This all can be measured directly as the mixed/central venous, arterial, pulmonary vein, and pulmonary artery saturation and can help to determine if the true cause is related to an imbalance of Qp and Qs. And if so is it ⬆️ Qp or ⬇️ Qs. 15/20

This ratio alone doesn’t tell the entire story. These are relative flows. With a Qp:Qs of 3, the Qp could be 9 L/min/m2 (overcirculation) and the Qs 3 L/min/m2 (normal) or the Qp could be 3 L/min/m2 (normal) and the Qs 1 L/min/m2 (low cardiac output). 16/20

The ratio is important, but so is the context. What are the clinical symptoms that lead to the concern and does that match with the data? 17/20

Qp:Qs can be manipulated by adjusting the relative resistances of the pulmonary and systemic circuits. By increasing the pulmonary vascular resistance (PVR) there will be less pulmonary blood flow and vice versa for systemic. 18/20

Therefore, with Qp>Qs if the PVR is increased or the systemic vascular resistance (SVR) is decreased the Qp/Qs should move closer to 1. 19/20

This is why the term overcirculation is confused and used frequently for both situations. If PVR is ⬆️ or SVR is ⬇️ it will drive more Qs. So if there is ⬆️ PBF or ⬇️systemic flow then ⬆️ PVR and ⬇️ SVR should help balance the Qp:Qs. 20/20