Cardiac output as a metaphor for flow through an urgent care setting

Flow through the urgent care setting, such as an emergency department (ED) is complex. There are a multitude of factors that affect flow through an urgent care setting. Systems research has shown that it is not enough to consider flow in only one part of the system.

But visualising the various problems, whilst at the same time considering the rest of the system is very difficult. It is even more challenging explaining this to external influencers (managers, executive teams, etc.) who sit outside the urgent care system, but has lots of control over the system.

We propose that cardiac output (stroke volume x heart rate) is an effective metaphor to help visualise flow through an urgent care setting. It is also likely to be effective at breaking down the key principles to these influencers.

It is particularly helpful in open systems, such as the ED, where there are many external drivers of flow, which cannot be directly controlled from within.

Stroke volume is a function of three factors: preload, contractility and afterload. Heart rate sits outside stroke volume, and compensates for fluctuations in stroke volume by increasing or decreasing.

cardiac output

The preload

Preload represents what is coming into the system, and the system’s capacity to receive. When the preload overwhelms the heart’s capacity to receive, we use venodilators to distribute the preload over a wider space in the venous system.

Prior to the pandemic this was done by placing patients in the ED’s corridors to create more space within the service. Thus, EDs were able to expand (or dilate) its available footprint by repurposing non-clinical space. In essence, EDs were already struggling with preload going into the pandemic.

At present, capacity to receive unscheduled patients into the urgent care system is restricted, due to the need for additional processes and systems required for infection prevention and control. This includes a reduction of the previous option to make use of corridor space to expand operations.

A restrictive cardiomyopathy does not do well with sustained, increased preload. It is therefore understandable that even a return to pre-pandemic numbers will easily outstrip existing infrastructure. The result has been ambulance ramping at unprecedented levels, causing substantial upstream delays in responding to 999 calls.


Contractility represents the muscle of the urgent care system: staff, resources and processes. When contractility is not optimal for whatever reason, we can use inotropes to improve it. But, managing a failing heart due to reduced contractility is challenging if inotropes are in short supply.

Resources and processes, such as access to rapid diagnostics, and flow of high risk and low risk COVID patients through the system, has improved since the start of the pandemic. But these still result in a more restrictive system than pre-pandemic. It is unlikely to return to pre-pandemic levels for the foreseeable future.

Resources and processes aside, workforce challenges already existed prior to the pandemic. This was exacerbated by the pandemic through increased sickness, immigration restrictions and early retirement. Mental health has taken a huge toll on urgent care staff. Recruitment to mid- and senior-tier clinical positions has been very challenging with substantial vacancies at most trusts.


Afterload represents flow of patients out of the system. Where the afterload is high, flow out of the system will be compromised. Afterload is generally reduced by any means that reduces the blood pressure.

Only a handful of acute care trusts were meeting NHSE urgent care performance targets pre-pandemic. It has not been met at all by any trust (with the exception of four specialised centres) for several months now. This during the summer, when performance historically tends to be better.

It is fair to say that reducing the pressure within the system has been less effective than hoped, despite substantial effort from hospital site teams, social care efforts and clinical commissioning groups. Most trusts report high numbers of patients that do not fulfil criteria to reside in hospital any longer. Like the NHS, the social care system reports substantial vacancies, further increasing systemic (vascular) resistance.

Julie Gibbs explains the problem as follow: whilst the front end of the system operates in minutes to hours, the backend of the system operates in days to weeks. Lack of capacity at the backend therefore disproportionately increases systemic (vascular) resistance, with increased afterload at the front end.

Whilst the front end of the system operates in minutes to hours, the backend of the system operates in days to weeks.

Julie Gibbs: emergency physician, Frimley Health NHS Foundation Trust

Stroke volume

It is important to understand that stroke volume is affected by preload, contractility and afterload at the same time. Addressing deficiencies in preload, without considering contractility or afterload will have a marginal effect on improving the stroke volume at best.

Furthermore, once you have filled the newly created capacity to compensate for preload, you’ll simply be back to where you started. Sites who have added space to their footprint without addressing staffing or exit block will have experienced this first hand.

The same applies to single focus solutions for contractility or afterload. The take away will be that you have failed, when in fact, you have simply not expanded your focus wide enough to be successful.

Heart rate

The heart rate represents the heart of the system: wellness, resilience, training, etc. Heart rate can compensate for fluctuations in cardiac output, but only to a point, and not for sustained periods of pressure.

We would usually see an increase in heart rate to ensure optimal perfusion of vital organs during exercise or illness. In exercise physiology we refer to the maximum heart rate (220 beats per minute minus age) as the optimal rate at which we can safely stress the system. This is because increasing the heart rate further decreases coronary perfusion, as the coronaries are only perfused during relaxation of the heart (diastole).

Simply put, with sustained stress, at some point the heart rate will not be able to further sustain optimal perfusion. We cannot carry on running indefinitely.

The urgent care system has been running on goodwill for decades; the pandemic has increased this relience on goodwill. Even with a perfect stroke volume (and all its components), a less than optimal heart rate will result in a reduced cardiac output.

Expanding on the model

You can present the cardiac output model as simple, or complicated as is needed. Further concepts that can be explored is the various cardiomyopathies, arrhythmias and Frank Starling’s law.

But the one that likely summarises how most urgent care staff feel about their systems at the moment is Takotsubo cardiomyopathy or broken heart syndrome. Takotsubo’s is usually due to severe emotional or physical stress, such the loss of a loved one, a serious accident, or a natural disaster such as a pandemic.

We welcome your input on how we can improve on this model, as well as general comments on whether you have found it useful to explain the urgent care system outside of urgent care.



Cite this article as: Stevan Bruijns, "Cardiac output as a metaphor for flow through an urgent care setting," in St.Emlyn's, October 26, 2021,

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