Doc 1: There’s a big swing on their A-line, shall I give more fluids?

Doc 2: Well given that they’re spontaneously breathing, there’s no real evidence to support PPV here 🤷‍♂️

- A recently overheard conversation, prompting this 🧵 on PPV in the spontaneously breathing patient.
The ∆ between systolic & diastolic pressures is the pulse pressure (PP) & it is determined by the compliance of the aorta and the ventricular stroke volume (SV).

Whilst aortic compliance reduces with age, beat-to-beat changes in PP occur predominately due to changes in SV.
When asking if a pt is fluid responsive (FR), we’re asking if ⬆️ preload will ⬆️ SV. Observing a ∆PP with a ∆preload can help us try to answer this.

Pulse pressure variation (PPV) is the diff. between the max/min PP (as a % of the mean), occurring over a respiratory cycle.
Ventilation requires changes in intra-thoracic pressure (ITP), and this ∆ITP, in turn affects preload.

PPV allows us to see if these ventilation-induced changes lead to a change in SV, hinting at where pts lie on their Starling curve, and if they are likely to be FR.
Spontaneous ventilation (SpV) causes a -ve ITP & tends to ⬆️ preload; mechanical ventilation (MV) causes a +ve ITP & tends to ⬇️preload.

In each case, the size of the ∆preload, relates to the size of the ∆ITP.

This needs to be considered when interpreting PPV in practice.
During MV, a PPV >12% has been shown to accurately predict FR (sens 88%; spec 89%) as long as:

🔹Vt >8ml/kg
🔹Sinus rhythm (SR)
🔹No spontaneous breathing

Understanding the physiology behind PPV, we can see why these preconditions must be met.

ccforum.biomedcentral.com/articles/10.11…
🔹Spontaneous breathing alters the ∆ITP unpredictably making interpreting PPV 😵‍💫

🔹In arrhythmias (i.e. AF) preload & thus PP can vary significantly beat-to-beat leading to false +ve’s

🔹Smaller Vt ➡️ Smaller ∆ITP leading to false -ve’s
Another important ⛔️ is pts with ⬇️ chest wall compliance (i.e. IAH). Here, the same Vt ➡️ a larger ∆ITP, causing a false +ve PPV.

Unrecognised this can lead to IV boluses, which can worsen CW compliance, exaggerating the PPV, leading to a 🌀 of crystalloid water-boarding 😨
PPV has also been studied in SpV where it’s been shown to have a diminished sensitivity but good specificity (see table from linked systematic review)

Sens will ⬆️ with forced insp efforts, or when there is increased WOB with larger -∆ITP

ncbi.nlm.nih.gov/pmc/articles/P…
Two of the mechanisms for ⬇️ sensitivity are:

1) In health the resp system is very compliant, so tidal breathing may cause inadequate ∆ITP

2) In hypovolaemic states, a ⬇️ITP may not ⬆️preload due to collapse of the great veins (maximal venous return is already achieved)
A final caveat… for both MV & SpV: in the presence of RV/LV impairment, ventilation can significantly affect SV irrespective of preload conditions, leading to high PPV/false +ve’s

Specificity is thus significantly reduced and PPV may be of limited value 😕
So back to the original question… in the absence of RV/LV dysfunction, a high PPV in a SpV pt means it’s highly likely they’ll be FR.

Of course the likes of @msiuba @icmteaching & @ThinkingCC won’t forgive me if I don’t mention that being FR doesn’t necessarily = needs fluid!
Hope this is of use, & may even help people avoid pitfalls I know I’ve fallen for!

For far more detailed explanations please read @PrXaMonnet review article:

annalsofintensivecare.springeropen.com/articles/10.11…

& @heart_lung ‘s book (everything worth knowing about Heart-Lung interactions I learnt here!)

• • •

Missing some Tweet in this thread? You can try to force a refresh
 

Keep Current with Tim Norfolk

Tim Norfolk Profile picture

Stay in touch and get notified when new unrolls are available from this author!

Read all threads

This Thread may be Removed Anytime!

PDF

Twitter may remove this content at anytime! Save it as PDF for later use!

Try unrolling a thread yourself!

how to unroll video
  1. Follow @ThreadReaderApp to mention us!

  2. From a Twitter thread mention us with a keyword "unroll"
@threadreaderapp unroll

Practice here first or read more on our help page!

Did Thread Reader help you today?

Support us! We are indie developers!


This site is made by just two indie developers on a laptop doing marketing, support and development! Read more about the story.

Become a Premium Member ($3/month or $30/year) and get exclusive features!

Become Premium

Don't want to be a Premium member but still want to support us?

Make a small donation by buying us coffee ($5) or help with server cost ($10)

Donate via Paypal

Or Donate anonymously using crypto!

Ethereum

0xfe58350B80634f60Fa6Dc149a72b4DFbc17D341E copy

Bitcoin

3ATGMxNzCUFzxpMCHL5sWSt4DVtS8UqXpi copy

Thank you for your support!

Follow Us on Twitter!

:(