Peak expiratory flow (PEF)

Peak expiratory flow (PEF) l/min, is an expiratory air rate. The reduction in airflow speed is of fundamental importance. The lower is the speed, the higher is pulmonary residual volume, i.e. there is decrease in the interrelations between alveolar volume and circulatory blood volume. Generally, decrease of the airflow is an evidence of the following diseases: bronchitis, pneumonias, lung neoplasms, abscesses.

Spirometry

This is a physiological test which measures respiratory performance as a function of time and volume, which therefore incorporates flow (as flow is volume over time).

Conventionally, this test is performed in the following manner:

The patient inhales to TLC

The patient then forcefully exhales into the spirometer nozzle, through their mouth

The patient continues to exhale until full expiration is achieved (for reliability, the ERS/ATS recommend recording at least six seconds of the expiratory time

The expiratory volume over time is graphed, and variables of spirometry are derived from the various features of that graph

That graph will be familiar to most:

If one were ever for some reason asked to reproduce this in their exam, three critically importal elements must be plotted along it, for maximum marks-scoring: the, FVC FEV1 and PEF.

FVC: Forced Vital Capacity: "the maximal volume of air exhaled with maximally forced effort from a maximal inspiration, i.e. vital capacity performed with a maximally forced expiratory effort".  This manoeuvre measures the difference between TLC and RV, which is VC. It is an "F" VC because it is forced, to discriminate between this method of measurement from other, more leisurely and less compelled methods of measuring the VC. A high value here may be a marker of lung overinflation 

FEV₁: Forced Expiratory Volume over 1 second: "the maximal volume of air exhaled in the first second of a forced expiration from a position of full inspiration". As a test of respiratory function it is made more meaningful by its use in a comparison with the FVC:

FEV₁/ FVC ratio: This is the ratio of gas expired over the first second to the total FVC. As such, it is an indicator of whether or not there is any airflow limitation. If the ratio is decreased, that means that there is some limitation to the rate of air egress from the lungs, which typically points to a diagnosis like COPD or asthma. Exactly what "decreased" means seems to vary. The GOLD criteria suggest we use a cut-off of 70%. The ATS instead use the "lower limit of normal" criteria from the fifth lowest percentile of spirometry data reported by the Third National Health and Nutrition Examination Survey (NHANES III).  An increased FEV1/FVC ratio is also possible, and this is usually associated with a restrictive lung disease pattern.

PEF is "the highest flow achieved from a maximum forced expiratory manoeuvre started without hesitation from a position of maximal lung inflation". It is the peak expiratory flow rate measured in L/s.  This parameter can be derived from the expiratory curve data; being the rate of volume change per unit time, one would logically expect this to be represented by the gradient of the expiratory curve. 

Alternatively, one could represent the PEF more effectively by reporting flow over time, which would produce a graphic like this one, stolen from the ERS statement on PEF measurement (Quanjer et al, 1997):

The couple of extra parameters here are the rise time (RT, the time it takes for the flow to get from 10% to 90% of the peak value), and the dwell time (DT,  the time spent at over 90% of peak flow). Together, these metrics have meaning in the scenario of long-term follow-up, but they are probably somewhat irrelevant in the impatient world of intensive care medicine, where instant gratification is all-important.  In any case, a discussion of flow-volume curves is somewhat outside of the scope of this chapter.

A low PEF suggests obstructive disease, but not necessarily so. It could also represent poor effort. In fact, a PEF value, when measured sequentially using a crude bedside instrument, is an excellent indication of whether or not somebody is about to develop the sort of respiratory muscle weakness that gets you intubated. An excellent example is the scenario of a Guillain-Barre syndrome patient whom one is monitoring. González et al (2016) report their experience, where GBS patients with a peak flow less than 194 ml/s (~41% of predicted) were inevitably intubated on the following day.

https://derangedphysiology.com/main/cicm-primary-exam/required-reading/respiratory-system/Chapter%20136/interpretation-lung-function-tests

https://www.gla.ac.uk/media/Media_678202_smxx.pdf

https://www.activ8rlives.com/support/data-collected/cardiovascular-and-respiratory/normal-peak-expiratory-flow-pef