Flow Volume Loops in MRCP

OK, if we talk about respiratory in MRCP, there are two important topics to learn before you decide to take your MRCP part 1 and 2. I think all MRCP candidates should learn by hard flow volume loop and spirometry because you will be expecting a lot MRCP questions about these two topics.

Today, we are going to learn about flow volume loop, a flow volume loop is produced by plotting flow on the y axis against volume on the x axis.

If a subject inspires rapidly from residual volume (RV) to total lung capacity (TLC) and then exhales as hard as possible back to residual volume, then a record can be made of the maximum flow volume loop.

1) Normal Flow Volume Loop


Normal. Inspiratory limb of loop is symmetric and convex. Expiratory limb is linear. Flow rates at the midpoint of the inspiratory and expiratory capacity are often measured. Maximal inspiratory flow at 50% of forced vital capacity (MIF 50% FVC) is greater than maximal expiratory flow at 50% FVC (MEF 50%FVC) because dynamic compression of the air-ways occurs during exhalation.

2) Obstructive disease ( asthma, COPD)


Although all flow rates are diminished, expiratory prolongation predominates, and MEF < MIF. Peak expiratory flow is sometimes used to estimate degree of airway obstruction but is dependent on patient effort.

3) Restrictive Disease ( interstitial lung disease)



The loop is narrowed because of diminished lung volumes, but the shape is generally the same as in nor-mal volume. Flow rates are greater than normal at comparable lung volumes because the increased elastic recoil of lungs holds the airways open.

You may find the graphs very confusing, just remember a few principles here,

1) In obstructive airway disease, due to airway obstruction, the PEF ( Peak expiratory flow rate) is lower than normal ( refer to above graph).

2) In restrictive lung disease, patient total lung capacity ( TLC) is compromised due to pathology ( such as fibrosis), therefore, you notice that, TLC in restrictive lung disease is smaller as compared to normal flow loop.

( One thing to remember, the value of X axis of the flow loop get smaller toward the right!!)



Source:
1) The Merck Manual