B737-800 first officer reported the captains airspeed indicator malfunctioned on the descent.

Attributes
ACN	1713772
Time
Date	201912
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.TRACON
State Reference	US
Environment
Flight Conditions	IMC
Aircraft 1
Make Model Name	B737-800
Operating Under FAR Part	Part 121
Flight Phase	Descent
Route In Use	Direct
Flight Plan	IFR
Component
Aircraft Component	Airspeed Indicator
Person 1
Function	Pilot Flying First Officer
Qualification	Flight Crew Multiengine Flight Crew Air Transport Pilot (ATP) Flight Crew Instrument
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Procedural Published Material / Policy Deviation - Procedural Clearance Deviation - Speed All Types

Narrative:

While descending through about 7000 ft. MSL in IMC; pm (pilot monitoring) noticed his airspeed immediately dropped to 100 kias. PF (pilot flying) airspeed was normal. Pm was also PIC. Pm immediately pushed power up and closed [the] throttles. At this point I saw the IAS disagree light and determined my IAS was correct. I disconnected [the] autopilot; said my aircraft and attempted to rejoin the STAR. During the event airspeed accelerated to almost 300 kias. We received a frequency change and slowed back to 250 kias. ATC did not notice the airspeed deviation. The ca (captain) airspeed indicator recovered after about 40 seconds and worked for the remainder of the flight.something caused the ca airspeed indicator to malfunction. The startle factor caused the captain to assume [the] worst case and push power up before we were able to determine that his indicator was malfunctioning. By the time we knew all the information we had accelerated above 250 knots.

Original NASA ASRS Text

Title: B737-800 First Officer reported the Captains airspeed indicator malfunctioned on the descent.

Narrative: While descending through about 7000 ft. MSL in IMC; PM (Pilot Monitoring) noticed his airspeed immediately dropped to 100 kias. PF (Pilot Flying) Airspeed was normal. PM was also PIC. PM immediately pushed power up and closed [the] throttles. At this point I saw the IAS disagree light and determined my IAS was correct. I disconnected [the] autopilot; said my aircraft and attempted to rejoin the STAR. During the event airspeed accelerated to almost 300 kias. We received a frequency change and slowed back to 250 kias. ATC did not notice the airspeed deviation. The CA (Captain) airspeed indicator recovered after about 40 seconds and worked for the remainder of the flight.Something caused the CA airspeed indicator to malfunction. The startle factor caused the Captain to assume [the] worst case and push power up before we were able to determine that his indicator was malfunctioning. By the time we knew all the information we had accelerated above 250 knots.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.