NSR
First Glance:
From across the room it looks like normal sinus rhythm.
Discussion:
Fine. You’re right. It’s normal sinus rhythm*.
We’re looking at lead II. The P wave axis is normal. There is a P before every QRS. We’re gained down to 50%, but even so there isn’t anything suspicious for chamber abnormalities. The T is normal. This is a normal strip.
The lower trace is marked ‘pleth’ which is short for ‘plethysmograph’ which is fancy-speak for pulse-ox. This is the waveform from the pulse oximetry monitor.
While a pretty basic thing, this is one of the most (in my limited experience) misunderstood readings on the monitor. The short story is that the waveform is critical in the interpretation of the Sp02 reading. If you don’t have a good waveform, you shouldn’t trust the number. Period.
There are dozens of ways to fake the machine out and give falsely high or low pulse ox numbers. Most commonly, the oximeter reads falsely low when it is actually just not reading much of anything, or is picking up movement artifact. If the waveform on the pulse ox is wacky, don’t trust the number. If the waveform is excellent (as above), then you can be sure it is actually reading the microscopic arteriolar flow, but it is still subject to false readings from carboxyhemoglobin, etc. If you have no waveform displayed (as on some portable fingertip Sp02 monitors) you might not be able to get this feedback!
Monitors sometimes look at the heart rate from both the EKG and the pulse ox (or A-line). In this strip we see that the machine has calculated the HR as 67 and the ‘pulse’ as 66. This is useful information. The machine knows that the fact that there are QRS complexes does not mean that there is a pulse. However the fact that there is a pulsatile Sp02 waveform does mean there is a pulse.
* True. You can never be 100% sure it’s truly sinus rhythm (vs ectopic atrial) from one lead because you need more than one lead to know the P wave axis. But we assume here, and we’re going to be right 95% of the time, and the other 5% probably don’t clinically matter. Also note that I made those last two numbers up. Choose your own adventure!
Final Impression?
NSR @ ~65 bpm.
Management implications:
Nothing particular.
The Take-home Point:
Pulse oximetry is an incredible tool, but there are ways to be lead astray. Carboxyhemoglobin, methemoglobin, various severe hemoglobinopathies, methylene blue administration, and raging anemias can all skew the reading, as well as mechanical problems such as nail polish or movement.
Further, in low peripheral flow states (shock, cold hands, etc) the saturation of the peripheral flow might actually be significantly lower than the central flow, and the central flow is what is perfusing the brain and liver and kidneys and that’s what you really care about. The skin will survive if the brain does.
One star strip. Students should identify the rhythm correctly.