The PWP Assessment consists of monitoring the patient’s heart rate variability for five minutes. By monitoring the patient’s heart rate variability (HRV), we can observe how the autonomic nervous system controls the heart muscle for this period.
The key to interpreting the PWP Assessment is ensuring that the patient’s autonomic nervous system is “at rest” during the exam. This is analogous to performing a static EMG exam and ensuring that the para spinal muscles are at rest. When we think of the autonomic nervous system being at rest, we mean in a non-aroused state. The autonomic nervous system can become aroused through either physical or emotional activity. The goal of the five-minute heart rate variability assessment is to keep the autonomic nervous system in a non-aroused state. If the system is aroused during the five minutes, then the interpretation of the data changes, because we are not observing how the heart muscle is controlled during rest, but rather during arousal.
There are three events that need to take place to keep the autonomic nervous system at rest during the five-minute protocol:
- Clearly if the patient is engaging with or observing the examiner, the autonomic nervous system will be activated. Once the examiner starts the protocol, he or she leaves the exam room for the duration of the exam.
- To keep the patient at rest during the five-minute exam, there is a configurable acclimation or rest period prior to starting the exam. This is selected by the examiner and can range from 30 seconds to 5 minutes. Directly after this period, the actual 5 minute HRV protocol begins. The configuration and use of this acclimation period is discussed below.
- Autonomic arousal is often not detectable from observation. Therefore, the PWP sensor has a built-in “arousal indicator”, which is displayed along with the HRV data. This is accomplished by monitoring the galvanic skin resistance of the index finger and ring finger, which is an independent indicator of sympathetic arousal. If during the exam, your patient has a physical activity (such as a sneeze), or an emotional activity (such as reflecting on a horrible traffic jam earlier in the day) this will cause a change in skin conductance which will be presented to you as an indication of autonomic arousal during the assessment.