1. Field of the Invention
This invention generally relates to health monitoring and, more particularly, to a system and method for determining when vital sign monitoring sensors are making poor contact with a patient being monitored.
2. Description of the Related Art
Electrocardiography (ECG or EKG) is the process of recording the electrical activity of the heart over a period of time using electrodes placed on a patient's body. These electrodes detect the tiny electrical changes on the skin that arise from the heart muscle depolarizing during each heartbeat. During each heartbeat, a healthy heart has an orderly progression of depolarization that starts with pacemaker cells in the sinoatrial node, spreads out through the atrium, passes through the atrioventricular node down into the bundle of His and into the Purkinje fibers spreading down and to the left throughout the ventricles. This orderly pattern of depolarization gives rise to the characteristic EC tracing.
A photoplethysmogram (PPG) is an optically obtained plethysmogram, a volumetric measurement of an organ. A PPG is often obtained by using a pulse oximeter which illuminates the skin and measures changes in light absorption. A conventional pulse oximeter monitors the perfusion of blood to the dermis and subcutaneous tissue of the skin. With each cardiac cycle the heart pumps blood to the periphery. Even though this pressure pulse is somewhat damped by the time it reaches the skin, it is enough to distend the arteries and arterioles in the subcutaneous tissue. If the pulse oximeter is attached without compressing the skin, a pressure pulse can also be seen from the venous plexus, as a small secondary peak. The change in volume caused by the pressure pulse is detected by illuminating the skin with the light from a light-emitting diode (LED) and then measuring the amount of light either transmitted or reflected to a photodiode. Each cardiac cycle appears as a peak.
Current ECG systems typically use wet electrodes and can detect a lead off condition by either measuring the signal noise level or measuring the impedance between the electrodes by periodically passing a small current between them. For wet electrodes on a patient's chest, the expected signal amplitude is known and noise levels are relatively small. Therefore, detecting unexpected noise typical of a lead off condition is relatively easy.
Consumer grade devices uses dry electrodes and they can be placed at numerous different locations on the user's body. Signal levels vary greatly and are dependent upon sensor location and skin dryness. Typically, the noise level can be 10 times greater or more than the signal level.
FIG. 1 is a magnified view of one ECG pulse as measured by a sensor with two dry electrodes. The thicker trace represents the ECG signal after filtering and rectification. Detecting an ECG poor contact condition when using dry ECG electrodes placed in positions with signals as low as those measured on the forehead is particularly challenging. Prior to filtering, valid ECG signals are almost completely masked by environmental noise. Also, a valid signal looks very similar to a poor contact condition.
FIG. 2 is a view representing the measurement of ECG and PPG pulses. In a healthy patient, when the sensors are making proper contact, a PPG pulse should follow each ECG pulse by approximately 250 milliseconds, which is about the time it takes blood to flow from the heart to an extremity plus the pre-ejection period time (PEP), PEP being the time between when the ECG signal peaks and the heart muscle contracts. If the PPG signal is lost, as shown, an alarm should be raised. A similar type alarm should be raised if the ECG signals are lost. The loss of these may indicate that a sensor has been removed.
FIG. 3 is a view representing a condition where environmental noise causes erroneous ECG detection. This condition becomes a problem when the environmental noise has characteristics similar to valid ECG pulses, and so cannot be filtered out.
FIG. 4 is a view of poor sensor contact resulting from the recent placement of the sensor. Recent sensor placement causes poor ECG and PPG event detection since the sensors are not yet stable.
FIG. 5 is a view depicting the lack of temporal correlation between ECG and PPG signals. This lack of correlation in a healthy patient may occur as a result poor sensor contact.
Since valid and invalid ECG signals look very similar, simply measuring the signal to noise ratio does not give a good indication of the poor contact condition. Large amounts of environmental noise can also cause false detections, and low signal quality can cause valid heart pulses to be missed. So, algorithms which only check for the existence or lack of detected pulses have poor contact quality detection in large noise environments.
It would be advantageous if more sophisticated detection processes existed that could determine when ECG and PPG signal errors are the result of poor sensor contact.