A photoplethysmogram is an optically obtained plethysmogram

Plethysmograph

A plethysmograph is an instrument for measuring changes in volume within an organ or whole body .-Lungs:...

, a volumetric measurement of an organ. A PPG is often obtained by using a pulse oximeter

Pulse oximeter

A pulse oximeter is a medical device that indirectly monitors the oxygen saturation of a patient's blood and changes in blood volume in the skin, producing a photoplethysmograph. It is often attached to a medical monitor so staff can see a patient's oxygenation at all times...

 which illuminates the skin

Skin

-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...

 and measures changes in light absorption (Shelley and Shelley, 2001). A conventional pulse oximeter monitors the perfusion of blood to the dermis and subcutaneous tissue of the skin.
With each cardiac cycle

Cardiac cycle

The cardiac cycle is a term referring to all or any of the events related to the flow or blood pressure that occurs from the beginning of one heartbeat to the beginning of the next. The frequency of the cardiac cycle is described by the heart rate. Each beat of the heart involves five major stages...

 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

Light-emitting diode

A light-emitting diode is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting...

 (LED) and then measuring the amount of light either transmitted or reflected to a photodiode. Each cardiac cycle appears as a peak, as seen in the figure. Because blood flow to the skin can be modulated by multiple other physiological systems, the PPG can also be used to monitor breathing, hypovolemia

Hypovolemia

In physiology and medicine, hypovolemia is a state of decreased blood volume; more specifically, decrease in volume of blood plasma...

, and other circulatory conditions (Reisner, et al., 2008). Additionally, the shape of the PPG waveform differs from subject to subject, and varies with the location and manner in which the pulse oximeter is attached.

Sites for measuring PPG

While pulse oximeter

Pulse oximeter

A pulse oximeter is a medical device that indirectly monitors the oxygen saturation of a patient's blood and changes in blood volume in the skin, producing a photoplethysmograph. It is often attached to a medical monitor so staff can see a patient's oxygenation at all times...

s are a commonly used medical device the PPG derived from them is rarely displayed, and is nominally only processed to determine heart rate. PPGs can be obtained from transmissive absorption (as at the finger tip) or reflective (as on the forehead).

In outpatient settings, pulse oximeters are commonly worn on the finger. However, in cases of shock, hypothermia

Hypothermia

Hypothermia is a condition in which core temperature drops below the required temperature for normal metabolism and body functions which is defined as . Body temperature is usually maintained near a constant level of through biologic homeostasis or thermoregulation...

, etc. blood flow to the periphery can be reduced, resulting in a PPG without a discernible cardiac pulse. In this case, a PPG can be obtained from a pulse oximeter on the head, with the most common sites being the ear, nasal septum

Nasal septum

The nasal septum separates the left and right airways in the nose, dividing the two nostrils.It is depressed by the Depressor septi nasi muscle.-Composition:The fleshy external end of the nasal septum is sometimes also called columella....

, and forehead.

PPGs can also be obtained from the vagina and esophagus

Esophagus

The esophagus is an organ in vertebrates which consists of a muscular tube through which food passes from the pharynx to the stomach. During swallowing, food passes from the mouth through the pharynx into the esophagus and travels via peristalsis to the stomach...

.

Uses

Monitoring heart rate and cardiac cycle

Because the skin is so richly perfused, it is relatively easy to detect the pulsatile component of the cardiac cycle. The DC component of the signal is attributable to the bulk absorption of the skin tissue, while the AC component is directly attributable to variation in blood volume in the skin caused by the pressure pulse of the cardiac cycle.

The height of AC component of the photoplethysmogram is proportional to the pulse pressure, the difference between the systolic and diastolic pressure in the arteries. As seen in the figure showing premature ventricular contraction

Premature ventricular contraction

A premature ventricular contraction , also known as a premature ventricular complex, ventricular premature contraction , ventricular premature beat , or extrasystole, is a relatively common event where the heartbeat is initiated by the heart ventricles rather than by the sinoatrial node, the...

s (PVCs), the PPG pulse for the cardiac cycle with the PVC results in lower amplitude blood pressure

Blood pressure

Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...

 and a PPG. Ventricular tachycardia

Ventricular tachycardia

Ventricular tachycardia is a tachycardia, or fast heart rhythm, that originates in one of the ventricles of the heart...

 and ventricular fibrillation

Ventricular fibrillation

Ventricular fibrillation is a condition in which there is uncoordinated contraction of the cardiac muscle of the ventricles in the heart, making them quiver rather than contract properly. Ventricular fibrillation is a medical emergency and most commonly identified arrythmia in cardiac arrest...

can also be detected.

Monitoring respiration

Respiration affects the cardiac cycle by varying the intrapleural pressure, the pressure between the thoracic wall and the lungs. Since the heart resides in the thoracic cavity between the lungs, the partial pressure of inhaling and exhaling greatly influence the pressure on the vena cava and the filling of the right atrium. This effect is often referred to as normal sinus arrhythmia.

During inspiration, intrapleural pressure decreases by up to 4 mm Hg, which distends the right atrium, allowing for faster filling from the vena cava, increasing ventricular preload, and increasing the stroke volume. Conversely during expiration, the heart is compressed, decreasing cardiac efficiency and reducing stroke volume. However, the overall net effect of respiration is to act as pump for the cardiovascular system. When the frequency and depth of respiration increases, the venous return increases, leading to increased cardiac output. (Shelley, et al., 2006)

Monitoring depth of anesthesia

Anesthesiologist must often judge subjectively whether a patient is sufficiently anesthetized for surgery. As seen in the figure if a patient is not sufficiently anesthetized the sympathetic nervous system response to an incision can generate an immediate response in the amplitude of the PPG.

Monitoring hypo- and hypervolemia

Shamir, Eidelman, et al. studied the interaction between inspiration and removal of 10% of a patient’s blood volume for blood banking before surgery (Shamir, Eidelman et al. 1999). They found that blood loss could be detected both from the photoplethysmogram from a pulse oximeter and an arterial catheter. Patients showed a decrease in the cardiac pulse amplitude caused by reduced cardiac preload during exhalation when the heart is being compressed.

External links

A student project: building a device for collecting PPGs

Database of real PPG data obtained with a finger pulse oximeter