Pressure, Flow, Resistance
December 05, 2009, 19:38 Filed in: Physics
Pressure, Flow, and Resistance Measurement
By Donald R. Elton, MD, FCCP
Lexington Pulmonary and Critical Care
Introduction
To study hemodynamics, it is necessary that you understand what you are measuring. Most hemodynamic measurements are based upon measuring pressures, flows, and resistances.
Pressure
Pressure is defined by physicists as force divided by area. Force can be thought of as mass or weight in simple terms. A given force applied to a small area will result in more pressure than that same force applied over a larger area. This is the basis for using snow shoes that spread out the weight of a man or woman over a larger area of snow to decrease the pressure applied to the snow thus preventing the snow-walker from falling into the snow.
Typical units for pressure include cm H2O and mm Hg which refer to the amount of pressure necessary to raise a column of a given height of either water or mercury. Hemodynamic pressures can be measured by three methods, column methods, occlusion methods, or transducer methods. In a column method, a column of fluid with the top open to the atmosphere is attached to tubing that leads to the place where you want to measure pressure.
Blood pressure was first measured, for example, by sticking a hollow cane pole into the carotid artery of a horse and watching to see how high in the pole the blood rose. Such a blood pressure might be measured in units of feet of blood or cm of blood or whatever.
Central venous pressure can be measured by the column method by using a vertical tube filled with water or IV fluid and attaching it to a catheter that's inserted into the right atrium and seeing how high the water or IV fluid rises in the column which usually has marks for cm of water (cm H2O). The column method has the advantage of low cost but the disadvantage that it can normally only be used to measure things of low pressure so the column size can be limited to something that would fit in the room with the patient.
The occlusion method of pressure measurement is how a sphygmomanometer (blood pressure cuff) works. The cuff is pumped up until flow is stopped and you note the pressure required on a gauge and assume that the occlusion pressure equals the vascular pressure you're interested in.
A transducer is an electrical device that converts pressure into electricity or a measurable electrical resistance. They work by the pressure of interest bending or distorting a strain gauge that changes its electrical properties based upon the degree of distortion. These are more expensive than the other methods but offer the advantage of accuracy and can be used over a wide range of pressures. They must be calibrated frequently during use and must be calibrated correctly if correct data is to be obtained from them. Some transducers can be placed directly into the vessel where pressure is to be measured but most are positioned outside of the patient and the patient's pressures are transmitted to the transducer via fluid filled non-compressible tubing. An external transducer must be placed at the same height above the floor as the place where you want to measure pressure (i.e. at the level of the heart if that's where you want to measure pressure) so the weight of the fluid conduit does not influence the pressure measured at the transducer.
Flow
Flow describes the movement of a volume fluid (gas or blood for example) over a given time. Typical units would be liters per minute or barrels per hour etc. The flow we are most frequently interested in is the cardiac output, normally expressed in liters per minute. We sometimes divide this value by a patient's body surface area (BSA) to come up with the cardiac index which is cardiac output normalized for variations in patient size. The cheapest way to measure cardiac output is to open the patient's chest and let the aortic outflow pour into a bucket for one minute. At the end of a minute you measure the volume of blood in the bucket and call that the cardiac output. This method has fallen out of clinical use. Most cardiac outputs today are measured by thermal dilution. In this method, cool water is injected into the circulation while the temperature is measured downstream. If one knows the distance downstream and can do some calculus, you can relate the rate of cooling and warming at the downstream location to the flow of blood by the temperature measuring point such that the temperature will rise and fall more quickly if the cardiac output is high. Radioactive dyes can also be used in place of cool water but cool water is cheaper, safer, and is easier to find. There are some high-tech ways to measure blood flow that are not yet in widespread clinical use and include doppler techniques and inductive plethysmography.
Resistance
Resistance describes the change in pressure that results from a given flow and is expressed in units of pressure over flow. Airway resistance would be in units of cm H2O per liter per second where cm H2O is the pressure and liters per second is the flow. The pressure part of resistance is found by subtracting the downstream pressure from the upstream pressure so you need to know three things to calculate a resistance: upstream pressure, downstream pressure, and flow. When dealing with vascular (blood vessel) resistances we normally use units of dynes.sec/cm5 by convention. To make life interesting, we first calculate a vascular resistance by taking the upstream - downstream pressures in mm Hg and then divide this result by the flow (normally cardiac output) in liters per minute. This results in a value of units mm Hg / liter / minute. To get this into the standard units of dynes.sec/cm5 we multiply the result by 80. Resistances are not measured directly but are calculated based on pressures and flows that are measured directly.
By Donald R. Elton, MD, FCCP
Lexington Pulmonary and Critical Care
Introduction
To study hemodynamics, it is necessary that you understand what you are measuring. Most hemodynamic measurements are based upon measuring pressures, flows, and resistances.
Pressure
Pressure is defined by physicists as force divided by area. Force can be thought of as mass or weight in simple terms. A given force applied to a small area will result in more pressure than that same force applied over a larger area. This is the basis for using snow shoes that spread out the weight of a man or woman over a larger area of snow to decrease the pressure applied to the snow thus preventing the snow-walker from falling into the snow.
Typical units for pressure include cm H2O and mm Hg which refer to the amount of pressure necessary to raise a column of a given height of either water or mercury. Hemodynamic pressures can be measured by three methods, column methods, occlusion methods, or transducer methods. In a column method, a column of fluid with the top open to the atmosphere is attached to tubing that leads to the place where you want to measure pressure.
Blood pressure was first measured, for example, by sticking a hollow cane pole into the carotid artery of a horse and watching to see how high in the pole the blood rose. Such a blood pressure might be measured in units of feet of blood or cm of blood or whatever.
Central venous pressure can be measured by the column method by using a vertical tube filled with water or IV fluid and attaching it to a catheter that's inserted into the right atrium and seeing how high the water or IV fluid rises in the column which usually has marks for cm of water (cm H2O). The column method has the advantage of low cost but the disadvantage that it can normally only be used to measure things of low pressure so the column size can be limited to something that would fit in the room with the patient.
The occlusion method of pressure measurement is how a sphygmomanometer (blood pressure cuff) works. The cuff is pumped up until flow is stopped and you note the pressure required on a gauge and assume that the occlusion pressure equals the vascular pressure you're interested in.
A transducer is an electrical device that converts pressure into electricity or a measurable electrical resistance. They work by the pressure of interest bending or distorting a strain gauge that changes its electrical properties based upon the degree of distortion. These are more expensive than the other methods but offer the advantage of accuracy and can be used over a wide range of pressures. They must be calibrated frequently during use and must be calibrated correctly if correct data is to be obtained from them. Some transducers can be placed directly into the vessel where pressure is to be measured but most are positioned outside of the patient and the patient's pressures are transmitted to the transducer via fluid filled non-compressible tubing. An external transducer must be placed at the same height above the floor as the place where you want to measure pressure (i.e. at the level of the heart if that's where you want to measure pressure) so the weight of the fluid conduit does not influence the pressure measured at the transducer.
Flow
Flow describes the movement of a volume fluid (gas or blood for example) over a given time. Typical units would be liters per minute or barrels per hour etc. The flow we are most frequently interested in is the cardiac output, normally expressed in liters per minute. We sometimes divide this value by a patient's body surface area (BSA) to come up with the cardiac index which is cardiac output normalized for variations in patient size. The cheapest way to measure cardiac output is to open the patient's chest and let the aortic outflow pour into a bucket for one minute. At the end of a minute you measure the volume of blood in the bucket and call that the cardiac output. This method has fallen out of clinical use. Most cardiac outputs today are measured by thermal dilution. In this method, cool water is injected into the circulation while the temperature is measured downstream. If one knows the distance downstream and can do some calculus, you can relate the rate of cooling and warming at the downstream location to the flow of blood by the temperature measuring point such that the temperature will rise and fall more quickly if the cardiac output is high. Radioactive dyes can also be used in place of cool water but cool water is cheaper, safer, and is easier to find. There are some high-tech ways to measure blood flow that are not yet in widespread clinical use and include doppler techniques and inductive plethysmography.
Resistance
Resistance describes the change in pressure that results from a given flow and is expressed in units of pressure over flow. Airway resistance would be in units of cm H2O per liter per second where cm H2O is the pressure and liters per second is the flow. The pressure part of resistance is found by subtracting the downstream pressure from the upstream pressure so you need to know three things to calculate a resistance: upstream pressure, downstream pressure, and flow. When dealing with vascular (blood vessel) resistances we normally use units of dynes.sec/cm5 by convention. To make life interesting, we first calculate a vascular resistance by taking the upstream - downstream pressures in mm Hg and then divide this result by the flow (normally cardiac output) in liters per minute. This results in a value of units mm Hg / liter / minute. To get this into the standard units of dynes.sec/cm5 we multiply the result by 80. Resistances are not measured directly but are calculated based on pressures and flows that are measured directly.