Pulmonary arterial pressure (PAP)

Pulmonary arteries and arterioles carry venous blood from the right side of the heart to the lungs with the purpose of replenishing the blood oxygen. Oxygenated blood then returns through the pulmonary veins to the left chambers (atrium and ventricle) of the heart to be pumped to the rest of the body. Pulmonary hypertension (PH) is a condition where the pressure inside the pulmonary artery is abnormally high. In response to this increased pressure in the pulmonary artery, the right side of the heart is required to pump with increased force to move blood toward this area of high Pulmonary Arterial Pressure (PAP). This response of the heart results in excessive muscle contraction, stretching of muscle fibers, ultimately leading to increased right atrium and ventricle size. Because the right side of the heart is working harder than normal, its walls thicken (hypertrophy), leading to heart failure and ultimately animal death. These mortality rates are typically very low even though cattle often experience PH. Currently, the impact of PH is primarily quantified in the form of death loss but increasingly there is evidence of reduced performance associated with PH.

Usage

The PAP test is used as an indicator of PH and as a selection/culling criterion. The procedure/technique for performing a Pulmonary Artery Catheterization (PAP test) begins by securing the animal within a chute with a moderate amount of squeeze applied so as to reduce body movement. The pressure applied is monitored throughout the test, adjusting to maintain animal comfort. The head is secured in place with a halter at the level of the point of the shoulder to expose the lateral jugular furrow. Care must be taken to avoid securing the head too high as the animal can often then manipulate its head, resulting in cutting of the catheter within the animal; in turn, resulting in pulmonary foreign body emboli. The proximal lateral aspect of the jugular furrow is scrubbed with a chlorhexidine/alcohol/water/soap solution, and all debris is removed making for clean needle insertion. The jugular vein at the level of the thoracic inlet or just proximal to this is occluded to allow blood flow resistance causing jugular vein distension. The proximal jugular vein is then punctured with a 12-13 gauge, 3.5-inch needle until there is free blood flow from the needle. The needle is then gently threaded down the jugular vein leaving 1 cm of the needle out of the skin to allow for needle control. An intramedic polyethylene catheter (internal diameter 1.19 mm, outside diameter 1.7 mm) is then passed through the needle and into the jugular vein. The catheter tubing is flushed with .9 % NaCl (approximately 1 ml) prior to placing in the needle and is also flushed while the catheter is being gently advanced down the jugular vein. The constant flushing of the .9 % NaCl aids in the movement of the catheter down the vein. The amount of total saline that is injected into the animal during the procedure is minimal and in most cases less than 1 ml. The external end of the catheter is then connected to a transducer via 3-way stopcocks after approximately 1-1.5 feet of the catheter has been passed into the jugular vein. The transducer must be placed at the level equal to the base of the heart, which is approximately just caudal to the point of the elbow. The transducer and catheter are again flushed to assure no air has entered the system. The catheter is then advanced to the distal jugular vein and the first pressure taken to help evaluate catheter location. The catheter is passed into the right atrium through the right A-V valve and into the right ventricle of the heart where the second measurement is taken. The catheter is then advanced from the right ventricle through the pulmonary valve and into the main pulmonary artery. The catheter is allowed to stay in this location until the pressure remains constant, approximately 5 seconds. The location of the catheter within the animal’s vasculature, as well as the pressure reading, is determined by monitoring the pressure changes and characteristics of the pressure wavelengths on a data scope. This allows accurate knowledge of the location of the end of the catheter at all times. Once the catheter reaches the pulmonary artery and the pressure is allowed to stabilize, the mean PAP, systolic and diastolic pressures are recorded. After the PAP is recorded, the catheter is slowly pulled back through the pulmonary valve into the right ventricle. A drop in the mean pressure is noted as well as a distinct pressure wave character change as the catheter is moved from the pulmonary artery into the right ventricle. This mean pressure drop is approximately 6-10 mmHg. If this ventricular drop in pressure is not noted, the catheter placement is re-evaluated. If the pressure remains unchanged, the animal was auscultated for the presence of valvular damage (murmurs) or other respiratory or cardiac pathology.

Hypoxic conditions (low atmospheric oxygen) that stimulate a pulmonary response are usually not seen until approximately 5,000 feet of elevation. For this reason, PAP measurements taken at low elevations (< 5,000 feet) have historically been used only to identify those animals that are very sensitive to hypoxic conditions. These cattle are hypertensive at these lower elevations, as evidenced by an elevated PAP measure, and therefore are not suitable for elevations above 5,000 feet. There is evidence; however, that PAP observations taken between 3,000 and 5,000 feet of elevation are genetically correlated to observations taken at higher elevations and therefore could be used as an indicator trait in the production of a high elevation PAP EPD. Currently, the number of observations of PAP taken below 3,000 feet is insufficient to determine the genetic relationship between these measures and measures taken above 5,000 feet of elevation.

Historically raw PAP measures have been used as a tool for deciding what animals should/could remain at elevation and likely survive. These phenotypic measures were often used as a tool to select animals for breeding as well. The growth of data sets over time, such as at the CSU John E. Rouse Beef Improvement Center and at other high elevation seedstock programs, and the subsequent accumulation of data in centralized locations has enabled the calculation of PAP EPD (e.g. www.angus.org). The release of PAP EPD and a long history of using PAP measures have brought about confusion for many breeders. Susceptibility to pulmonary hypertension as indicated by PAP has a considerable genetic influence with heritability estimates for PAP ranging from .25 to .46 but also indicating that environment has a considerable influence on PAP measures. Therefore, non-genetic factors (i.e. environment) may result in a high PAP score and unsuitability for high elevation environments. These non-genetic effects could include lung damage from bovine respiratory disease and other factors, PAP measurements taken under extreme cold, and feeding of rations for rapid weight gain among others.

Pulmonary Arterial Pressure Measurement Considerations

Cattle moved from low elevations to high elevations should remain at the altitude for a period of 4 weeks or more to accurately PAP test for adaptation to high altitude. Hypoxic pulmonary changes have been seen immediately in cattle as they have been moved to high altitude. However, to most accurately test those that are to be used for breeding, a longer stay at high altitude is recommended. In practicality, it is important for the rancher to know at what elevation the PAP measurement was performed as it has been shown that the PAP measurement will increase as the animal is moved or travels to higher elevations. It is not uncommon to see clinical PH in animals that have a PAP measurement of greater than 49 mmHg. Using repeated measures on the same animal it has been shown that the PAP measurement generally increases 1-2 mmHg per 1,000 feet rise in elevation. For simple guidelines to the rancher and veterinarian, an animal should be tested at an elevation of at least 5,000 feet after 4 weeks if testing for suitability for residing at high elevations. The higher the elevation of the test the more accurate and reliable the outcome.

The equipment, materials and process needed to perform PAP testing in cattle are listed below:

• Cattle chute equipped with a squeeze and scissor head catch is best. Other types of head catches tend to bind the head and neck, making passing of the catheter difficult. The animal should be secured within the chute with a moderate amount of squeeze applied to reduce body movement. Monitoring the squeeze is very important, too little squeeze allows too much movement, making measurement difficult. Too much squeeze can increase thoracic pressure and result in a false high PAP measure. The chutes must allow securing the head with a halter and turning of the head to allow for jugular exposure
• Adequate labor for handling cattle efficiently and safely. This is best done by having one person responsible for running the chute, catching the head, and applying a consistent level of squeeze to the animal during the procedure.
• Two people securing the halter (nose tongs cannot be used) and turning the heads. This allows for one person to pull the head to the side while the other tightens the halter safely securing the head into the correct position.
• Movement and animal handling processes should be conducted in a low stress manner. The use of dogs and cattle prods should be limited to those animals in a life-threatening situation or those that have refused movement. Minimal use of these tools is recommended.

PAP Scheduling Considerations

• No vaccinations should be given in the 2 weeks prior to PAP testing although vaccination can be given at the time of PAP test.
• Avoid testing two weeks after weaning to avoid stress on the animal and resulting potential for elevated PAP.

PAP Data Recording

• Name and address of owner/owners
• Test Location and elevation. Note: If other breeders bring animals to the testing location from other nearby ranches then the location of the cattle’s origin and elevation should be recorded.
• Breed of each animal
• Sex of each animal (e.g. steer, bull, heifer, and cow)
• Date (to enable calculation of age at PAP test)
• Mean PAP, systolic and diastolic pressures
• Sire identification if possible, including the registration number of sire if available
• If the sire is not known, then this should be noted (may occur under commercial production circumstances in an effort to determine whether an animal should remain/reside at altitude).

Inherent risks associated with Pulmonary Arterial Pressure Testing

Complications and accidents are very rare but may include infection at needle site as well as systemic septicemia and/or endocarditis, chute injury, loss of catheter in the vascular system, pulmonary cardiac trauma and emboli, acute cardiac failure, local abscess formation, and death resulting from one of the previously described conditions.

Utilization of the PAP Test for Varying Elevation

Age of Animal Being Tested

When evaluating a PAP score in the context of animals residing at elevation, the age of the animal at the time of testing should always be considered. The PAP test is less predictable as an indicator of disease for animals tested at younger than 10 months of age. An exception to this inconsistency is in those animals that have a high PAP measurement (>49 mmHg). High scores, even at young ages, suggest an animal should not remain, nor be moved to elevations above 5000 feet. For example, if the animal scores a measurement of 49 mmHg or greater there is still a great degree of confidence that this animal is a high-risk candidate for high altitude problems while conversely, animals with low PAP scores may not be indicative of high-altitude suitability. From the perspective of EPD generation, research continues on the genetic relationship between PAP taken at younger ages and those observations taken after 10 months of age. Recent research indicates that while less predictable at ages less than 10 months, there is a genetic relationship between measures taken after 10 months of age and therefore younger measures are useful for genetic prediction.

PAP has been shown to increase with age(Thomas and Speidel, pers. Comm.) and therefore indicate a need for adjusting for age in the context of genetic evaluation.

Phenotypic PAP Score Evaluation

The following chart has been developed to help the breeder evaluate a PAP measurement on an animal for suitability for residing at high altitudes. This must be viewed in the context of animal survival at elevations. An animal may have a high PAP value for reasons other than genetic predisposition (e.g. lung damage as a result of respiratory disease) and therefore not be adapted to living in high altitude environments. As such the chart reflects recommendations for use at elevation based on phenotypic PAP measures rather than genetic recommendations and should be used only for selecting animals that are likely themselves to survive at the elevations listed. In contrast, PAP EPD should be used in the context of genetic improvement and production of progeny likely to survive at higher elevations.

PAP Risk Factor Low Elevation Test Chart

PAP Measurements taken between 3,000- and 4,000-feet elevation should be considered a screening measurement only.

PAP test conducted at elevation <4,000 feet.

PAP	Use at Low Elev	Use at Moderate Elev.	Use at High Elev.	Use at Extreme
Score	< 4,000 feet	4,000-5,000 feet	5,500-7500 feet	>7,500 feet
34-39	Low Risk	Low Risk	Moderate Risk	ModerateRisk
40-45	Low Risk	Moderate Risk	High Risk	High Risk
46-49	Moderate Risk	High Risk	Do Not Use	Do Not Use
>=50	Moderate Risk	High Risk	Do Not Use	Do Not Use

PAP Risk Factor Moderate Elevation Test Chart

PAP test conducted at elevation 4,000-5,500 feet.

PAP	Use at Low Elev	Use at Moderate Elev.	Use at High Elev.	Use at Extreme
Score	< 4,000 feet	4,000-5,000 feet	5,500-7500 feet	>7,500 feet
34-39	Low Risk	Low Risk	Low Risk	Low Risk
40-45	Low Risk	Low Risk	Moderate Risk	Moderate Risk
46-49	Moderate Risk	High Risk	Do Not Use	Do Not Use
>=50	Moderate Risk	High Risk	Do Not Use	Do Not Use

PAP Risk Factor High Elevation Test Chart

PAP test conducted at elevation 5,500-7,000 feet.

PAP	Use at Low Elev	Use at Moderate Elev.	Use at High Elev.	Use at Extreme
Score	< 4,000 feet	4,000-5,000 feet	5,500-7500 feet	>7,500 feet
34-39	Low Risk	Low Risk	Low Risk	Low Risk
40-45	Low Risk	Low Risk	Low/Moderate Risk	Low/Moderate Risk
46-49	Moderate Risk	Moderate Risk	Moderate Risk	High Risk
>=50	Moderate Risk	Moderate Risk	High Risk	High Risk

PAP Risk Factor High Elevation Test Chart

PAP test conducted at elevation 5,500-7,000 feet.

PAP	Use at Low Elev	Use at Moderate Elev.	Use at High Elev.	Use at Extreme
Score	< 4,000 feet	4,000-5,000 feet	5,500-7500 feet	>7,500 feet
34-39	Low Risk	Low Risk	Low Risk	Low Risk
40-45	Low Risk	Low Risk	Low Risk	Low Risk
46-49	Moderate Risk	Moderate Risk	Moderate Risk	Moderate Risk
>=50	Moderate Risk	Moderate Risk	High Risk	High Risk

• Special consideration should be given to the amount of time the animal was exposed to elevation (>5,500 feet) prior to testing. The predictability and repeatability of the PAP measurement improves with longer the exposure to higher elevation (minimum of 4 weeks is required).
• This chart is based on animals greater than 10 months of age.
• Testing of younger animals (<10 months) may result in a greater variability to the predictive ability of the measurement.
• Risk—Defined as the likelihood of an animal developing pulmonary hypertension themselves or being at risk for having a genetic predisposition for the disease.

Original Attribution

R. Mark Enns, Colorado State University
Tim Holt, Colorado State University
Scott E. Speidel, Colorado State University
Milton G. Thomas, Colorado State University