Techniques For Pressure Measurement Esophageal Gastric and Transdiaphragmatic Pressures

Scientific basis. Transdiaphragmatic pressure (Pdi) is defined as the difference between Ppl and Pab (13) and, in practice, is generally equated to the difference between Pes and Pga, so that Pdi = Pga — Pes (where Pes is usually, but not always, negative). This is contrary to most pressures across a structure, which are taken at a direction such that positive pressures inflate (e.g., positive transpulmonary pressures inflate the lung). For this reason Pdi is also sometimes defined as Pdi = Pes — Pga. As the diaphragm is the only muscle in which contraction simultaneously lowers Pes and increases Pga, an increase in Pdi is, in principle, the result of diaphragmatic contraction unless there is passive stretching. An inspiratory effort produced with a completely passive unstretched diaphragm is associated with a negative change in Pes and Pga but no change in Pdi. This assumes that changes in Pes or Pga induced by mechanisms other than diaphragm contraction are uniformly transmitted across the diaphragm from one compartment to the other. This is probably true when the diaphragm is relaxed (6, 13) at functional residual capacity (FRC), but may be modified when the diaphragm is stretched, as at low lung volumes.

Methodology. Pes and Pga are most often measured by passing a pair of probes, generally balloon catheters (see previous passages), through the nose, following local anesthesia of the nasal mucosa and pharynx. Their position is usually assessed by asking the subject to perform sharp sniff maneuvers while monitoring the signal on an oscilloscope or computer screen. A simple technique is to advance both probes well into the stomach, as judged by a positive deflection during a sniff and then to withdraw one of them until the sniff-related pressure deflection first becomes negative, indicating that the balloon has entered the esophagus. It is then withdrawn a further 10 cm. The validity of the Pes measurement can be checked by matching Pes to Pao during static Mueller (inspiratory) maneuvers (the dynamic occlusion test) (6, 12, 14). Displacement of balloons is minimized by taping the catheters to the nose. The distance between the nostril and the tip of the balloons varies with the size of the subject, but is usually 35-40 cm for Pes and 50-60 cm for Pga in adults.

Placing the probes becomes more difficult when the subject cannot perform voluntary inspiration (e.g., with anesthetized patients, diaphragmatic paralysis, cognitive impairment, or muscle incoordination). The pressure signals during a swallow can then be useful: A balloon is positioned in the esophagus if swallowing is associated with a slow, powerful rise in pressure, whereas if this does not occur the balloon is likely to be in the stomach. Measurement of balloon distance from the nostril can be a useful indication of its position.

It is advisable to measure Pes and Pga separately by using two pressure transducers, with Pdi derived from a third differential pressure transducer or reconstructed electronically offline. This allows the investigator to monitor balloon position and detect confounding events such as esophageal spasms, as well as recording the three pressures independently. Resting Pga is usually positive with respect to atmosphere due to hydrostatic pressure in the abdomen. For respiratory muscle measurements Pga is conventionally taken as zero at resting end expiration.

Advantages. Pdi is specific for diaphragm contraction (see previous passages). Separate measurements of Pes and Pga provide information on the components of this contraction and Pes on the inspiratory driving pressure (Pes/Pdi ratio).

Disadvantages. The procedures require the subject's cooperation and occasionally untrained healthy volunteers can fail to increase Pdi because of lack of coordination, in the absence of any diaphragmatic abnormality (25). This is, however, unusual during the inspiratory phase of quiet breathing at rest. The measurements are mildly uncomfortable, both initially (when swallowing the catheters) and during studies. However, the discomfort of swallowing a thin catheter is small compared with other established medical procedures and scarcely "invasive." Good-quality equipment and adequate practice minimize the discomfort, but some skill is necessary and passing the probes can be time-consuming. Particular care must be taken in patients with impaired swallowing, as well as esophageal diseases, or disorders at the level of the gastro-esophageal sphincter.

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