Mechanical Work on the Lungs and Work of Breathing with Positive End-Expiratory Pressure and Continuous Positive Airway Pressure

doi:10.1378/chest.76.3.251

Chest

Volume 76, Issue 3, September 1979, Pages 251-256

https://doi.org/10.1378/chest.76.3.251 Get rights and content

The mechanical work on the lung required during spontaneous breathing with positive end-expiratory pressure (PEEP) was compared with different methods of continuous positive airway pressure (CPAP) in nine young healthy athletes (surfers) at levels of 5, 10, 15, and 20 cm H₂O. At the level of 20 cm H₂O, PEEP increased the mean total work per minute by 116 percent and the total work per liter by 121 percent. The percent increase rose linearly with the level of PEEP. In contrast, with methods of CPAP that maintained the airway pressure (Paw) constant, the total work per minute decreased by 45 percent at a PEEP of 10 cm H₂O and remained at this level with PEEP of 15 and 20 cm H₂O. Use of PEEP did not increase the functional residual capacity (FRC) in these spontaneously breathing subjects. In contrast, CPAP resulted in a rise in FRC proportional to the level of CPAP. This suggests that CPAP must be applied in a manner that maintains Paw constant to provide optimal assistance to ventilation.

Section snippets

Materials and Methods

The protocol for this experiment and the safeguards taken were approved by the Committee on Investigations and Activities Involving Human Subjects. Informed consent was obtained from ten male paid volunteers between 21 and 24 years of age. All were in excellent condition, as evidenced by their active pursuit of the vigorous sport of surfing. Nine of the ten were certified scuba divers.

A history was taken, and a physical examination and screening studies of ventilatory function were done on all

Rationale for Computations

The computer calculated the respiratory rate, the TV, and the minute ventilation as previously described.⁸ The calculations of work were modified for this experiment. We were interested in the work done on the lung by the subject’s effort. The work done by the subject and the system for CPAP equals:

$\int_{I}^{E} (P_{awo} - P_{P 1}) dv$

where P_awo is the pressure at the airway opening, P_pl is the pleural pressure, and dv is the flow into and out of the patient. The work done by the system for CPAP is:

$\int_{I}^{E} (P_{awo} - P_{bs}) dv$

Group 1: Unconditioned Response

In group 1, in which no attempt was made to control the respiratory rate or TV, the subjects responded in two different ways to PEEP and to CPAP with an air flow of 60 L/min with the “elastic bellows.” Four of the subjects maintained their respiratory rate, TV, and minute ventilation fairly constant while receiving PEEP and CPAP. Two of the subjects increased their respiratory rate, TV, and minute ventilation with increasing levels of CPAP. Studies on these two subjects were discontinued at

Discussion

The effects of the alteration in methods of ventilation on work can be measured in two ways, ie, (1) by measuring the change in oxygen consumption, and (2) by calculating mechanical work. In this study, we were interested in changes in mechanical work done on the lungs by the subject, not by the breathing apparatus used to assist ventilation. As discussed under the rationale, we ignored the force provided by systems for CPAP. In addition to providing the force to do work on the lung, the

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Resident.

Professor of Surgery and Bioengineering.

Assistant Clinical Professor of Surgery.

Supported by Public Health Service grants GM 17284 and HL 13172 from the National Institutes of Health.

Manuscript received January 26; accepted January 29.

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Chest

Clinical InvestigationsMechanical Work on the Lungs and Work of Breathing with Positive End-Expiratory Pressure and Continuous Positive Airway Pressure

Section snippets

Materials and Methods

Rationale for Computations

Group 1: Unconditioned Response

Discussion

J Thorac Cardiovasc Surg

Chest

Chest

Treatment of the idiopathic respiratory distress syndrome with continuous positive airway pressure

N Engl J Med

Clinical Investigations
Mechanical Work on the Lungs and Work of Breathing with Positive End-Expiratory Pressure and Continuous Positive Airway Pressure