REPLY
Weaning Patients from Mechanical Ventilation Using Gastric pH
Z. Mohsenifar, MD;
Spencer K. Koerner, MD; and
Michael I. Lewis, MD
1 March 1994 | Volume 120 Issue 5 | Pages 438-439
IN RESPONSE:
Dr. Rochester raises several interesting questions and makes many insightful comments. Gastric PCO2 is the main determinant of gastric intramural pH. As we stated [1], a rising gastric PCO2 would have predicted 9 of 11 weaning failures and an elevated baseline gastric PCO2 would have predicted the remaining 2 failures. Independent of weaning, hypotension or low cardiac states would be expected to be detrimental to splanchnic blood flow, which would eventually result in a rise in gastric CO2. Dead space in the nasogastric tube and bicarbonate flux are possible sources of error in measuring real-time gastric PCO2.
We believe that the blood flow demanded by the respiratory pump to support the excessive work of breathing during weaning trials was primarily responsible for the redistribution of blood flow from the splanchnic bed. Such diversion can occur in states other than hypotension or low cardiac output. In an animal model of oleic acid-induced pulmonary edema, Magder and colleagues [2] reported significantly increased respiratory muscle blood flow, coupled with a reduction in splanchnic blood flow without a significant decrease in blood pressure. In support of our hypothesis, three patients who failed weaning trials had restoration of gastric intramural pH to normal when assisted mechanical ventilation was resumed.
We will respond to Dr. Tobin's major points: We designed our study to investigate the utility of gastric intramural pH in predicting weaning success or failure and clearly stated that our protocol differed from Yang and Tobin's [3]. Of interest, Lee and colleagues [4] found the ratio of frequency/tidal volume not to be highly predictive using their protocol.
As stated [1], we used a 7200 A ventilator (Puritan-Bennett, Carlsbad, California), which measures BTPS-corrected tidal volume using an eight-breath running average. All patients received the same FIO2 as that was provided by assisted mechanical ventilation.
Nathan and colleagues [5] found an overcompensation of pressure support related to upper airway obstruction and increased work of breathing after extubation. The level of pressure support (7 to 8 cm H2O), calculated based on the product of airway resistance and peak spontaneous flow rate, has been reported to overcome the added respiratory work resulting from an endotracheal tube and circuit resistance [6].
Patients were managed in a standard way by physicians blinded to the results of gastric intramural pH and gastric PCO2 to prevent any bias. Of importance, however, they showed a high false-positive rate (even in Yang and Tobin's report), which unfortunately results in failed extubations.
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Author and Article Information
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Cedars-Sinai Medical Center; Los Angeles, CA 90048
1. Mohsenifar Z, Hay A, Hay J, Lewis MI, Koerner SK. Gastric intramural pH as a predictor of success or failure in weaning patients from mechanical ventilation. Ann Intern Med. 1993; 119:794-8.
2. Magder S, Erian R, Roussos C. Respiratory muscle blood flow in oleic acid-induced pulmonary edema. J Appl Physiol. 1986; 6:1849-56.
3. Yang KL, Tobin MJ. A prospective study of indexes predicting the outcome of trials of weaning from mechanical ventilation. N Engl J Med. 1991; 324:1445-50.
4. Lee KH, Hui KP, Chan TB, Tan WC, Lim TK. Frequency/tidal volume ratio did not predict extubation outcome (Abstract). Am Rev Respir Dis. 1993; 147:A873.
5. Nathan SD, Ishaaya AM, Koerner SK, Belman MJ. Prediction of minimal presure support during weaning from mechanical ventilation. Chest. 1993; 103:1215-9.
6. Fiastro JF, Habib MP, Quan SF. Pressure support compensation for inspiratory work due to endotracheal tubes and demand for continuous positive airway pressure. Chest. 1988; 93:499-505.
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