TEMA 4. ALTERACIONS DE LA CAPNOMETRIA

Com hem comentat prèviament, el CO₂ és un producte directe del metabolisme cel·lular, despesa cardíaca i ventilació, portat als pulmons mitjançant la circulació venosa i espirat; així doncs, canvis sobtats sistèmics del CO₂ –alteracions en la ventilació o circulació- provocarà canvis sobtats en els valors d'EtCO₂.

Això permet que la capnografia sigui usada com a mètode per a la detecció precoç de compromís ventilatori o circulatori. Avaluant la tendència de l'EtCO₂ permet la valoració de la severitat de l'esdeveniment o la resposta al tractament d'aquest.

Diversos autors han citat com a variació significativa de l'EtCO₂ canvis del 10% del valor basal del pacient, EtCO₂ igual o més gran de 50 mmHg o pèrdua d'ona de 15 segons o més.

Les principals causes que disminueixen l'EtCO₂ són a nivell respiratori la hiperventilació o depressió respiratòria amb baixos volums tidal, apnea, fuga al sistema ventilatori i la disminució de la despesa cardíaca; causes de baixa despesa cardíaca podrien ser una hemorràgia, insuficiència cardíaca, hipovolèmia, hipotensió o hipotèrmia. Les maniobres de RCP no efectives també disminueixen l'EtCO₂.

A nivell respiratori, la hiperventilació pot ser conseqüència del dolor, la por, l'ansietat o la hiperventilació pel mateix respirador. El tromboembolisme pulmonar (TEP) també es pot manifestar amb un EtCO₂ baix perquè augmenta l'espai mort ventilat.

Canvis sobtats a una línia plana del capnograma amb lectura quantitativa propera al zero hauria d'alertar d'un esdeveniment significatiu, com ara intubació en esòfag, desplaçament del TET o parada cardíaca. Al pacient amb ventilació mecànica invasiva podria indicar obstrucció del TET o desconnexió del ventilador.

Un augment o elevació de l'EtCO₂ indica retenció de CO₂ a causa d'hipoventilació i/o disminució de la freqüència respiratòria, augment de la despesa cardíaca, millora de la perfusió pulmonar, augment de la temperatura corporal, sedació excessiva.

Augments transitoris de l'EtCO₂ poden ser deguts a convulsions (augment del metabolisme musculoesquelètic), retorn de circulació espontània o reperfusió tissular per exemple en alliberar un torniquet.

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