Dott. Lillehei
Dott. Lillehei

Nel 1954, Lillehei riparò un difetto interventricolare di un bambino sfruttando il cuore e i polmoni del padre. L’evoluzione del bypass cardiopolmonare da ieri a oggi.

Circa 60 anni fa, non era affatto raro l’utilizzo dell’ipotermia negli interventi cardiaci in quanto permetteva un abbassamento delle funzioni metaboliche e vitali. In questo modo il paziente subiva l’operazione chirurgica della durata massima di 10 minuti dopo i quali non era più possibile mantenere le funzioni a livelli così bassi senza provocare ingenti danni da ischemia.

Era il 1954 quando a Gregory Glidden, un neonato di 13 mesi del Minnesota, venne diagnosticato un difetto interventricolare (DIV).  Il difetto interventricolare è un foro tra i due ventricoli che li mette in comunicazione. Spesso si risolve da solo se di piccole dimensioni, ma in caso contrario è necessaria la correzione chirurgica con un patch autologo, omologo o porcino. Se non corretto, il DIV può provocare un iperafflusso al ventricolo destro (shunt sinistro-destro) e quindi ai polmoni che subiscono un trauma pressorio manifestando vasculopatia polmonare ostruttiva. La condizione a questo punto diventa irreversibile ed è quindi importante correggere il difetto celermente per evitare anche che il ventricolo destro sia sottoposto a pressioni sistemiche e ne sopperisca ipertrofizzandosi.

Clarence Walton Lillehei, un cardiochirurgo che aveva partecipato al primo intervento a cuore aperto in ipotermia, decise di attuare quella che all’epoca era vista come un follia. La sua idea era quella di sfruttare le funzioni cardiopolmonari del padre del bambino per curare il difetto cardiaco di quest’ultimo, ma come?

Schema della circolazione sanguigna crociata
Schema della circolazione sanguigna crociata

In anestesia totale per entrambi, la vena cava del neonato veniva collegata tramite delle cannule alla vena femorale del padre per la circolazione venosa, l’arteria femorale paterna alla carotide neonatale. In questo modo il sangue venoso del neonato giungeva ai polmoni paterni dove subiva gli scambi gassosi e poi tramite la pompa cardiaca paterna (il cuore), tornava nella carotide del bambino. Il chirurgo aveva tutto il tempo per correggere il difetto cardiaco e con un campo esangue. Sotto gli occhi scettici dei cardiochirurghi contemporanei, l’intervento andò a buon fine e il neonato non subì lesioni ischemiche. Era nata la circolazione crociata o cross circulation. Sfortunatamente, Gregory morì 11 giorni dopo per una sospetta polmonite.

Lillehei e la sua equipe proseguirono e operarono con la stessa tecnica altre 45 persone tra cui la prima Tetralogia di Fallot. La tetralogia di Fallot è una cardiopatia congenita  cianotica risultante dalla combinazione di DIV, stenosi della valvola polmonare (o agenesia), trasposizione aortica e ipertrofia ventricolare destra (conseguenza delle prime tre caratteristiche). La terapia consiste nella ricostruzione anatomica sempre nei tempi più brevi possibili onde evitare ipertensione ventricolare ed embolie paradosse.

Nel frattempo, in altre parti del mondo, furono inventati i primi ossigenatori a gorgogliamento, dei cilindri solitamente di vetro dentro i quali veniva fatto passare il sangue del paziente che veniva a contatto con bolle di ossigeno. In questo modo il sangue si sarebbe ossigenato ma lo scoppio delle bolle avrebbe traumatizzato i globuli rossi, talvolta rompendoli e provocando quindi più danni che benefici.

Più tardi, l’ossigenatore a gorgogliamento fu sostituito da quello a membrana e/o a fibre cava, costituito da materiali capaci  di mettere in contatto sangue e una miscela di ossigeno senza provocare la denaturazione dei globuli rossi. In questo modo la funzione di trasporto di ossigeno non veniva a mancare.

Ma il problema della circolazione extracorporea non aveva a che fare solamente con la sostituzione dei polmoni. Le prime pompe per la sostituzione della funzione cardiaca erano cilindri pressurizzati che pompavano il sangue nel circolo arterioso tramite l’uso di manovelle. Con l’avvento dell’elettricità e l’avanzare della tecnologia,  le pompe cardiache divennero via via più piccole ed essenziali: la pompa a rulli, la pompa centrifuga e la pompa d impeller.

Pompa a rulli. Consiste di  due rulli che comprimono un tubo alternativamente in una struttura a forma di cavallo. In base alla velocità di rotazione dei due rulli, si ottiene una portata diversa (calcolata sulla base di quella del paziente). E’ necessario che i rulli siano occlusivi a sufficienza ma allo stesso tempo non troppo: nel primo caso, se non sono abbastanza occlusivi si avrò il fenomeno del backflow cioè un rigurgito di flusso in senso contrario a quello della pompa; nel secondo caso, cioè se i rulli sono troppo occlusivi, gli elementi ematici, prevalentemente i globuli rossi, subiranno un trauma con gravi conseguenze sull’ossigenazione e sulla coagulazione.

Pompa centrifuga. E’ formata da più coni sopravvoposti che ruotano grazie alla pressione del sangue in entrata e che generano un flusso preload-dipendente (il preload è il precarico). La pompa non è occlusiva ma in caso di flusso in entrata debole o nulla, la pompa si ferma.

Pompa ad impeller. Di aspetto esteriore simile alla pompa centrifuga, contiene delle lame disposte a raggiera che ricevono una spinta proporzionale al flusso in entrata (come quando si soffia su una girandola!). Allo stesso modo della pompa centrifuga, la pompa si ferma se non riceve flusso in entrata.

E’ ovviamente importante ricordare che il sangue, forse uno dei tessuti più importanti del corpo umano, quando viene a contatto con una superficie diversa da quella in cui si trova abitualmente (quindi i vasi sanguigni), scatena una reazione infiammatoria che provoca in prima istanza la coagulazione. Ed è questo infatti il problema più grande associato alla circolazione extracorporea: è necessario che il paziente sia scoagulato in modo da prevenire la formazione di trombi che possono raggiungere facilmente il circolo cerebrale, il circolo coronarico e quello renale (e ovviamente anche tutti gli altri organi) provocandone ischemia. La scoagulazione di solito prevede l’uso di eparine e antitrombine e viene controllata frequentemente grazie all’uso di sistemi come l’ACT (tempo di coagulazione attivato). Anche l’emogasanalisi è di grande rilevanza in quanto permette di regolare e controllare la funzionalità polmonare e renale: emoglobina ed ematocrito, pressioni parziali di ossigeno e anidride carbonica, potassio, lattati, acidi e basi sono i essenziali.

Il bypass cardiopolmonare è ora più studiato e più frequente e nei centri con poli cardiochirurgici è una pratica quotidiana.

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