equation = (-0.4692*cprop+1.4919)*(pco2-tc)+(so+a/(po2-po))*(pco2-tp)+dw == k*vco2/(pco2*(1-vdanat-vdalvratio))-(psv-vth*r-peepi)*fr/(1/cl + 1/ccw);

fpco2[r_,dw_,a_,fr_,po_,tc_,tp_,cprop_,po2_,peepi_,so_,vth_,psv_,cl_,ccw_,k_,vco2_,vdanat_,vdalvratio_] := Evaluate[equation];

Pane[
Manipulate[
    Column[{
        Pane[{"Test =",solution}]}],
    {{cl,0.075,"CL"},0,2,Appearance->"Labeled"},
    {{ccw,0.125,"CCW"},0,2.5,Appearance->"Labeled"},
    {{r,5,"R"},0,10,Appearance->"Labeled"},
    {{peepi,0,"PEEPi"},0,20,Appearance->"Labeled"},
    {{po2,100,"PO2"},60,140,Appearance->"Labeled"},
    {{vth,0.2,"VTH"},0,1,Appearance->"Labeled"},
    {{psv,0,"PSV"},0,15,Appearance->"Labeled"},
    {{cprop,0.5,"CPROP"},0,3,Appearance->"Labeled"},
    {{dw,7,"DW"},0,15,Appearance->"Labeled"},
    {{a,16.7,"A"},0,20,Appearance->"Labeled"},
    {{fr,20,"FR"},0,30,Appearance->"Labeled"},
    {{po,30,"PO"},0,50,Appearance->"Labeled"},
    {{tc,34,"TC"},0,50,Appearance->"Labeled"},
    {{tp,34,"TP"},0,50,Appearance->"Labeled"},
    {{so,0,"SO"},0,2,Appearance->"Labeled"},
    {{k,0.75,"K"},0,5,Appearance->"Labeled"},
    {{vco2,0.25,"VCO2"},0,3,Appearance->"Labeled"},
    {{vdanat,0.12,"VDANAT"},0,1,Appearance->"Labeled"},
    {{vdalvratio,0.3,"VDALV"},0,1,Appearance->"Labeled"},
    ContentSize->Automatic]
]

fpco2[5,7,16.7,20,30,34,34,0.5,100,0,0,0.2,10,0.075,0.125,0.75,0.25,0.12,0.3]

7 + 1.49587 (-34 + pco2) == -8.4375 + 0.323276/pco2

NSolve[fpco2[5,7,16.7,20,30,34,34,0.5,100,0,0,0.2,10,0.075,0.125,0.75,0.25,0.12,0.3],pco2]

{{pco2 -> 23.6891}, {pco2 -> -0.00912287}}