THE l ' l ~ Y ~ I l ~ 1 4 ~ G Y
OF ASCIDIANS
69
critirizcd by Krijgstiit~n oil t,h(a grouiidrt of over-simplification and that
'' they adjusted their (wbitxary) c~olirttants to tit the observations . . . ".
Kriebel ( 1 Nib) hus recently rwived interest in the back pressure
theory and p i ~ t ~ s
forward several pieces of' evidence in its favour :
(a) During rcversiil pausc Mood tnay be seen to flow back into the
heart indicating a prcssrire build-up in the arterial end.
(b) Ciiierntttogrc~phic~ ima1,ysirt of beat ing hearts shows that during
the first fcw bwts after roverstit thtw i H a fall-off in the stroke volume
of blood s~rggosting a full in V ~ I I O U R pressure.
(c) A loweritig of intrib-carditrc pressure by removal of blood causes
a change in thv pulsation period (interval between reversals).
(d) Raising the intra-cardiac pressure by occluding the artery causes
a reversal.
(e) In experiments with cannulated hearts i t can be shown that
pacemakers are very sensitive to changes in pressure (Fig. 17) and
pacemaker frequency may be altered by adjusting the blood volume or
intra-cardiac pressure.
Critics of the back pressure theoiy (set. Millar, 1952, 1953b ; Krijgsman, 1 ! 6 6 ; Krijgsrnan und Krijgsmim, 1957 ; Anderson, 1968) point out
that isolatcd Iitwts opcmed to eiisur(. (~quiL1 pressure inside and outside
will still exhibit rcversuls. Furt hernwre, if hearts are ligatured in the
middle in situ the two end ptwt~makcis will beat toward the centre of
the heart, for srvcrnl hours and in spit(> of the prcssure build-up do not
exhibit reversds.
Although back pressure csnnot bc mtirely respondble for heart
beat reversal it! may be ;L contributoray ftic+tor b,v inducing mechanical
Rtimulation of changing frequenc*,y in 1)accmakers (Andormn, 19118).
Pacemaker fatigue hart been siiggcstetl MI thc cause of rcvcrHal by
Krijgsman (1966) ant1 Krijgsmaii and Krijpman (1967). Their conclusions are bused on the fact that isolated hearts continue to reverse and
that in half-he& preparations there are periods of activity alternating
with periods of rest. Since there is no opposing pacemaker in a half
Itcart it is assurncd thiit the periods of rest are the result of fatigue of
the active paceniuker.
In natural conditions such priotls of rest following fatigue would
be very brief indeed as during dominance of any one pacemaker the
other end of the heart is being driven and continues to contract.
Krijgsman and Krijgsman concluded that, pacemaker activity and
contractility art' separate functions and that, fatigue is not in the
contractile elenitwt8 but in the active pacemaker component. They
suggested thereforc that thc paccmalters respond to Rome Hecretion or
OF ASCIDIANS
69
critirizcd by Krijgstiit~n oil t,h(a grouiidrt of over-simplification and that
'' they adjusted their (wbitxary) c~olirttants to tit the observations . . . ".
Kriebel ( 1 Nib) hus recently rwived interest in the back pressure
theory and p i ~ t ~ s
forward several pieces of' evidence in its favour :
(a) During rcversiil pausc Mood tnay be seen to flow back into the
heart indicating a prcssrire build-up in the arterial end.
(b) Ciiierntttogrc~phic~ ima1,ysirt of beat ing hearts shows that during
the first fcw bwts after roverstit thtw i H a fall-off in the stroke volume
of blood s~rggosting a full in V ~ I I O U R pressure.
(c) A loweritig of intrib-carditrc pressure by removal of blood causes
a change in thv pulsation period (interval between reversals).
(d) Raising the intra-cardiac pressure by occluding the artery causes
a reversal.
(e) In experiments with cannulated hearts i t can be shown that
pacemakers are very sensitive to changes in pressure (Fig. 17) and
pacemaker frequency may be altered by adjusting the blood volume or
intra-cardiac pressure.
Critics of the back pressure theoiy (set. Millar, 1952, 1953b ; Krijgsman, 1 ! 6 6 ; Krijgsrnan und Krijgsmim, 1957 ; Anderson, 1968) point out
that isolatcd Iitwts opcmed to eiisur(. (~quiL1 pressure inside and outside
will still exhibit rcversuls. Furt hernwre, if hearts are ligatured in the
middle in situ the two end ptwt~makcis will beat toward the centre of
the heart, for srvcrnl hours and in spit(> of the prcssure build-up do not
exhibit reversds.
Although back pressure csnnot bc mtirely respondble for heart
beat reversal it! may be ;L contributoray ftic+tor b,v inducing mechanical
Rtimulation of changing frequenc*,y in 1)accmakers (Andormn, 19118).
Pacemaker fatigue hart been siiggcstetl MI thc cause of rcvcrHal by
Krijgsman (1966) ant1 Krijgsmaii and Krijpman (1967). Their conclusions are bused on the fact that isolated hearts continue to reverse and
that in half-he& preparations there are periods of activity alternating
with periods of rest. Since there is no opposing pacemaker in a half
Itcart it is assurncd thiit the periods of rest are the result of fatigue of
the active paceniuker.
In natural conditions such priotls of rest following fatigue would
be very brief indeed as during dominance of any one pacemaker the
other end of the heart is being driven and continues to contract.
Krijgsman and Krijgsman concluded that, pacemaker activity and
contractility art' separate functions and that, fatigue is not in the
contractile elenitwt8 but in the active pacemaker component. They
suggested thereforc that thc paccmalters respond to Rome Hecretion or