Electrical power amplifier for continuous cardiac output monitoring

A continuous cardiac output monitor includes a general-purpose monitoring console with local display and communication facilities, and a module removably interfacing with the console to configure the latter for performing continuous cardiac output monitoring. The module includes a switch-mode high efficiency power amplifier for providing electrical heating power at a selected voltage, frequency, and wave form to a heating element of a continuous cardiac output monitoring catheter, which catheter at a distal end portion thereof is immersed in the blood flow of a patient. The catheter effects a temperature transient in the patient's blood flow by the controlled application of electrical resistance heating utilizing electrical power from the power amplifier, and this temperature transient is sensed and used to derive a value for the patient's cardiac output.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to power amplifiers or electrical power 
supplies. More particularly, the present invention is in the field of 
power amplifiers for supplying electrical power at a chosen frequency or 
frequencies and at a selected and variable voltage level or levels. Still 
more particularly, the present invention is in the field of regulated 
electronic power amplifiers or supplies which supply a selected level of 
electrical power at a controlled frequency and variably controlled voltage 
to an electrical load. With particularity to the application environment 
of such power amplifiers or power supplies, the present invention relates 
to such a power amplifier which is particularly configured and constructed 
for use in the medical environment to supply resistive heating power to a 
continuous cardiac output monitoring catheter. Thus, the present invention 
also is in the field of apparatus and method for monitoring the cardiac 
output of a human patient. 
2. Related Technology 
Conventionally, cardiac output monitoring for patients experiencing a 
cardiac crisis, such as may occur over a period of time following a 
coronary occlusion, is to periodically inject a quantity (or bolus) of 
chilled saline solution into the patient's circulatory system at a 
selected location. A temperature monitoring catheter is used at another 
selected location to sense the temperature-versus-time relationship of the 
blood flow so that a value of cardiac output can be derived. This 
technique is known as thermodilution, and provides a good signal to noise 
ratio of the pulmonary blood flow as it is cooled by the chilled saline 
solution as compared to the normal temperature of blood flow in the 
pulmonary artery prior to and after the injection of the bolus of saline. 
A relationship known as the Stewart-Hamilton equation is used to derive 
the cardiac output value. 
Unfortunately, this conventional technique is dependent on the skill of the 
person who performs the saline injection. That is, the rate and uniformity 
over time with which the bolus of saline solution is injected can 
influence the accuracy of the result. Consequently, a number of such tests 
over a period of time are used to determine an average value of cardiac 
output. Detection of a trend or long-term change (over a period of hours, 
for example) in cardiac output is very difficult with this conventional 
technique. Also, the injection of chilled saline may have the disadvantage 
for some patients of adding a relatively large quantity of water to the 
blood stream. This water must be removed by the patient's kidneys. 
Another conventional cardiac output monitoring technique utilizes a 
catheter instilled through the right atrium and right ventricle of the 
heart, and from the heart into the pulmonary artery. A resistance heating 
element externally carried by this catheter is used to intermittently 
slightly heat the pulmonary blood flow from the heart as this blood flows 
toward the patient's lungs. Downstream of the heating element, the 
catheter carries a temperature sensing element. The 
temperature-versus-time relationship of the sensed blood flow can 
similarly be used to derive a value for cardiac output. This technique has 
the advantage of providing substantially continuous monitoring of cardiac 
output. However, the signal-to-noise ratio of the heated blood temperature 
in comparison to the normal body temperature of blood flow existing prior 
to and after an interval of heating is very low. This must be the case 
because the blood cannot be heated excessively or damage will result to 
formed blood cells. Consequently, techniques have been developed to heat 
the pulmonary blood flow on a pseudo-random basis, so that the resulting 
temperature variations can be detected and distinguished from the 
otherwise normal slight variations in temperature of the pulmonary blood 
flow. 
For reasons of patient safety and avoidance of electromagnetic interference 
with or effect upon other monitoring and treatment apparatus which may 
also be in the medical environment around a patient, a frequency of 100 
KHz has been recognized as the most desirable for use in powering the 
resistance heating element of the monitoring catheter. With this fixed 
frequency of applied power for the resistance heating element of the 
catheter, a variable voltage level is used to control the power level of 
energy liberated at the heating element into the patient's pulmonary blood 
flow. This control on the level of heating energy liberated into the 
patient's blood flow must be carefully controlled because the actual rate 
of blood flow circulation for the patient may be decreased or impaired, so 
that overheating must be avoided. 
In addition to the above, it is increasingly recognized that the modern 
medical environment is restrictively complex. That is, the complexity of 
medical monitoring and treatment apparatus which must be used with some 
seriously ill or injured patients restricts access to the patient and 
presents the risk of error or malfunction of the apparatus. Additionally, 
hospitals and clinics face a significant burden in maintenance, service, 
storage, and logistical planning of the availability of this complex and 
expensive medical apparatus. As a result, an increasingly popular trend in 
the hospital, clinical, and portable medical treatment environments (fire 
departments, emergency medical teams, and military portable field 
hospitals, for example) is to use a general purpose monitoring device 
which can be electronically configured to serve a variety of monitoring 
functions. 
Configuration of the monitoring device is accomplished by simply plugging 
into the console of the general purpose monitoring device one or more 
modules containing the circuitry and stored information necessary to 
accomplish particular monitoring functions. In the hospital and clinical 
environment, for example, this technology has the advantage that the 
general purpose monitors may be installed in or left in the patient rooms 
and in the emergency or critical care areas, for example. These monitors 
need not be moved about the hospital or clinic. The monitors are simply 
configured to perform various monitoring functions as are necessitated by 
the condition of the particular patient by plugging the appropriate 
modules into the monitor consoles. Only the modules need to be moved about 
the hospital or clinic. The modules themselves are comparatively small, 
light and inexpensive. Storage of the modules when they are not in use 
requires far less space than does the conventional monitoring equipment. 
Also, movement of the necessary configuration modules about the hospital 
or clinic environment does not present nearly the burden for hospital 
staff as does the movements of conventional monitors. 
That is, conventional monitors are relatively large, heavy, and expensive 
pieces of equipment, which are generally mounted on wheeled carts. Each 
time a monitor is moved from one location to another within a hospital, 
for example, there is a certain risk that it will be damaged in the 
process of movement. Also, the physical movement of the monitor requires 
the services of a relatively strong member of the hospital service 
personnel, for example, to move the wheeled cart and monitor onto and off 
of hospital elevators. On the other hand, the configuration modules of 
modular type monitoring equipment are small enough to be carried by hand 
from one location to another. In fact, several of these modules can be 
carried at a time by one person if necessary. A single wheeled cart of a 
size comparable to one conventional monitor can carry several to several 
dozen of the configuration modules for a modular monitoring system. 
With respect to the conventional continuous cardiac output monitors, the 
monitor includes a linear electronic power amplifier capable of supplying 
a variable power level and fixed frequency of electrical alternating 
current power output, and which provides electrical heating power to the 
resistance heating element of the continuous cardiac output monitoring 
catheter. This conventional linear power amplifier is physically too large 
to be accommodated within the envelope of a monitoring module of the newer 
modular-type of monitoring apparatus. Also, the conventional power 
amplifier is of a power efficiency so low that although only about fifteen 
watts of power is dissipated into the patient's blood flow on an 
intermittent basis, about thirty to forty-five watts, or more, of power is 
liberated as heat into the console of the conventional continuous cardiac 
output monitor. That is, the efficiency of these conventional linear power 
amplifiers may be as low as 25 percent. Were this level of heat to be 
liberated within a monitoring module, assuming that the conventional 
linear power amplifier could somehow be physically fitted into the module, 
the conventional plastic casing of the module could be warped or melted by 
the resulting high temperatures. 
SUMMARY OF THE INVENTION 
In view of the deficiencies of the related technology as explained above, 
an object for the present invention is to provide a power amplifier which 
avoids one or more of these deficiencies. 
More particularly, an object of the present invention is to provide a power 
amplifier for a continuous cardiac output monitoring apparatus which 
allows the power amplifier to be physically fitted within a configuring 
module for a modular-type of monitoring system. 
Still another object for the present invention is to provide such a power 
amplifier having such a high level of efficiency that a permissibly small 
level of heat energy is liberated from the power amplifier, allowing the 
power amplifier to be housed in a module compatible with conventional 
modular monitoring apparatus. 
Particularly, the present invention relates to apparatus and method for 
electrically heating cardiac blood flow within the heart of a human 
patient, and for sensing the temperature versus time relationship of the 
blood flow in the pulmonary artery. The power amplifier provides 
alternating current electrical power at a particular frequency chosen 
because of the particular safety of this frequency for the patient, and 
relative freedom of this frequency from the production of electromagnetic 
interference which could affect other medical apparatus being used in the 
treatment of the patient. The alternating power supplied is of variable 
voltage level to control the energy dissipated in a resistive load from 
which heat energy is applied to cardiac blood flow. The heat energy is 
intermittently according to a pseudo random algorithm to provide a 
temperature transient in the patient's cardiac pulmonary blood flow, which 
transient is sensed in order to derive a value for cardiac output of the 
patient. 
Accordingly, the present invention provides a cardiac output monitor for 
use with a cardiac output monitoring catheter immersed in the blood flow 
of a patient, the catheter having a heating element in heat transfer 
relation to the blood flow to provide a temperature transient therein in 
response to the controlled application of electrical resistance heating, 
and a temperature sensor disposed on the catheter down stream of the 
heating element in the blood flow for providing a response to the 
temperature transient, the monitor comprising a console having provision 
for connection with the catheter for supplying electrical power thereto 
and for receiving therefrom the response to the temperature transient in 
the patient's blood flow, the monitor further including an output device 
for locally displaying indicia relative to the cardiac monitoring of the 
patient, including a value for cardiac output; and a module removably 
attaching to the console, the module providing an electrical power 
amplifier circuit for selectively supplying electrical heating power of a 
selected frequency and voltage level to the console for connection through 
the console to the heating element of the catheter. 
According to a further aspect, the present invention provides an 
alternating current switch-mode electrical power amplifier for supplying 
alternating current of a selected frequency; the power amplifier 
comprising a reference oscillator providing a reference asymmetrical pulse 
train signal having a frequency which is a multiple of the selected 
frequency; divider means for first dividing the reference pulse train by a 
value which is equal to the multiple divided by two to provide a resulting 
pulse train, and then dividing the resulting pulse train by two to provide 
a final square wave pulse train having a 50 percent duty cycle and which 
toggles between a first higher value and second lower value at the 
reference frequency; and switching means for switching electrical power 
alternatingly at the selected frequency in response to the final pulse 
train. 
An advantage of the present invention resides in the freedom from harmonic 
interference in the regulated power supplied by the power amplifier. That 
is, such harmonic interference frequencies are substantially not present 
at all in the alternating current power supplied by the present power 
amplifier. The present power amplifier supplies essentially pure sine wave 
alternating current electrical power. Further, the small size, light 
weight, low cost, frequency stability, fault tolerance (actually, 
redundant fault tolerance), and good accuracy of the power level 
regulation provided by the present power amplifier are individually and in 
combination better than can be achieved with conventional power supplies. 
Additional objects and advantages of the present invention will be apparent 
from a reading of the following description of a particularly preferred 
exemplary embodiment of the invention taken in conjunction with the 
appended drawing Figures, which are described below.

DETAILED DESCRIPTION OF AN EXEMPLARY PREFERRED EMBODIMENT OF THE INVENTION 
FIG. 1 shows a human patient 10, who, for example, may have suffered a 
coronary occlusion or heart attack, or who may have suffered severe trauma 
such as may occur from a motor vehicle accident. For the heart attack 
patient, because damage to the patient's heart 12 occurs not only as a 
result of an initial occlusion, or blockage of a coronary artery, but also 
over a period of several hours or several days as portions of the 
patient's heart become necrotic as a result of the occluded coronary blood 
supply, the patient's coronary capacity is at risk of failure over this 
considerable time period following the heart attack. For the trauma 
patient, who may have lost a considerable quantity of blood, and who may 
be in shock, dilation of peripheral vasculation of the body may result in 
a decrease in blood pressure while the coronary capacity of the patient is 
decreased over a period of time. During these time periods, early 
detection of an impending heart failure is very important so that early 
interventional measures can be taken while these measures can have their 
best effect. 
Experience has shown that monitoring of the patient's blood pressure alone, 
or monitoring of blood oxygen saturation levels at the extremities of the 
patient 10, for example, are not adequate indications of impending heart 
failure. As a result, conventional technology has been developed which 
effects a monitoring of the patient's pulmonary blood circulation, as well 
as other related factors, such as oxygen saturation of the blood flowing 
to the patient's lungs, directly at the pulmonary artery 14 of the patient 
10. 
This monitoring of pulmonary blood flow is effected by instilling a 
monitoring catheter 16 into the patient's right jugular vein. A distal end 
portion 18 of this catheter is advanced down the vein into the right 
atrium 20 of the heart 12. From the right atrium 20, the distal end 
portion 18 is advanced through the tricuspid valve 22 and into the right 
ventricle 24 of the heart 12. Subsequently, the distal end portion 18 of 
the catheter 16 is advanced through the pulmonary valve 26 and into the 
pulmonary artery 14. As those who are ordinarily skilled in the pertinent 
arts will appreciate, usually, an inflatable balloon portion 28 of the 
catheter 16 will be inflated for this introduction procedure so that the 
prevailing blood flow helps in moving or floating the catheter along to 
its desired location. 
Externally of the patient 10, the catheter 16 is connected at a 
plug-and-socket interface 16'/30' to a multi-conductor electrical cable 
30. This cable 30 provides similar plug-and-socket connection to 
electrical circuitry located at a general purpose variably-configurable 
modular-type of monitor console 32. This monitoring console 32 includes a 
display screen 34 upon which information about the patient's condition can 
be locally displayed. Also, this monitor 32 includes a data output 
facility, such as a computer system RS-232 port (schematically indicated 
with the arrowed numeral 36), and by which patient information is provided 
to a remote location, such as to a nurse's station, central patient 
monitoring and data recording computer system, or to a physician who may 
wish to receive the information at his home or office via a telephone line 
interconnection with such a hospital's central patient monitoring computer 
system. 
The console 32 includes a plurality of electrical interconnection apertures 
or ports 38 into which configuration modules may be received in order to 
configure the monitor to perform those monitoring functions which are 
required by particular patients. In the present case, the console 32 has 
received a module 40 for monitoring cardiac output of the patient 10. The 
cable 30 has direct plug-and socket connection to the module 40, which 
provides interface between the catheter 16 and the console 32. The module 
40 includes a second cable connector 41 which is configured to receive the 
connector 30' at the distal end of the cable 30 where catheter 16 connects 
for monitoring of the patient 10. As will be seen, the module 40 can also 
be used to verify the correct operation of the cable 30 by connection of 
both ends of the cable 30 to the module 40. 
With attention now to FIG. 2, it is seen that the module 40 has connection 
to the monitor 32 via a data bus, generally indicated by the double-headed 
arrow 42, and via a number of electrical connections which supply 
electrical power to the module 40 from the console 32. It will be 
appreciated that FIG. 2 is very schematic, and that the electrical 
conductors depicted outwardly of catheter 16 are actually of a fine gauge 
and over a portion of their length are disposed within the elongate and 
comparatively thin shaft of the catheter. Another portion of the length of 
the illustrated conductors will be understood to be provided by the cable 
30. Two of the conductors 46 and 48 connect to a resistance heating 
element 50 which is outwardly disposed on the distal end portion 18 of the 
catheter 16. The heating element 50 may actually be configured as a 
flexible thin metallic film element having a high coefficient of 
resistance change with change in temperature. 
The catheter 16 will preferably be configured so that this heating element 
50 is actually disposed in the right ventricle of the patient 10. The 
turbulent blood flow in this ventricle resulting from the pumping action 
of the heart assists in distributing heat energy from the heating element 
50 uniformly throughout the pulmonary blood flow. Downstream of the 
heating element 50 with respect to the direction of blood flow (indicated 
with arrows 52) is disposed a temperature measuring sensor 54. The sensor 
54 may be a small bead thermistor, for example, and is connected to the 
cable 30 and console 32 via conductors 56 and 58. Within the module 40, 
the conductors 56 and 58 supply the temperature signal from sensor 54 to a 
microprocessor based control system 56, including a microprocessor 58 and 
power amplifier circuit 60. The microprocessor 58 has a two-way control 
and data interface with the power amplifier circuit 60, as is generally 
indicated by the control and data bus arrow 62. This general interface 
reference numeral (62) is used throughout the following explanation to 
refer to the interface of information and control signals in one or both 
directions between the power amplifier 60 and the microprocessor 58. 
FIG. 3 shows that the power amplifier 60 includes a programmable 
selectively variable voltage source section 64, a frequency source section 
66, and a switch-mode amplifier section 68 which receives as inputs both 
electrical power at a selected programmable voltage level from the section 
64, and a reference frequency signal from the section 66, and which 
combines these inputs to provide frequency-controlled alternating current 
electrical power to an isolated patient-connected section 70 with a pure 
sine wave form at a selected and variable voltage level. Electrically, the 
patient-connected section 70 is defined in part by the catheter 16 and the 
cable 30. The programmable selectively variable voltage source 64 has 
connection, as is depicted at 72, to a 28 volt direct current power source 
(not shown). A power cut off relay 74 is under the control of the 
microprocessor 58, as is indicated by the interface connection 62. As will 
be seen, this control of relay 74 by microprocessor 58 is redundant, and 
is further backed up by control of the voltage source 80 by the 
microprocessor 58 so that the relay 74 can be opened or voltage source 80 
may be commanded to provide a zero output voltage, all in order to 
safeguard the patient 10 from inadvertent injury by excessive heating at 
catheter 16. This relay 74 supplies electrical power to a pair of 
semiconductor switches 76 and 78. Switches 76 and 78 are controlled by a 
voltage regulator circuit 80, which is also under the control of the 
microprocessor 58, as is indicated by the interface connections 62. 
As will be further explained below, the applicants have adapted a 
conventional semiconductor integrated circuit voltage regulator, which is 
designed to provide a steady regulated voltage output level even if its 
supply voltage varies, and have created a programmable selectively 
variable direct current voltage supply. In the present case, the 
programmable voltage supply has a resolution of 2.sup.12, or 4095 
different incremental voltage levels which may be individually selected by 
the microprocessor 58 in order to control the level of resistance heating 
and energy dissipation at the heating element 50 of catheter 16. 
Accordingly, the level of electrical power which is supplied by the module 
40 to the heating element 50 of the catheter 16 is under very fine control 
by the microprocessor 58. 
Direct current electrical power of finely controlled voltage level is 
supplied by the voltage source section 64 to the amplifier section 68, as 
is indicated by the schematic conductor 82. The power amplifier section 68 
also receives a precisely regulated frequency signal from the frequency 
source section 66, as is indicated by the schematic conductor connections 
82 and 84. As can be seen from the schematic illustration of FIG. 3, the 
frequency source section 66 includes a 1 MHz crystal reference oscillator 
88. This oscillator 88 provides an asymmetrical (i.e., positive-going 
only) output signal at a precise 1 Mhz rate with a high-signal duty cycle 
of about 40 percent. This signal is provided to a divider circuit 90, 
which first effects a division by 5 to provide a 200 KHz signal, which is 
still only positive going, and has a low duty cycle. As an aid to the 
reader at this point in the circuit description, small graphical signal 
wave-form illustrations have been added to FIG. 3. 
Next, the divider circuit 90 effects a division by 2 to provide a 100 KHz 
signal which toggles between a signal-high value and signal-low (zero) 
value 100 thousand times a second. While this signal is a square wave form 
and is only positive-going, it is the basis for a pure symmetrical 
alternating current wave form which will be provided by the power 
amplifier 60. As those who are ordinarily skilled in the pertinent art 
will be aware, a pure square wave form of 50 percent duty cycle has only 
odd-factored harmonics added to a pure sine wave form when analyzed by 
Fourier analysis. As will be seen, the wave form provided by the divider 
90 is used to effect a switching of direct current into a pure square wave 
form from which the odd-factored harmonics are removed, to provide a pure 
sine wave form of alternating current output from the power amplifier 60. 
In order to prevent simultaneous conduction, and a resulting short 
circuit, during switching of the direct current power from the voltage 
source section 64, a dead-time generator is provided to provide a pair of 
oppositely-going square wave signals which still have a 50 percent duty 
cycle. These signals are provided to respective ones of a pair (94, 96) of 
switch drivers. These switch drivers 94, 96 in turn control switching of 
respective ones of a pair of semiconductor (MosFet) switches 98, 100, 
which are part of the power amplifier section 68. 
Considering now the power amplifier section 68, it is seen that the 
switches 98, 100 control current flow through opposite sides of a 
center-tapped transformer 102. Preferably, this transformer has a turns 
ratio of substantially 1:1.6 As mentioned above, this center tapped 
transformer 102 provides a square wave output at a frequency of 100 KHz 
with a 50 percent duty cycle. Consequently, this square wave has the 
characteristics of a pure sine wave with only odd-numbered harmonic 
factors added. Power amplifier section 68 includes first and second series 
tuned circuits 104 and 106, which are respectively tuned to present a very 
high impedance to the third and fifth harmonic factors of the 100 KHz 
signal from transformer 104. That is, circuit 104 is tuned to present a 
very high impedance so as to effectively block the 300 KHz component of 
the signal from transformer 102, while the circuit 106 is tuned to block 
the 500 KHz component. This blockage of the third and fifth harmonic 
components of the selected frequency signal from the transformer 102 is an 
important aspect of the present power amplifier because these first two 
odd-numbered (third and fifth) harmonics carry the most energy. By 
presenting a high impedance at the circuits 104 and 106, a large portion 
of this energy of the third and fifth harmonics will be reflected and will 
not be lost to inefficiency in the power amplifier 60. 
Next, the power amplifier section 68 includes a series tuned circuit 108, 
which presents a very low impedance to the 100 Hz selected frequency, 
while presenting a high impedance to higher order harmonic frequencies of 
the 100 KHz selected frequency. The remaining portions of the higher order 
harmonics which pass the tuned circuit 108 are shunted to ground by a 
high-order shunt tuner circuit 110. This shunt tuner circuit 110 drives a 
resulting sine wave voltage signal into the primary winding of an 
isolation transformer 112 having a turns ratio of substantially 3:1. As is 
seen on FIG. 3, a voltage sensing circuit 114 is associated with the 
connection between the shunt tuner circuit 110 and the primary winding of 
the transformer 112 in order to provide a feed back value of voltage 
supplied to the catheter 16 to the microprocessor 58, as is indicated by 
the interface arrow 62. 
Importantly, the ground side of the transformer 112 is connected to the 
ground indicated at 116 via a second 100 KHz series tuner circuit 118. The 
series tuned circuits 108 and 118 are separated from one another by the 
high reflected impedance appearing at the primary winding of the isolation 
transformer 112 by virtue of its 3:1 turns ratio so that the inductances 
and capacitances of these tuners do not simply add with one another to 
result in a composite tuned circuit which would have a tuned frequency 
other than 100 KHz. Accordingly, the tuned circuits 108 and 118 can each 
participate in insuring that substantially only a pure sine wave voltage 
form at 100 KHz is effective at the primary winding of transformer 112. 
FIG. 3 also shows that the power amplifier section 68 of power amplifier 
60 has connection to the microprocessor 58 via a sense common circuit 120, 
and a current flow sensing circuit 122. 
The isolated patient-connected circuit section 70 effectively floats with 
respect to ground potential because no ground connection is made across 
the isolation transformer 112. This isolated patient-connected section 70 
of the power amplifier circuit 60 includes the resistive heating element 
50 of the catheter 16, and a calibration resistor 124. Each of these 
resistances has a value of substantially thirty nine ohms, so that the 
reflected impedance at the primary winding of the isolation transformer is 
substantially 350 ohms (i.e., 39 ohms multiplied by the square of the 
turns ratio across transformer 112). Also, isolated patient-connected 
circuit section 70 includes a relay 126 switching the connection of the 
secondary winding of transformer 112 between the heating resistor 50 and 
the calibration resistor 124. This switching relay 126 has a connection 
with the microprocessor 58, as is indicated with interface arrow 62. 
Accordingly, the microprocessor 58 can not only control the condition of 
relay 126, but can also verify this condition (as will be further 
explained), so that the microprocessor 58 can verify that electrical power 
is applied to the heating resistor 50 only when commanded by the 
microprocessor 58, and cannot be applied otherwise without the 
microprocessor down the positive action to shut down the entire module 40. 
Considering now FIG. 4, a more greatly detailed presentation of the 
frequency source circuit'section 66 is presented. This circuit section 
includes the crystal oscillator 88, which provides the square wave 
positive-going signal seen in FIG. 5A to the divider 90 via a conductor 
128. Divider 90 effects first a division by five to produce the signal 
seen at FIG. 5B. This signal varies between zero and a positive value upon 
each fifth positive-going signal transition of the signal seen in FIG. 5A. 
The value of the signal seen in FIG. 5B drops back to zero after a 
comparatively short time interval, which is considerably shorter than the 
time required for five cycles of the signal of FIG. 5A to pass. 
Importantly, the signals seen in FIGS. 5A and 5B are not square waves with 
a 50 percent duty cycle, and will not satisfy the relationship explained 
above with respect to having only odd-ordered harmonics with respect to a 
pure sine wave signal. The signal seen in FIG. 5B is output by circuit 90 
on a conductor 130, which returns this signal to the circuit 90 to a 
terminal at which a division by two is effected. The division by two 
results in a signal seen in FIG. 5C which toggles between zero and a 
positive value at a rate of 100 KHz. This signal is a square wave with a 
50 percent duty cycle, but is positive-going only. That is, the signal of 
FIG. 5C is not symmetrical about the zero voltage axis. 
This signal of FIG. 5C is provided by a conductor 132 to the dead time 
generator circuit 92. This circuit 92 includes a pair of 
oppositely-connected exclusive-0R (XOR) gates 134, and 136. The gate 134 
is connected at one input terminal to the positive input V.sub.cc 
(indicated with the numeral 138), and at the other input terminal receives 
the signal of FIG. 5C. 
Consequently, the gate 134 conducts only while the signal of FIG. 5C is 
positive. On the other hand, the gate 136 is connected at one input 
terminal to the signal of FIG. 5C, and at the other input terminal is 
connected to ground (indicated with the numeral 140). This gate 136 
consequently conducts only while the signal of FIG. 5C is zero. The result 
is that the gates 134 and 136 each provide respective ones of a pair of 
time-matched square-wave signals which are positive-going only, and are 
oppositely time sequenced, as is seen in FIG. 5D. The reader will note 
that the time scale of FIG. 5D is considerably compressed in comparison to 
that of FIGS. 5A through 5C, so that several cycles of the time-matched 
oppositely-sequenced signals can be shown. These signals of FIG. 5D are 
provided on respective conductors 142 and 144 to respective 
resistor-capacitor networks 146 and 148, each one of which also includes a 
respectively oriented diode conducting toward ground potential so that the 
signal provided by the gates 134 and 136 can transition high to charge the 
capacitor (as seen on a respective conductor 150 and 152), but can 
transition low only with the added effect of the resistor-capacitor time 
constant resulting from the networks 146 and 148. The resulting signals at 
the conductors 150 and 152 are seen in FIG. 5E. These signals are still 
only positive-going, and between the two signals have a duty cycle of 50 
percent. 
The signals of FIG. 5E are provided to a pair of exclusive-OR gates 154 and 
156, which are connected together at one of their input connections by a 
conductor 158. The conductor 158 has a ground connection, indicated by 
numeral 160. Consequently, the gates 154 and 156 each individually only 
conduct when the signal received from the conductors 150 and 152 (the 
signal of FIG. 5E) are high. However, these gates do not switch off as 
soon as the signal of FIG. 5E drops below it greatest high value. Instead, 
these gates switch off at some voltage level intermediate of the high and 
low (zero) signal levels. Because of the resistor-conductor time constants 
effective on the zero-going portion of the signals seen in FIG. 5E, the 
gates 154 and 156 do not switch off simultaneously with the signals seen 
in FIG. 5D, but have their switching off delayed until some lower but 
non-zero voltage value, indicated with the numeral 162 is reached. As a 
result, the gates 154 and 156 provide on respective conductors 164 and 166 
respective signals as are indicated in FIG. 5F. These signals are still 
only positive-going, and have a 50 percent duty cycle between the two of 
these signals. However, the switching-off transition (i.e., the negative 
going part of the square wave form) of each wave form is delayed slightly 
with respect to the positive-going transition of the companion wave form 
seen in FIG. 5F. Conductors 164 and 166 provide the signals seen in FIG. 
5F to a switch driver circuit 168, which inverts each of these signals so 
that the positive-going turn-on part of the signal is delayed with respect 
the negative-going turn-off portion of the signal. The switch driver 
circuit 168 provides respective inverted signals of the same wave form as 
is seen in FIG. 5F, but of respectively inverted shape and transposed time 
sequencing, on conductors indicated with the numerals 170 and 172. The use 
made of these signals with be further explained below. 
Viewing now FIGS. 6-8 in conjunction with one another, the voltage 
regulator 80 and switches 76, 78 of the programmable selectively variable 
voltage source circuit 64 of FIG. 3 is shown in greater detail. 
Particularly, FIG. 6 schematically depicts the overall circuit schematic 
for a regulated voltage supply using a voltage regulator integrated 
circuit 174. This circuit 174 provides an output voltage on a conductor 
175 which is equal to a constant multiplied by the sum of one plus the 
quotient of the value of resistor R.sub.1 divided by the value of resistor 
R.sub.x. According to the present preferred embodiment of the invention, 
this constant is 1.25. As is seen in FIG. 6, resistor R.sub.x is a 
variable resistor according the present invention. Further, as will be 
seen, the present invention includes provision for the value of resistor 
R.sub.x to be digitally programmable and to be controlled by 
microprocessor 58. The value of resistor R.sub.x is programmable with a 
resolution of 2.sup.12 incremental values. Accordingly, it will be seen 
that the regulated and controlled voltage level provided by the voltage 
regulator circuit 174 is controllable by the microprocessor 58 with a fine 
degree of control. 
Turning now to FIGS. 7 and 8 in combination, it is seen that the voltage 
regulator circuit 80 includes a resistor designated R.sub.1, which is seen 
in FIG. 8, and which functions as the resistor designated in the same way 
and seen in FIG. 6. The voltage regulator circuit and switches 76 and 78 
are also seen in FIG. 8. Also, this voltage regulator circuit section 
includes an array of four digitally-controlled analog switches 176, 178, 
180, and 182, which are seen in FIG. 7. The analog switches 176-182 are 
each controlled by the microprocessor 58 via the interface connections 
designated with the numeral 62, and seen along the left side of FIG. 7. 
That is, the microprocessor 58 can drive up the signal level at individual 
ones or groups of as many as all of the signal leads to these analog 
switches. A signal-high value on any one of the conductors indicated 
results in the associated switch 176-182 switching a respective one of the 
resistors indicated below into a parallel resistance relationship to 
ground. 
To further explain the above, it is seen that the analog switches 176-182 
also have connection individually with an array of twelve resistors, which 
are designated R.sub.2 through R.sub.13, and which are individually 
switched into connection with a grounded conductor 188 when the respective 
one of the leads indicated with the interface numeral 62 are driven 
signal-high by the microprocessor 58. These resistors R.sub.2 through 
R.sub.13 collectively function as the variable resistor R.sub.x seen in 
FIG. 6. These resistors have increasing values ranging generally from 
about 200 ohms to about 422 Kohms. Particularly, the resistors R.sub.2 
through R.sub.13 have values in ohms of: 200, 402, 806, 1.62K, 3.24K, 
6.49K, 13.0K, 26.1K, 52.3K, 105K, 210K, and 422K. An example of an analog 
switch which has proved to be acceptable for use as the switches 176 and 
178 for switching the resistors having the lower values (i.e., in the 
range from about 200 ohms to about 1.6 Kohms, is the Siliconix 9956DY. 
This analog switch has a very low resistance when switched on. Thus, the 
resistance of the switches 176 and 178 themselves does not itself add 
appreciable to the resistances of the resistors R.sub.2 through R.sub.5. 
On the other hand, for the analog switches 180 and 182, a Harris DG412DY 
has shown to be acceptable. This analog switch has a very low leakage 
current when switched off so that the comparatively small incremental 
change in current flow which results when the higher-valued resistors 
R.sub.6 through R.sub.13 are switched in parallel into the circuit can be 
easily distinguished from the leakage current through the switches 180 and 
182 themselves. The resistors R.sub.6 through R.sub.13 have values greater 
than 1.6 Kohm, up to or greater than about 422 Kohm. Preferably, the 
resistors R.sub.2 through R.sub.13 are precision 0.1 percent, 50 PPM 
resistors in order to enable closer calibration of the voltage supplied by 
the voltage regulator 174. 
It will be noted viewing FIG. 7 that a resistor R.sub.14 is provided to 
adjust the maximum resistance value appearing on conductor 190 along with 
a resistor R.sub.15 and a trimming resistor R.sub.16 adjusting the 
effective resistance appearing at conductor 192. The value of the resistor 
R.sub.15 sets the maximum voltage which the voltage regulator 174 will 
supply even when all of the resistors R.sub.2 through R.sub.13 are 
switched to ground by closing of all of the analog switches 176-182. The 
conductor 192 appears at the left side of FIG. 8, and has connection with 
the voltage regulator circuit 174. This conductor 192 is analogous to the 
conductor schematically seen at the upper end of resistor R.sub.x of FIG. 
6, and having connection with the regulator circuit 174. The effective 
resistance value to ground from conductor 192 controls the voltage level 
output by voltage regulator circuit 174. As is seen, the microprocessor 58 
controls this effective voltage level by effecting switching of resistors 
R.sub.2 through R.sub.13 into connection with grounded conductor 188 via 
the analog switches 176-182. Also, the microprocessor 58 can control the 
voltage regulator circuit 174 through the indicated interface line 62 so 
that the voltage regulator 174 is turned off to provide no output power, 
or is turned on to provide output power of the voltage level selected by 
the switched condition of the analog switches 176-182. Accordingly, the 
microprocessor 58 can control the power level of electrical heating 
effected at the heating element 50 of the cathode 16 from zero to the full 
wattage capacity of this heater. 
Viewing FIG. 8, another feature of the present voltage source circuit 
section will be seen. An example of an integrated circuit acceptable for 
use as the voltage regulator 174 is the Linear Technology LTC1149. This 
voltage regulator has a constant off time architecture, rather than a 
fixed switching frequency. Consequently, operating frequency of this 
voltage regulator will vary with output voltage. For the present 
application, the output voltage can vary between 1 volt and 26 volts DC. 
Recalling FIG. 6 it is seen that the voltage regulator circuit 174 
requires a capacitance to ground connected in common with the conductor 
connecting to the reference resistor R.sub.1. In order to avoid the use of 
large high-voltage capacitors and preserve the small size, weight, and 
cost goals of the present power amplifier, this capacitance is provided by 
a capacitor array indicated with the numeral 194 on FIG. 8. The capacitor 
array includes a plurality of capacitors connected between the conductor 
82 (which is the regulated-voltage power output conductor for the 
selectively-variable voltage source circuit section 64, recalling the 
description of FIG. 3), and ground. The capacitor array 194 includes a 
number of equally valued resistors 196, which serve as voltage sharing 
resistors among the capacitors of the array 194, distributing the voltage 
drop equally across these capacitors and preventing an excessive current 
flow in any one of the capacitors. 
Viewing now FIG. 9, the power amplifier circuit section 68 and isolated 
patient-connected circuit section 70 are shown in greater detail. 
Recalling the description of the power amplifier circuit section 68, it is 
seen that this circuit section receives the signals of FIG. 5F on 
conductors 170 and 172. This circuit section also receives the selectively 
varied voltage output from voltage source circuit section 80 on conductor 
82. The signals on conductors 170 and 172 drive MosFet switches 98 and 100 
alternately into conduction, with the indicated dead time preventing 
simultaneous conduction of through these switches, so that the creation of 
a short from conductor 82 to ground connection 198 is prevented. The 
alternate current conduction through the switches 98 and 100 drives the 
center-tapped transformer 102 to provide an essentially symmetrical square 
wave output of 50 percent duty cycle into the first 300 KHz trap 104. This 
trap 104 includes a capacitor 200 and inductor 202 which in combination 
are tuned to present a high impedance to a 300 KHz frequency. Similarly, 
the 500 KHz trap 106 includes a capacitor 204 and inductor 206 which in 
combination are tuned to provide a high impedance to a 500 KHz frequency. 
The 100 KHz series tuner 108 of FIG. 3, is formed by the interaction of 
the two inductors 104 and 106 in series with a capacitor 208. These 
components are tuned to present a low impedance to a 100 KHz frequency and 
a comparatively high impedance to higher order (i.e., the 7th, 9th, etc.) 
harmonics of the 100 KHz selected frequency). The shunt tuner 110 of FIG. 
3 is actually formed by a capacitor 210, and a pair of parallel connected 
inductors 212, 214, which pass higher order harmonics (now of 
comparatively low energy level) to ground connection 116. Series tuner 118 
is formed by the cooperation of a capacitor 218 and an inductor 220, 
allowing the selected 100 KHz frequency to reach ground 116 with little 
impedance. Accordingly, the primary winding of isolation transformer 112 
receives essentially alternating current power of essentially pure sine 
wave characteristic. It will be noted that the voltage drop occurring 
across a resistor 222, which is exposed essentially only to the selected 
100 KHz frequency of the electrical power delivered into the resistive 
load 50 is available to the microprocessor 58 via the interface 
connections 62 bridging this resistor. Accordingly, the microprocessor 58 
can verify when and if electrical power of the selected frequency is being 
delivered to heater 50 of catheter 16. 
In order to further understand the control and safeguard features of the 
present invention, it should be noted that at the isolation transformer 
112, is established a virtual isolation barrier, indicated with the dashed 
line 224. To the right hand side of the barrier 224 is the isolated 
patient connected portions of the power amplifier circuit 60, module 40, 
and catheter 16, with heater 50. No physical electrical connection is 
effected across the barrier 224. In order to control the relay 126, an 
additional isolation transformer 226 is provided. This isolation 
transformer is powered by a 100 KHz power supply circuit 228 seen on FIG. 
4. Viewing the power supply circuit 228 on FIG. 4, it is seen that a 
divider 230 receives the signal of FIG. 5A from the oscillator 88, and is 
connected just like the divider 90 to provide a signal like that 
illustrated by FIG. 5C to a transistor 232. The transistor 232 toggles on 
and off in response to the signal from the divider 230, and similarly 
causes a second transistor 234 to toggle on and off. This second 
transistor 234 drives a pair of oppositely connected PNP (236) and NPN 
(238) transistors. The transistors 236 and 238 toggle on and off in 
opposition to one another to provide a low-power 100 KHz alternating 
current power supply at conductor 240. 
Returning to FIG. 9, it is seen that the conductor 240 delivers this 
low-power 100 KHz alternating current power to the primary winding of the 
transformer. Thus, the same frequency of alternating current power which 
has been determined to offer the greatest level of patient safety is used 
to effect control of the isolated patient-connected circuit section 70. 
The secondary winding of transformer 226 drives a rectifier 242 providing 
direct current power on the isolated patient-connected circuit section 70. 
In order to control the relay 126, the microprocessor 58 can command 
illumination of a light emitting diode 244. Light from this LED (arrow 
246) crosses the barrier 224, and causes a photodiode 248 to become 
conductive. The photodiode 248 controls current flow through the coil 250 
of the relay 126 so that this relay is under the control of the 
microprocessor 58 with no physical connection across the barrier 224. 
In order to inform the microprocessor 58 that power is being dissipated on 
the isolated patient-connected side of the barrier 224, the voltage drop 
occurring across the calibration resistor 124 is used to drive a 
transistor switching circuit into conductivity. It will be noted that the 
calibration resistor 124 is in fact formed by two resistors connected in 
parallel. Conductivity at switching circuit 252 illuminates a LED 254. 
Again, light from the LED 254 is beamed across the barrier 224 (arrow 
256), and causes a photodiode 258 to become conductive. Conductivity of 
the photodiode 258 pulls low the signal on conductor 260, which has 
connection with the microprocessor 58, as is indicated by interface arrow 
62. 
Returning to a consideration of FIG. 3, it is seen that the microprocessor 
58 has control over the relay 74, from which power is received to operate 
the entire power amplifier 60. In the event that the microprocessor 58 is 
informed that electrical power is being dissipated into the isolated 
patient-connected section 70 when this power dissipation has not been 
commanded, then the relay 74 will be opened to shut down the power 
amplifier. On the other hand, if after commanding the voltage source 80 
(regulator 174) to provide a selected level of voltage and the closing of 
the relay 126, the microprocessor is not informed within a selected time 
interval (only a fraction of a second) that power is being dissipated in 
the isolated patient-connected section 70, then a fault is assumed. In 
this event also, the relay 74 will be opened, or the voltage regulator 174 
is alternatively commanded to provide a zero voltage output, and the 
patient is protected from any and all inadvertent injury which might 
result from operation of the heating element 50 in the pulmonary artery 14 
without adequate control. 
In order to provide the desired degree of safeguarding over unintended or 
inadvertent operation of the heating element 50, the power amplifier 
circuit 60 includes the circuit sections 262 and 264, which are 
illustrated in FIGS. 10 and 11, respectively. Particularly viewing the 
circuit portion 262, it is seen that the conductor 266 is seen in FIG. 7 
to be connected to the regulated voltage output of the voltage source 174 
at conductor 82 via a pair of voltage dividing resistors indicated with 
the numeral 268. The voltage level appearing at the conductor 266 is an 
indication of the voltage level actually provided by the voltage regulator 
174. This voltage level is provided to a unitary gain buffer 270. This 
buffer 270 is an operational amplifier which provides an output to the 
microprocessor 58, indicated with the general interface numeral 62. 
Accordingly, the microprocessor 58 can read the voltage level provided by 
the voltage source 80 using voltage regulator 174. In the event that a 
fault in the regulator 174 or some other portion of the power amplifier 60 
causes a voltage other than an acceptable and expected value to appear 
from the buffer 270, then the microprocessor 58 will effect a shutdown of 
the power amplifier 60. 
Turning now to FIG. 11, the circuit section 264, which effects control of 
the relay 74 (recalling FIG. 3), is depicted. The circuit section 264 
includes the relay 74 itself, and a transistorized switching circuit 
indicated with the numeral 272. This transistorized switching circuit 272 
places the relay 74 under the control of the microprocessor via an 
interface effected by conductor 274. That is, a signal-high value at the 
conductor 274 provided by the microprocessor 58 will result in the switch 
circuit 272 closing and in the closing of the relay 74 to power the power 
amplifier 60. However, this closing of the switching circuit 272 can be 
effected only so long as the validity of this action is verified by an 
internal watch dog timer (not shown), which is associated with the 
microprocessor 58. In other words, if an internally repeated diagnostic of 
the microprocessor 58 is not successfully completed, the watch dog will 
reset and reboot the microprocessor 58 and will effect a shut down of the 
power amplifier 60 by pulling the signal on conductor 274 signal-low. This 
pull down of the signal on conductor 274 is effected through the diode 
276, which has connection to the watch dog portion of the microprocessor 
58 as is indicated by the interface numeral 62. 
Turning once again to FIG. 1, it will be recalled that the module 40 
includes provision to verify the correct functioning of the cable 30. That 
is, the cable 30 could be damaged during use of the monitoring apparatus 
including console 32, catheter 16, module 40 and cable 30. The console 32 
and module 40 are durable components, while the catheter 16 is a single 
use apparatus. Consequently, a new catheter 16 is used with each patient. 
However, the cable 30, although it is a durable component is by far the 
most subject to damage from rough use, or by being stepped on, for 
example, in the medical treatment use environment for the apparatus 
described. In order to test and verify correct operation of the cable 30, 
the connector portion attached to the cable 30 at the plug-and-socket 
connection 16'/30' is connected back into the module 40 at the connector 
41 provided on this module 40. 
FIG. 9 illustrates with dashed lines 278 the electrical configuration 
effected by this connection of the cable 30 back into connector 41. In 
other words, with the cable 30 connected into connector 41, the resistance 
heating element 50 of a catheter 16 is not connected to the output of the 
relay 126. However, the calibration resistors 124 are still connected at 
the contacts of this relay 126 to which they ordinarily connect, and are 
also now connected via cable 30 to the contacts of this relay at which the 
resistance heating element 50 of the catheter 16 is connected in the use 
configuration of the apparatus. Consequently, with the cable 30 connected 
to the connector 41, if the console 32 is used to effect a calibration of 
the module 40 and catheter 16, the module 40 will read the calibration 
resistors 124 in the calibration sequence, and then will read these 
calibration resistors 124 again via the cable 30 as though it were testing 
the operational readiness of a catheter. In this test sequence, if there 
is more than a predetermined difference in resistance between the 
calibration resistors 124 and the heating resistor 50 of a catheter, the 
module 40 will signal via console 32 that the catheter is bad. However, in 
the cable test configuration described, this "bad catheter" signal will 
mean that the cable 30 itself is defective. 
While the present invention has been depicted, described, and is defined by 
reference to a particularly preferred embodiment of the invention, such 
reference does not imply a limitation on the invention, and no such 
limitation is to be inferred. The invention is capable of considerable 
modification, alteration, and equivalents in form and function, as will 
occur to those ordinarily skilled in the pertinent arts. The depicted and 
described preferred embodiment of the invention is exemplary only, and is 
not exhaustive of the scope of the invention. Consequently, the invention 
is intended to be limited only by the spirit and scope of the appended 
claims, giving full cognizance to equivalents in all respects.