While writing this paper, I often stopped
to take a breather. While waiting to see if it would be
accepted, I was breathless with anticipation. I hoped I
wouldn't choke while presenting it. When the presentation
was finished, I could breathe easy. These metaphors
demonstrate the close connection between the physical act of
breathing and the mental states of anxiety and its
Anxiety isn't the only influence on breathing patterns; all
emotions affect respiration. Psychologists investigating this
relationship require some form of electronic patient-monitoring
equipment, partially because the very act of watching one's
breathing changes its pattern. Reliable and accurate respiration
monitors assist psychologists in the evaluation and treatment of
patients' mental and emotional states. Also, by revealing a
patient's breathing patterns over a period of time, the respiration
monitor can be a useful investigative tool in the diagnosis of
neurotic and psychotic individuals.
New IC technologies from Maxim Integrated Products provide an
accurate, low-cost solution for the design requirements of high-end
applications such as respiration monitors.
A Respiration Monitor with Smart-Sensor Technology
The respiration monitor of Figure 1 displays breathing
patterns, giving an approximation of the amplitude of respiration.
Several important parameters used to detect anxiety are visible on
this monitor: rate of breathing, regularity of the breathing
pattern, and the duration of pauses after exhalation and before
inhalation. Because calm, positive emotions usually produce a
pattern of longer exhalation than inhalation, the ratio of
inhalation to exhalation time can serve as an anxiety
Figure 1: This block diagram depicts a respiration
The monitor depicted in Figure 1 uses a silicon
piezoresistive transducer (PRT) to detect the decreases and
increases of pressure corresponding to inhalation and exhalation.
The PRT output is fed to a signal-conditioning IC, which corrects
for errors inherent in the PRT and then passes a compensated
voltage signal to the analog/digital converter (ADC). The ADC
output (a digitized version of the pressure signal) is then fed to
a PC interface and converted to RS-232 levels. These in turn are
passed to a PC, which displays the respiration waveform and allows
for analysis of the parameters.
The PRT Sensor
PRTs are commonly configured as a closed Wheatstone bridge. When
pressure is applied to an active-bridge PRT (Figure 2a),
resistances of the diagonally opposed legs change equally and in
the same direction. As the resistances of one pair of diagonally
opposite legs increases with pressure, the resistances of the other
pair decreases. A half-active-bridge PRT (Figure 2b)
exhibits resistance changes in only half of the bridge. The
advantages of full- or half-active PRT sensors include high
sensitivity (>10mV/V), good linearity at a constant temperature,
and the ability to track pressure changes without signal hysteresis
up to the destructive limit.
Figure 2: (a) All four legs of an active-bridge PRT
respond to pressure. (b) For a half-active-bridge PRT, only two
legs respond to pressure.
Engineers have always employed PRTs in low- and medium-accuracy
applications, but designers of higher-end applications have
traditionally been forced to use strain gauges-despite the higher
cost. However, new IC technologies that allow for the accurate
correction of PRT sensors enable the use of these devices in
PRT Sensor Errors
The chief obstacle in correcting a PRT sensor is its wide range
of error. The variety in PRT sensor manufacturing methods produces
various types of errors and a range of error magnitudes. Even
within a given model, these error magnitudes vary appreciably from
one transducer to the next.
Common PRT errors include ". . .strong nonlinear dependence of
the full-scale signal on temperature (up to 1%/°Kelvin), large
initial offset (up to 100% of full scale or more), [and] strong
drift of offset with temperature" (Konrad
et al, 1999, 12).
At a given temperature, both of the PRT types in Figure 2
maintain their bridge resistances (between
VCC and ground) at a level that is fairly
constant over a wide range of pressures. But as the temperature
increases, the bridge resistance increases significantly. If the
bridge is powered with a constant-current source, the result is an
increasing bridge voltage.
PRT sensitivity increases as the bridge voltage increases with
temperature. When the bridge voltage is held constant, a PRT's
sensitivity to pressure decreases with the temperature. Thus,
sensitivity is a function of two opposing factors: temperature and
the temperature-dependent bridge voltage. These changes in bridge
resistance or bridge voltage can be exploited by modern
signal-conditioning ICs to correct the sensitivity errors over
temperature in a PRT. ICs use the change in bridge resistance to
correct for variations in sensitivity and also offset, over
A Traditional Correction Scheme
The circuit in Figure 3 compensates PRTs to a reasonable
level of accuracy. It allows for the adjustment of offset, offset
drift with temperature, and sensitivity drift with temperature.
Related to sensitivity drift is the full-span output drift over
temperature; these two parameters change proportionally in response
to temperature. The relationship between offset and full-span
output is shown in Figure 4.
Figure 3: A traditional correction scheme for PRTs
features temperature-sensitive resistors.
The circuit's zero-trim resistors compensate the sensor's offset
voltage at room temperature, and the resistors
Rts and Rtz (or
R'tz) correct for temperature errors. As
described earlier, bridge resistance rises with increasing
temperature, which increases the voltage across the sensor. That
additional voltage increases the sensor's sensitivity, making its
output voltage higher for a given pressure.
Figure 4: A PRTs offset and full-span output constitute
the full-scale output.
When the voltage across the sensor is held constant, the
sensor's sensitivity decreases with increasing temperature. Because
the positive-going sensitivity coefficient caused by increasing
bridge resistance with temperature is greater than the
negative-going sensitivity coefficient, the full-span output tends
to increase with temperature. Resistor
Rts negates this effect by shunting
increasing amounts of the bridge current as temperature rises. In a
similar manner, Rtz or
R'tz corrects the offset drift. Either
Rtz or R'tz is
included in the circuit, depending on the direction of the offset
drift with temperature.
The chief problem with this compensation scheme is circuit
interaction among the compensation components, which makes
calibration cumbersome and limits the achievable accuracy.
Additionally, electronic trimming is not feasible with this
A Modern Correction Scheme
In Maxim's MAX1457, a signal-conditioning IC (Figure 5)
drives the respiration monitor's sensor and corrects the sensor
errors. The IC contains a controlled current source that drives the
sensor and an ADC that digitizes the sensor's bridge voltage. This
voltage is a product of current from the current source and the
temperature-dependent bridge resistance.
Figure 5: A specialized IC (MAX1457) that provides
current-source excitation and compensation for the pressure sensor
yields 0.1% accuracy.
Also included in the MAX1457 are a programmable-gain amplifier
(PGA) that amplifies the sensor's differential output and five
digital/analog converters (DACs) that correct various sensor
errors. Because the sensor output is a low-level signal, the PGA
output voltage is not sufficient to drive the ADC. For that reason,
the MAX1457's internal op amp is used to boost the PGA output to a
Bridge voltage increases with temperature, so temperature
dependence can be used to compensate full-span temperature errors.
With constant-voltage bridge excitation, the full-span output (FSO)
decreases with temperature, resulting in a full-span output
temperature coefficient (FSOTC) error. When the bridge voltage is
made to increase with temperature at a rate that compensates for
the decrease in full-span sensitivity with temperature, the FSO
Figure 6: This circuitry within the MAX1457 compensates
for offset and full-span temperature errors.
Figure 6 demonstrates how the MAX1457 implements this
scheme for correcting full-span output errors due to temperature.
Using the digitized bridge voltage from the ADC output, the chip
determines which previously calculated correction coefficient
stored in EEPROM should be applied to the FSOTC DAC. The resulting
DAC output voltage then changes the current level feeding the
bridge. This new current level compensates the FSO by adjusting the
bridge voltage to compensate for the change in the sensor's
sensitivity at a particular temperature. The chip applies the
analog bridge voltage to the FSOTC DAC's reference input, providing
an additional correction between each successive pair of digital
numbers supplied by the ADC to the EEPROM.
The same technique compensates offset over temperature, except
the OffsetTC DAC voltage is fed to a summing junction at the PGA
output instead of to the MAX1457 current source.
Temperature coefficients are calculated and stored in EEPROM. In
most instances, the sensor data is taken at various pressures with
the sensor and MAX1457 at the lowest temperature, then the same
data in taken with the sensor and MAX1457 at the highest
temperature. Using this data from the temperature extremes,
software written for the MAX1457 calculates the four correction
coefficients (FSO, FSOTC, Offset, and OffsetTC). These four
coefficients correct the PRT's first-order errors. For the accuracy
level required by this respiration monitor, a fifth coefficient to
correct pressure nonlinearity is considered unnecessary.
To achieve 0.1% accuracy, the MAX1457 allows compensation at
specific temperatures, with recalculation of the FSOTC and the
OffsetTC at each temperature. The user may select up to 120
calibration points. If sensor output errors were perfectly
repeatable, the accuracy of a sensor-MAX1457 combination would be
better than 0.1%.
The MAX1457 compensation technique has a significant advantage
over the traditional approach shown in Figure 3. The MAX1457
eliminates interaction between compensation components by
separating the offset and span adjustments; it compensates offset
at the PGA, and separately adjusts the FSO via the current source.
Another advantage is the extra accuracy made available by specific
adjustments at various temperatures. That method is inherently more
accurate than one based on external resistors, whose values cannot
compensate the sensor precisely at specific temperatures.
The MAX1457 compensation technique was developed by MCA in 1994.
Maxim acquired MCA in 1997, and the original MCA staff continues to
develop the technology with the enhancement of Maxim's own
technology and worldwide distribution network.
Simpler Compensation ICs
The 16-bit resolution of the MAX1457's correction DACs is more
than is necessary for a respiration monitor. The part was chosen
because it includes the extra op amp needed to boost the monitor's
low-level sensor signals.
Although the MAX1457 offers greater precision than is warranted
for this application, its ability to compensate for temperature
errors is needed even for modest variations of temperature. A
change of 10°C commonly produces a 3% change in the FSO of a
PRT. Because the MAX1457 enables the respiration monitor to operate
over a wide temperature range, the monitor's potential applications
could include space exploration and scuba diving.
Figure 7: A MAX1450 signal conditioner operating with
external laser-trimmed resistors provides 1% accuracy.
The functions performed by the MAX1450 signal conditioner
(Figure 7) are similar to those of the MAX1457, but
resistors rather than DACs are used to set the error correction.
Because the MAX1450 uses far fewer calibration points than does the
MAX1457, its accuracy is 1%. MAX1450 chips are commonly included in
hybrids with laser-trimmed resistors to provide a low-cost
Figure 8: A MAX1458/MAX1478 signal conditioner operating
with internal 12-bit DACs provides 1% accuracy.
A third IC, the MAX1458/MAX1478 in Figure 8, provides the
same basic compensation techniques as the other two ICs, but
includes 12-bit compensation DACs. MAX1458/MAX1478 devices also
include an EEPROM for onboard storage of the compensation
coefficients. The MAX1458/MAX1478 signal conditioner provides the
same 1% accuracy as the MAX1450.
MAX1450/MAX1458/MAX1478 devices compensate a sensor by
calculating the four correction coefficients using pressure data
measured at two temperatures-usually the extremes of the
operating-temperature range. Unlike the MAX1457, these devices do
not allow for additional corrections of temperature errors at
user-selected temperature levels. For a more detailed discussion of
these compensation schemes, refer to Konrad
et al, 1999, and Dancaster et al,1997.
Respiration and Its Connection to Emotional States
The following section shows the connection between emotion and
respiration, and demonstrates that the breath-pattern monitor is a
useful investigative and possibly therapeutic tool. Another aim is
to point out the specific aspects of the waveforms that correspond
to particular emotional states.
Links between Respiration and Emotion
Anthropologists have concluded that certain basic emotions
appear in every culture. Frans Boiten, a psychological researcher,
lists the six basic emotions as "joy, sadness, fear, anger,
surprise, and disgust" (Boiten, 1998, 31).
According to one theory, each of these six emotions evokes distinct
autonomic responses-including changes in respiration. Other
theorists posit the idea that particular emotions per se do
not specifically cause changes in breathing. Instead, emotions land
somewhere along a dimension such as calm vs. excited or passive vs.
active coping, and the placement along the dimension determines the
breathing pattern (Boiten, 1998, 30).
Boiten's study demonstrated that both theories have merit, but
only in certain situations. The breath holding that occurs when one
experiences disgust or pain is best explained by the theory that
specific emotions cause certain respiratory responses. The same
holds true for the quick inhalation that occurs when one is
On the other hand, the placement of an emotion along the
negative vs. positive affect dimension explains other results. A
negative emotion enhanced the variability of the breathing pattern;
especially affected was the volume of air inhaled and exhaled.
Decreases in the pause after exhalation occurred to some extent
when a negative affect was elicited. Mild positive emotion seemed
to decrease the volume of air breathed.
Other results were best interpreted by whether the subject was
engaged in active or passive coping while the breathing pattern was
recorded. Active coping produced a "relatively fast, shallow and
regular breathing pattern. Contrarily, unavoidable aversive
stimulation…has been known to engender a passive coping
attitude, which is in the study reflected by a relatively deep and
irregular breathing pattern. Thus we have shown that detection of
and distinguishing between respiratory responses to a variety of
affective and cognitive demanding tasks requires measurement of the
fine structure of the respiratory cycle" (Boiten, 1998, 48, 49).
Through a comprehensive review of the literature discussing
emotions and respiration, Boiten and other researchers drew the
following conclusions about what emotions, behaviors, and thought
processes accompany different types of breathing (Boiten et al, 1994, 119-122).
Fast and Deep Breathing
When the depth and rate of breathing increases, it is usually
associated with "states of excitement. By 'excitement,' we mean the
state that generates undirected, aimless behavior under conditions
in which directed action is blocked or restrained. Feelings of
excitement can be understood as the felt urge towards such
undirected action." This form of breathing is closely associated
with a fight or flight response. (Boiten
et al, 1994, 119).
Fast and Shallow Breathing
"…this pattern appears to blend readiness for action with
some degree of inhibitory control…. [It]…seems typical
for attempts to tune in to behavioral demands that may necessitate
a restrained, precise, and goal-directed (as opposed to massive)
type of physical action" (Boiten et
al, 1994, 120).
Slow and Deep Breathing
A relaxed, resting state typically evokes slow, deep breathing.
Also an "unrestrained emotional readiness to act" may bring out
this type of breathing (Boiten et
al, 1994, 120, 121).
Slow and Shallow Breathing
"…primarily indicative of states that are characterized by
withdrawal from the environment and by passiveness, and might be
encountered during depressed and unhappy, as well as happy, but
unexcited moods" (Boiten et al,
Thoracic dominance breathing occurs when movement of the chest
dominates while in abdominal dominance breathing, abdominal
movement dominates. "Thoracic dominance appears primarily to
correspond to unpleasant affect, tenseness or anxiety, and
abdominal dominance to pleasant emotional states or relaxation" (Boiten et al, 1994, 121).
Irregularity of Breathing
"A number of studies have shown that breathing may become
irregular under conditions of emotional upset, excitement, and task
involvement." Changes in the volume of air breathed, the rate of
breathing, and slope of the volume of air breathed vs. time occur
from breath to breath in normal breathing. "...little is known
about the mechanisms that augment the occurrence of respiratory
irregularities during emotional behaviour" (Boiten et al, 1994, 122).
Respiration Patterns in Normal, Neurotic, and Psychotic
Psychologists use a number of diagnostic tools to evaluate the
mental and emotional status of patients. In addition to the
numerous tests that can be administered, a psychologist's trained
eye often picks up behavioral nuances usually missed by the
One diagnostic aid is the breathing pattern of an individual.
Certain features of these patterns distinguish mentally healthy
people from those who are less healthy. A number of those features
discovered or confirmed by one study of the breathing patterns of
normal, neurotic, and psychotic individuals are listed below (Clausen, 1951).
Clausen measured the timing of respiration patterns and created
a rough estimate of its amplitude using a pneumograph. He measured
both the thoracic and abdominal breathing of normal, neurotic, and
psychotic people for a brief period on each of five days. Five
indicators of neurosis were found within breathing waveforms: a
shorter duration of the breath cycle, a sharper abdominal peak vs.
thoracic peak, a triangular shape to the abdominal peak, a
rectilinear shape of the abdominal inhalation, and a sharp
abdominal transition from inhalation to exhalation. The latter gave
a particularly strong indication of neurosis. Strong signs of
neurosis in men include dissimilarity in the recordings over the
five days and a bell shape to the thoracic peak. In women, a
lightly concave abdominal exhalation was indicative of neurosis. A
somewhat less significant indicator for both men and women was an
irregular-breathing pattern. Also, a lack of exhalation pauses also
points to a neurotic condition.
Clausen comments on the variability of breathing patterns of
neurotics: "Some authors…have claimed that neurotic patients
have more deliberate controlled respiration which should be a means
of suppressing emotions. The above findings may fit in with this
assumption. The greater variability from day to day could indicate
that neurotics do not breathe naturally and automatically, i.e.,
they have no characteristic basic type of response to which they
might revert in a relaxed state. At the same time the deliberate
control will guarantee a limited fluctuation around the level they
are trying to maintain for that particular day" (Clausen, 1951, 51). Later in the paper,
Clausen further summarizes: "Mental disturbance per se, whether
neurosis or psychosis, changes the breathing pattern. More
specifically, it could be that people with chronic emotional
conflicts develop a cautious respiration, characterized by fast and
shallow respiration of thoracic type as a means of suppressing the
emotions" (Clausen, 1951, 60).
Respiration and Anxiety
While anxiety is the hallmark of a fully developed neurosis,
many without a diagnosable neurotic condition suffer from it daily.
Fritz Perls, the founder of Gestalt therapy, proposed a theory that
explains anxiety in terms of inhibited breathing. One author (Fesmire, 1994) quotes Perls et al:
"Anxiety is the experience of breathing difficulty during any
blocked excitement" (Perls et al,
1951,128). In other words, we tend to block emotion by constricting
our breathing. Perls felt that the societal taboo on showing fear
causes us to block that emotion, resulting in an emotion closely
related to fear-anxiety. When angry, we also try to keep control by
holding our breath. Again Fesmire quotes Perls et al: "The
anxiety occurs when we try to rein in the excitement within the
limits of decorum" " (Perls et al,
The link between anxiety and breathing was clearly shown in the
following experiment. The respiratory efficiency of one group that
suffered from anxiety and a control group free of any anxiety
disorders were measured. Respiratory efficiency, which is "a
measure of the respiratory effort required by the subject to
extract 100ml of oxygen from the respired air" (Coppen and Mezey, 1960, 54), was significantly
lower for the anxious group than for the control group. The anxious
group was then injected with sodium amytal, a drug very effective
in reducing anxiety. The respiratory efficiency of the anxious
group became similar to that of the control group, suggesting a
direct link between anxiety and respiration.
Some relief from anxiety may occur through the retraining of
one's breathing patterns. One paper states that the retraining of
one's breathing patterns can be used to treat sleeping
difficulties, high/low muscle tension, and nervousness (Buckholtz, 1994, 147). Many psychologists
suggest psychotherapy instead; the resulting emotional healing can
then be demonstrated by changes in the breathing pattern.
Imke Buckholtz (1994, 145) discusses the
origin of breathing problems, stating that damaged breathing
muscles are an obvious source. He further explains that "many
breathing disorders have their origin in trauma, fear, shock, pain,
grief, or other stressful events in both the early and adult years
of life." And by suppressing one's breathing, a person can diminish
the "pain, stress, or grief of a situation."
Buckholtz describes a method for retraining the breathing
created by Elsa Gindler, who "experimented with movements to
strengthen the deeper layers of the muscular system and improve the
circulation of oxygen, movements that reduced tensions that had
been preventing the breathing muscles from functioning properly"
(Buckholtz, 1994, 141). He claims that
breathing retraining often results in improved emotional stability
(Buckholtz, 1994, 145). Thus the reverse of
the axiom that emotions play a role in breathing patterns seems
possible; changing the breathing results in an emotional
Buckholtz advises that Gindler's breathing retraining "does not
directly address psychological problems" (Buckholtz, 1994, 146). He suggests that
reduction of physical problems through breathing retraining may be
what a person needs to make long-standing improvements in everyday
life. However, he cautions: "But sometimes, especially with early
childhood trauma, or if the client constantly finds himself falling
back into the same restrictive patterns, the help of a
psychotherapist is required. The patient needs to become aware of
the primary experience that caused the physical block and 'undo' it
by consciously going through it again" (Buckholtz, 1994, 146).
Changes in breathing that are consciously induced or conditioned
by operant or classical means "can affect both our thoughts and our
feelings" (Ley, 1994, 96). Ley also
suggests "the implications of this premise are relevant to theory,
diagnosis, and treatment of stress and anxiety-related disorders
(e.g., panic disorder, phobias, test anxiety, occupational strain,
and related psychosomatic disorders), and to basic and applied
research in the psychophysiology of breathing" (Ley, 1994, 95).
Buckholtz's and Ley's ideas lend credence to the notion that
changing the breath pattern aids in controlling emotional states.
However, neither statement directly suggests creating this change
through the use of monitoring devices such as the one described
Presented at Sensor Expo, Cleveland, Ohio, September 14-16,