The document describes a study that measured intrathoracic pressure (ITP) in an anesthetized rat undergoing volume-controlled mechanical ventilation. A pressure catheter was inserted into the rat's diaphragm to measure ITP during ventilation. The rat was ventilated with a preset tidal volume and respiration rate. ITP measurements during five ventilation cycles showed that ITP decreased during inhalation and increased during exhalation in response to the ventilator. The change in ITP (delta) and time periods varied slightly between cycles. Future studies should measure transpulmonary pressures and compare ITP during spontaneous breathing.

1. INTRATHORACIC PRESSURE (ITP)
MEASUREMENT IN ANESTHETIZED RAT
Pressure catheter diaphragm-access
measurements during invasive
volume-controlled mechanical
ventilation

2. RAT VENTILATION
Animal ventilated using Small Animal Ventilator 683 Harvard Apparatus by means of invasive
volume-controlled mechanical ventilation. Anesthesia induced by mixed gas (Oxygen with
0.5% isoflurane) delivered as a pre-set tidal volume (Vt) and respiration rate (RR) in
strokes/min; while allowing passive exhalation with (I/E ratio 1:1)*
Endotracheal tube was inserted through tracheostomy
Tidal Volume and Respiration Rate were calculated based on rat’s body weight:
In this case: Vt = 2.2 ml and RR= 70 ventilation cycles/min
Ventilated volume rate per kg and minute was: 5.3ml/kg/min
5 ventilation cycles are followed by Table
* During spontaneous breathing (I/E ratio is 1:2); hence the exhalation is twice as long as inhalation

3. VENTILATOR’S CYCLIC EFFECT ON ITP
inspiration
expiration
During all 13 breaths the mean intrathoracic pressure (-3.177) mmHg was detected
Ventilator’s cyclic effect on ITP
Channel 1, Rat intrathoracic (ITP) / (intrapleural pressure) during controlled mechanical ventilation
Channel 2, Mean ITP
(ventilation parameters has not been adjusted)
Rat Intrathoracic pressures (ITP)/intrapleural pressure measured using pressure catheter inserted through
diaphragm. Diaphragme later re-sutured and airtightly sealed
pneumothorax prevented by using invasive volume-controlled mechanical ventilation with pre-set Vt and RR

4. ITP during Inhalation cyclic effect of Ventilator (cycle 1)
During 0.094 sec time difference from peak to trough; ITP has dropped during inhalation from -1.416 to -6.356 that is (-4.94mmHg)
Ventilator’s Ratio I/E set to 1:1
Not adjusting Vt or RR

5. ITP during Exhalation with cyclic effect of Ventilator (cycle 1)
During 0.056 sec time difference from trough to peak; ITP has increased during exhalation from -6.356 to -1.161 that is (5.195mmHg)
Ventilator’s Ratio I/E set to 1:1
Not adjusting Vt or RR

6. ITP during Inhalation cyclic effect of Ventilator (cycle 2)
During 0.092 sec time difference from peak to trough; ITP has dropped during inhalation from -1.161 to -5.656 that is (-4.495mmHg)
Ventilator’s Ratio I/E set to 1:1
Not adjusting Vt or RR

7. ITP during Exhalation with cyclic effect of Ventilator (cycle 2)
During 0.056 sec, time difference from trough to peak; ITP has increased during exhalation from -5.656 to -1.041 that is (4.615mmHg)
Ventilator’s Ratio I/E set to 1:1
Not adjusting Vt or RR

8. During 0.088 sec time difference from peak to trough; ITP has dropped during inhalation from -1.041 to -3.501 that is (-2.46mmHg)
ITP during Inhalation with cyclic effect of Ventilator (cycle 3)
Ventilator’s Ratio I/E set to 1:1
Not adjusting Vt or RR

9. ITP during Exhalation with cyclic effect of Ventilator (cycle 3)
During 0.056 sec, time difference from trough to peak; ITP has increased during exhalation from -3.501 to -1.093 that is (2.409mmHg)
exhalation
Ventilator’s Ratio I/E set to 1:1
Not adjusting Vt or RR

10. ITP during Inhalation with cyclic effect of Ventilator (cycle 4)
Ventilator’s Ratio I/E set to 1:1
Not adjusting Vt or RR

11. ITP during Exhalation with cyclic effect of Ventilator (cycle 4)
Ventilator’s Ratio I/E set to 1:1
Not adjusting Vt or RR

12. ITP during Inhalation with cyclic effect of Ventilator (cycle 5)
inhalation
Ventilator’s Ratio I/E set to 1:1
Not adjusting Vt or RR

13. ITP during Exhalation with cyclic effect of Ventilator (cycle 5)
Ventilator’s Ratio I/E set to 1:1
Not adjusting Vt or RR

14. ITP during
inspiration
From peak
(mmHg)
To trough
(mmHg)
Delta (mmHg) Time (sec)
Cycle 1 -1.416 -6.356 4.94 0.094
Cycle 2 -1.161 -5.656 4.495 0.092
Cycle 3 -1.041 -3.501 2.46 0.088
Cycle 4 -1.903 -3.685 2.592 0.090
Cycle 5 -1.942 -6.035 4.092 0.106
ITP during
expiration
From trough
(mmHg)
To peak
(mmHg)
Delta (mmHg) Time (sec)
Cycle 1 -6.356 -1.161 5.195 0.056
Cycle 2 -5.656 -1.041 4.615 0.056
Cycle 3 -3.501 -1.093 2.409 0.056
Cycle 4 -3.685 -1.942 1.742 0.060
Cycle 5 -6.035 -1.394 4.641 0.056
If not using controlled mechanical volume ventilation, during
spontaneous inspiration the ITP decreases (higher negative number
as compared to atmospheric pressure) due to thoracic volume
increase while during expiration, the ITP increases (lesser negative
number) as thoracic volume decreases, getting closer to atmospheric
pressure.
During controlled mechanical volume ventilation, the ITP trough
during 5 inspiration cycles varied (from -6.356 to -3.501). When ITP
was measured close to -6mmHg during inspiration it resembled
spontaneous breathing as compared to ITP at -3.5 mmHg.
Similarly, the ITP peak during expiration varied from (-1.041 to -
1.942).
In this case increase of transpulmonary pressures might have limiting
effect on both the expiration and inspiration ITPs. As the inspiration
time was not been able to be pre-set, increased inspiration times
might have temporary increased Vt that could have led to auto-PEEP
in following cycles.
Time during which ITP has risen during exhalations stayed the same
(0.056sec) except at cycle 4 where the highest subatmosperic peak
ITP was detected (-1.942) at exhalation, most likely due to
cumulative increase of transpulmonary pressures not allowing full ITP
return (closer to atmospheric pressure).
Future experiements: To perform better analysis of volume
ventilation and its effects on ITP, employing measurement of
transpulmonary pressures at the same time as ITP are necessary.
Additionally for comparison, ITP measurement using solid state
pressure sensor might need to be performed to detect ITPs during
spontaneous breathing.