Effects of frequency and airway pressure on gas exchange during interrupted high-frequency, positive-pressure ventilation in ponies.

The research paper presents a comparative study between conventional mechanical ventilation (CMV) and interrupted high-frequency, positive-pressure ventilation (IHFPPV) in ponies. The study investigates cardiovascular effects and pulmonary gas exchange during both methods, revealing that IHFPPV, when adjusted in frequency and airway pressure, can provide equivalent or better gas exchange compared to CMV.

Study Design and Procedure

The study was conducted on six anesthetized ponies laid in dorsal recumbency. The researchers compared the effects of two different ventilation methods: conventional mechanical ventilation (CMV) and interrupted high-frequency, positive-pressure ventilation (IHFPPV). They manipulated the parameters of the IHFPPV, including the ventilator frequency (ranging from 2.5 to 12.5 Hz) and the peak airway pressure (Paw), which stayed constant at control values obtained during CMV (18 to 20 cm of H2O).

Observations

During the experiment, the researchers observed some significant changes from control values, notably in:

• The ratio of dead-space volume to tidal volume (VD/VT)
• The respiratory minute volume (VE)

The carbon dioxide excretion (VCO2) measured during IHFPPV averaged at 2.12 ml/kg/min. With an increase in the ventilator frequency, the researchers documented a direct increase in both dead-space ventilation (which ranged from 40 to 73.7% of total ventilation) and VE (which increased from 89 ml/kg/min at 2.5 Hz to 145 ml/kg/min at a frequency of 12.5 Hz).

Alterations in IHFPPV Frequency and Paw

The researchers also experimented by keeping the IHFPPV frequency constant at 12.5 Hz while manipulating the Paw from 5 to 30 cm of H2O. This modification led to significant differences in:

• Arterial partial pressure of O2 (PaO2)
• VCO2
• Pulmonary shunt fraction (QS/QT)
• VE
• Arterial-alveolar differences in oxygen tension (AaDO2)
• VD/VT
• Cardiac output

Findings suggest that PaO2 and VCO2 increase linearly when Paw increased. Moreover, VD/VT declined with the rising Paw, decreasing from 98 to 69.3%.

Comparison of IHFPPV and CMV

Comparing the effects of the two ventilation methods, the study discovered that gas exchange at a Paw of 15 cm of H2O during IHFPPV was comparable to conditions at a Paw of 20 cm of H2O under CMV. When the Paw was increased during IHFPPV, gas exchange improvements above control values were witnessed.