MOLOKHIA, HUFFMAN, ROBINSON, A 
latter have been reported to require two hours 
postoperatively before resumption of spontane- 
j ous breathing.^" Halothane was preferred to 
other inhalation anesthetics since it is non- 
flammable, causes only mild respiratory depres- 
sion, and in the concentrations used produces 
little cardiac and circulatory depression. Al- 
though mixtures of nitrous oxide and a more 
potent anesthetic have been used in the calf,-'" 
NoO was not employed during our procedures 
since the high concentrations required would 
restrict the fraction of oxygen available in the 
inspired gas mixture. In fact, although clinical 
observations suggest a better postoperative 
course, there is no definite evidence that nitrous 
oxide plus a more potent anesthetic is prefer- 
able to the volatile agent alone.'' 
Ventilation was the largest problem. Tradi- 
tional methods of estimating tidal volumes by 
observing chest wall excursion during inspira- 
tion are unreliable. The calf's chest wall is ex- 
tremely thick and inelastic. During spontaneous 
respiration, the chest wall exhibits little motion 
with diaphragmatic movement predominating. 
All animals were ventilated with a Bennett 
(Bennett Respirator Products, Santa Monica, 
California) PR-1 pressure-cycled respirator 
with attached spirometer, delivering 50% oxy- 
gen at a rate of 15 breaths/minute. In an initial 
series of eight calves the respirator was set at 
inspiratory pressures of 15-20 cm HoO. Tidal 
volumes ranged between 1000 and 1200 ml ini- 
tially. Animals in this group developed hypoxia, 
moderate hypercarbia and acidosis soon after 
thoracotomy, and the tidal volumes dropped to 
600-800 ml. Large amounts of sodium bicar- 
bonate were necessary to maintain pH. Arterial 
pOo fell as low as 75 mmHg during assisted 
ventilation intraoperatively and 70 mmHg dur- 
ing spontaneous oxygen breathing postopera- 
tively. Corresponding values of O2 saturation 
and pCOo were 93% and 92%, and 42 and 48 
mmHg, respectively. These values may be com- 
pared to normal pressures and saturation at 
37° C and at body temperature determined by 
Donawick^'^ in calves of comparable size (Table 
I). A more detailed representation is given in 
Table II and graphically in Figure 1. The ab- 
normal blood gases encountered at low ventila- 
tory pressures were not correctable by manual 
MACOPOULOS, PONN AND NORMAN 385 
deep inflation of the lungs at 10-15 minute in- 
tervals. Intermittent use of 100% oxygen in- 
halation was also ineffective in producing sus- 
tained increases of pOo to acceptable levels. 
In a second series of eight calves inspiratory 
pressures were raised to 30-40 cm HoO. Tidal 
volumes were 1400-1600 ml initially. In con- 
trast to the earlier group, PO2 was maintained 
above 150 mmHg and tidal volume above 1200 
ml during the entire procedure, with Oo satura- 
tions above 96%). pCOo decreased to a minimum 
of 29 mmHg but generally remained within 
acceptable limits (Table II, Figure 1). These 
results indicate that ventilatory pressures of 
Table I. — Values for pH and blood gases in normal 
calves (from Donawick et al.^') 
Minimum Maximum Mean ± S. D. 
pH 
37°C - 7.37 7.4G 7.40 ± 0.02 
Body temp _ 7.37 7.44 7.37 ± 0.02 
Arterial pCOu (mmHg) 
37°C 34.7 44.0 39.5 ± 3.06 
Body temp. 36.9 48.5 42.8 ± 3.28 
Arterial pOs (mmHg). 
37°C 72.5 94.5 83.9 ± 83.9±6.30 
Body temp. 79.0 108.6 93.6 ± 7.68 
INSPIRATORY PRESSURE 35-40 CM HjO 
INSPIRATORY PRESSURE 15-20 CM H^O 
Figure 1. — Arterial pOs levels in calves during LVAD 
implantation under different ventilatory pressures. 
