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BACKGROUND Apnoeic oxygenation with high-flow nasal oxygen prolongs the safe apnoeic period during induction of general anaesthesia. However, central haemodynamic effects and the characteristics of central gaseous exchange remain unexplored. OBJECTIVE To describe mean pulmonary arterial pressure along with arterial and mixed venous blood gases and central haemodynamic parameters during apnoeic oxygenation with low-flow and high-flow nasal oxygen in pigs. DESIGN Experimental crossover study. SETTING Animal study of 10 healthy Swedish landrace pigs at Karolinska Institutet, Sweden, April-May 2021. INTERVENTION The pigs were anaesthetised, their tracheas intubated and their pulmonary arteries catheterised. The animals were preoxygenated and paralysed before apnoea. Apnoeic periods between 45 and 60 min were implemented with either 70 or 10 l min-1 100% O2 delivered via nasal catheters. In addition, seven animals underwent an apnoea without fresh gas flow. Cardiopulmonary parameters and blood gases were measured repeatedly. MAIN OUTCOME MEASURES Mean pulmonary arterial pressure during apnoeic oxygenation with high-flow and low-flow oxygen. RESULTS Nine pigs completed two apnoeic periods of at least 45 min with a PaO2 not lower than 13 kPa. The mean pulmonary arterial pressure increased during 45 min of apnoea, from 18 ± 1 to 33 ± 2 mmHg and 18 ± 1 to 35 ± 2 mmHg, at 70 and 10 l min-1 O2, respectively (P < 0.001); there was no difference between the groups (P = 0.87). The PaCO2 increased by 0.48 ± 0.07 and 0.52 ± 0.04 kPa min-1, at 70 and 10 l min-1 O2, respectively; there was no difference between the groups (P = 0.22). During apnoea without fresh gas flow, the SpO2 declined to less than 85% after 155 ± 11 s. CONCLUSION During apnoeic oxygenation in pigs, the mean pulmonary arterial pressure increased two-fold and PaCO2 five-fold after 45 min, while the arterial oxygen levels were maintained over 13 kPa, irrespective of high-flow or low-flow oxygen.